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Noclevername
2015-Oct-07, 04:50 AM
1. From the Hubbles Law vs. Universal Acceleration (http://cosmoquest.org/forum/showthread.php?158709-Hubbles-Law-vs-Universal-Acceleration) thread:


If the Universe is infinite, then only a finite portion of it was involved in the Big Bang.

Is this true or not, and why?

2. Can an infinite space experience internal expansion?

3. What evidence is there for universal homogeny?

Jeff Root
2015-Oct-07, 06:08 AM
I'm Jeff Root, and I approve these questions.

Not that I need to, of course. Just want you to know I do.

-- Jeff, in Minneapolis

grapes
2015-Oct-07, 07:46 AM
1. From the Hubbles Law vs. Universal Acceleration (http://cosmoquest.org/forum/showthread.php?158709-Hubbles-Law-vs-Universal-Acceleration) thread:



Is this true or not, and why?

I think even Jeff admits in that thread that this is not a mainstream view, no?


2. Can an infinite space experience internal expansion?

You mean, mathematically? Just take any "center" (Cx, Cy, Cz) and let any point (x,y,z) move to
( (x - Cx)ht + Cx, (y - Cy)ht + Cy, (z - Cz)ht + Cz )

Did I do that right? It's early :)


3. What evidence is there for universal homogeny?
That was also answered, I think--the evidence that the known universe is "flat". Or do you mean something else?

Jens
2015-Oct-07, 08:34 AM
If the Universe is infinite, then only a finite portion of it was involved in the Big Bang.

The first question involves that statement. I think it's an interesting proposal. In fact, I basically agree with Jeff. In a previous thread I started by arguing that something that is finite cannot become infinite if only a finite amount of time has elapsed, but actually it was pointed out to me that it is possible mathematically for a finite number to increase to infinity in a finite amount of time, so that for example at T=1 it reaches a limit of infinity.

The problem I have with that is, it is difficult to fathom what this would mean physically. Either time keeps going after T=1, and you wonder what happens to the extent of space at that time, or perhaps the increasing speed of the particles means that time slows down (in a sort of Zeno-ish way) so that it never reaches T=1.

For the second question, the answer seems intuitively to be yes. Making an analogy, if you have an infinitely sized plane and then you make folds in it, you can double the area of the already infinite plane.

And on the third, I'm not sure there is clear evidence of homogeneity or non-homogeneity. It is generally assumed that the large-scale structure of the universe is homogeneous, but there have been debates about this. The latest big paper I am aware of (2012) claims that it is homogeneous.

Noclevername
2015-Oct-07, 09:21 AM
I think even Jeff admits in that thread that this is not a mainstream view, no?


I wrote it because I would like the mainstream view explained.

Noclevername
2015-Oct-07, 09:23 AM
In a previous thread I started by arguing that something that is finite cannot become infinite if only a finite amount of time has elapsed, but actually it was pointed out to me that it is possible mathematically for a finite number to increase to infinity in a finite amount of time, so that for example at T=1 it reaches a limit of infinity.

What if it starts out infinite?

Noclevername
2015-Oct-07, 09:43 AM
I'm Jeff Root, and I approve these questions.

Not that I need to, of course. Just want you to know I do.

-- Jeff, in Minneapolis

Jeff, I know you think it's true. Could you explain the reasoning behind that? This is the perfect place to spell out your own ideas.

Jetlack
2015-Oct-07, 10:49 AM
isn't expanding infinities sort of a contradiction? I´m sort of confused by the concept. How does something infinite become less or more so infinite?

Noclevername
2015-Oct-07, 10:58 AM
isn't expanding infinities sort of a contradiction? I´m sort of confused by the concept. How does something infinite become less or more so infinite?

It doesn't, it just gets more stuff in it (including more empty spaces between the stuff.)

Jens
2015-Oct-07, 11:08 AM
isn't expanding infinities sort of a contradiction? I´m sort of confused by the concept. How does something infinite become less or more so infinite?

Suppose you think of an infinite plane, and then make it into a cube. The cube is clearly infinitely bigger than the already infinite plane.

Jens
2015-Oct-07, 11:10 AM
What if it starts out infinite?

If it starts out infinite, then there isn't any problem to talk about.

Noclevername
2015-Oct-07, 11:15 AM
The example I often hear is the Infinite Hotel, technically the name is Hilbert's Paradox of the Grand Hotel (https://en.wikipedia.org/wiki/Hilbert%27s_paradox_of_the_Grand_Hotel). A hotel with infinite rooms can be made to accommodate infinite new guests by having every guest move to leave empty rooms between them.

I can visualize this, but I'm not sure how it works technically.

Ken G
2015-Oct-07, 12:16 PM
The first thing to understand is that science makes models, and tests them. Science can make a model of an infinite homogeneous universe, then find that model tests out well. Indeed, that is precisely the situation we find ourselves in. However, this does not mean science could ever answer the question "is the universe really infinite", or "is the universe really homogeneous." I think it's pretty clear those could never be answered in the affirmative in a demonstrable way. So if Jeff wants to say that the universe cannot be infinite and homogeneous, there is certainly never going to be a scientific outcome that can refute his belief. There could be evidence it is finite, or inhomogeneous in some fundamental way on very large scales, but no evidence of either of those exist yet. A fairly normal thing for a scientific thinker to do, in this situation, is to continue to adopt models that are infinite and homogeneous, while looking for evidence that the models break down somewhere. How that search will play out is anyone's guess, but I'd say there is already pretty good evidence that such a search may never yields results, since we've already seen pretty much as far as we are going to.

mkline55
2015-Oct-07, 12:22 PM
If the universe is infinite, then how could the rate of expansion be affected by gravity? Effectively, the pull of gravity should be equal in all directions, shouldn't it?

Noclevername
2015-Oct-07, 12:36 PM
If the universe is infinite, then how could the rate of expansion be affected by gravity? Effectively, the pull of gravity should be equal in all directions, shouldn't it?

I thought gravity was local. Isn't that what the inverse square law means?

Ken G
2015-Oct-07, 12:56 PM
If the universe is infinite, then how could the rate of expansion be affected by gravity? Effectively, the pull of gravity should be equal in all directions, shouldn't it?Ah, a very important question, because it is exactly what Newton thought-- but he was wrong. Newton thought that an infinite homogeneous universe (which was the model he apparently favored) could be static, for just the reason you say-- the force of gravity would not know what direction to point, so had to be zero. This is wrong for two reasons:

1) Such a situation would never be stable. That means, start out with this picture, and Newton's model for gravity, and the universe still would not be static because any tiny perturbation on a large enough spatial scale, for any reason at all, would grow with time. Indeed, this is the reason we have galaxies in the first place. Newton would have been fine with galaxies as long as they were sprinkled around in a static way, but that wouldn't work either-- the instability only gets worse on larger scales. The way our universe gets around that problem is expansion.

2) Newton's model of gravity held that the force of gravity was an absolute, so it absolutely had to point in some particular direction at some point, or it had to be zero at that point. Had Newton simply relaxed that single requirement, and allowed the magnitude and direction of the force of gravity to depend on your perspective, he could have devised a Big Bang model himself, without needing general relativity. You just pick an arbitrary point, and regard the force of gravity to be zero at that point, but then there's a force of gravity pointing toward that point at all other points, following Newton's formula. If you set it up like that, it works just fine, and you get something quite close to the modern Big Bang model with a flat universe, if you start out with the appropriate initial condition (which Newton would have had no reason to know, because he hadn't seen the Hubble Law, but then again, neither had Edgar Allen Poe when he conceptualized that initial condition). You just don't let it bother you that if you had picked a different point of reference to be your apparent "center" of the universe, all the forces would be different, because the observable behavior of the universe you are predicting would have been just the same.

Ken G
2015-Oct-07, 01:03 PM
I thought gravity was local. Isn't that what the inverse square law means?Believe it or not, an inverse-square force is regarded as an "infinite range" force, for reasons quite similar to the "Olber's paradox" in static cosmology. The finite age of the universe solves both problems.

Noclevername
2015-Oct-07, 01:09 PM
https://en.wikipedia.org/wiki/Olbers%27_paradox

OK, so if I have this right, the reason gravity doesn't pull in all directions equally is that it's a light speed phenomenon, and we're simply outrunning its effects?

Jeff Root
2015-Oct-07, 01:12 PM
If the Universe is infinite, then only a finite portion of it
was involved in the Big Bang.
If that is not mainstream -- and I agree that it seems
not to be -- I'd like to know why it isn't.



2. Can an infinite space experience internal expansion?
I'm not sure what is being asked, here. Grapes and Jens
both gave purely mathematical answers. A real answer
has to be physical, not mathematical, but the question
needs to be more defined.

I have no problem with the idea of expansion of already
infinite space, but I can't envision how such a situation
could come about. It seems completely fanciful, not
like something that might actually occur.



3. What evidence is there for universal homogeny?
If I understand both the question and the theory correctly,
I believe the best evidence is the uniformity of the cosmic
background radiation. It shows that the hot gas which filled
the Universe some 380,000 years after the Big Bang had a
very uniform temperature. The sphere that we can see the
CBR from was only about 42 million light-years in diameter
at the time it was emitted, but the matter which emitted it
is now well beyond the most distant galaxies we can see,
so it has grown to a really gignormous volume. One can
extrapolate from gignormous to infinite, but that seems
pretty iffy.

Very strangely, even though the CBR is incredibly smooth,
the slight variations in it are said to be greater than can
be explained without some mechanism to create them.

More definitely, the very large-scale structure of galactic
filaments and walls is said to be more advanced than can
be explained by existing theory. There hasn't been enough
time since the Big Bang for the structure to be so evolved.
I have not yet seen any suggestion that dark energy might
contribute to that evolution somehow. My suggestion is
that mutual gravitational repulsion ("antigravity") between
ordinary matter and antimatter could provide the needed
extra push to form the filaments and walls rapidly. As well
as causing acceleration of the expansion. And expansion
in general.

It is thought that about one particle in a billion escaped
annihilation of matter and antimatter in the first second
of the Big Bang. All that would be required to have equal
amounts of matter and antimatter in the Universe would
be for one particle of matter and one particle of antimatter
in a billion to have escaped annihilation. Gravitational
repulsion between regions of matter and antimatter could
then provide the impetus for the very slight temperature
variations in the CBR, cosmic expansion, rapid evolution of
large-scale structures, and acceleration of the expansion.

-- Jeff, in Minneapolis

Noclevername
2015-Oct-07, 01:15 PM
I have no problem with the idea of expansion of already
infinite space, but I can't envision how such a situation
could come about. It seems completely fanciful, not
like something that might actually occur.


Why not?

I hear you say this objection a lot, can you lay out all the whys and wherefores so I can understand the reasoning behind your objection?

Jetlack
2015-Oct-07, 01:17 PM
Suppose you think of an infinite plane, and then make it into a cube. The cube is clearly infinitely bigger than the already infinite plane.

Okay yes that makes sense :-) Cheers.

mkline55
2015-Oct-07, 01:25 PM
Suppose you think of an infinite plane, and then make it into a cube. The cube is clearly infinitely bigger than the already infinite plane.

I cannot conceptualize that. You mean take the edges of the infinite plane and fold them together?

Jetlack
2015-Oct-07, 01:28 PM
On the issue of Olber´s paradox. That also confuses me because even on the most clear night sky we can´t see dim or very distant stars even when we know they are there, at the centre of our visual focus. We can only see as many stars as the effectiveness or our instruments permit. I´m not saying the universe is not finite, but is Olber´s paradox really any proof it is?

Grey
2015-Oct-07, 02:35 PM
If that is not mainstream -- and I agree that it seems
not to be -- I'd like to know why it isn't.It's not that it's not possible that this is the case. It's that it's not accepted as the only possibility. Suggesting it as an alternative (though at this point, one without any real evidence to support it, as Ken G points out) is fine, but continuing to insist that it must be true is the problem. (And as Ken G also correctly points out, it's not that there's direct evidence that the universe is infinite either, although our observations are certainly consistent with that; we really are on the edge of what may be even theoretically observable.)


I have no problem with the idea of expansion of already
infinite space, but I can't envision how such a situation
could come about. It seems completely fanciful, not
like something that might actually occur.This just seems to be a failing of your imagination. Suggesting that the universe can't be this way because you can't imagine it is never a good position to take.


It is thought that about one particle in a billion escaped
annihilation of matter and antimatter in the first second
of the Big Bang. All that would be required to have equal
amounts of matter and antimatter in the Universe would
be for one particle of matter and one particle of antimatter
in a billion to have escaped annihilation. Gravitational
repulsion between regions of matter and antimatter could
then provide the impetus for the very slight temperature
variations in the CBR, cosmic expansion, rapid evolution of
large-scale structures, and acceleration of the expansion.I think the burden would be on you to show that this actually works to create a universe like we see (I don't think it would). And it relies on the assumption that matter and antimatter repel each other gravitationally. We've had discussions about that before, and there are some very good theoretical reasons to think that's not the case. ALPHA will probably give us a definitive answer in a few years, but I wouldn't bet your whole cosmological model on it.

profloater
2015-Oct-07, 02:38 PM
On the issue of Olber´s paradox. That also confuses me because even on the most clear night sky we can´t see dim or very distant stars even when we know they are there, at the centre of our visual focus. We can only see as many stars as the effectiveness or our instruments permit. I´m not saying the universe is not finite, but is Olber´s paradox really any proof it is?
Here is a related thought experiment.
Imagine a room with perfect white walls that scatter and reflect all the light.
Start in the dark and light a lamp, what happens?
Now expand the room to let's say one light second size, you are very small in comparison but light the same light.
What happens?

profloater
2015-Oct-07, 02:39 PM
[QUOTE=Jetlack;2317645]On the issue of Olber´s paradox. That also confuses me because even on the most clear night sky we can´t see dim or very distant stars even when we know they are there, at the centre of our visual focus. We can only see as many stars as the effectiveness or our instruments permit. I´m not saying the universe is not finite, but is Olber´s paradox really any proof it is?[/

Jeff Root
2015-Oct-07, 02:40 PM
I have no problem with the idea of expansion of already
infinite space, but I can't envision how such a situation
could come about. It seems completely fanciful, not
like something that might actually occur.
Why not?

I hear you say this objection a lot, can you lay out all the
whys and wherefores so I can understand the reasoning
behind your objection?
There isn't much reasoning involved.

First, I must say that I left an important qualifier out
of that statement, but I expect you interpreted it as if
I had stated it. I should have said:

I have no problem with the idea of uniform expansion
everywhere throughout already infinite space, but I can't
envision how such a situation could come about. It seems
completely fanciful, not like something that might actually
occur.

The idea of local expansion within an infinite space is
relatively trivial. Using the imagery suggested by Jens,
take an infinite plane, grasp a point on it and pull it out,
stretching it. You have increased the area of the plane
in that small region. Or add a lump of matter to the
Universe.

But you are undoubtedly talking about uniform expansion
of an infinite volume. The question is: How can you get
something to happen everywhere throughout an infinite
volume of space, more-or-less simultaneously? It doesn't
sound remotely possible to me. If you think it is possible,
I'd say it is incumbent on you to explain how it could come
about, not on me to say why it could not come about.

Again, I can imagine and even sort of visualize an infinite
universe expanding uniformly everywhere simultaneously
as a fantasy idea, but I can't imagine such an expansion
as being physically possible. Something like how I can
imagine and even sort of visualize invisible pink unicorns,
but can't imagine how they could actually be physically
possible.

-- Jeff, in Minneapolis
.

Noclevername
2015-Oct-07, 03:08 PM
But you are undoubtedly talking about uniform expansion
of an infinite volume. The question is: How can you get
something to happen everywhere throughout an infinite
volume of space, more-or-less simultaneously? It doesn't
sound remotely possible to me. If you think it is possible,
I'd say it is incumbent on you to explain how it could come
about, not on me to say why it could not come about.


I only know that I don't know enough about the physics involved to say whether it's possible or not.

You seem to be assuming that some kind of coherent signal is needed to get things to coordinate over distances (correct me if I'm wrong). I can think of other ways to get even expansion, such as the energy substrate just taking the same amount of time to cool down everywhere. Or, expansion as it occurs is simply an innate property of spacetime reacting to energy, so everywhere that has spacetime and energy is going to expand at the same rates.


Again, I can imagine and even sort of visualize an infinite
universe expanding uniformly everywhere simultaneously
as a fantasy idea, but I can't imagine such an expansion
as being physically possible. Something like how I can
imagine and even sort of visualize invisible pink unicorns,
but can't imagine how they could actually be physically
possible.


The universe is often counter-intuitive, maybe more often than it matches our intuitive views, so what we "feel" is physically possible is simply a bad guide to what actually happens.

Jeff Root
2015-Oct-07, 03:45 PM
If the Universe is infinite, then only a finite portion of it
was involved in the Big Bang.

If that is not mainstream -- and I agree that it seems
not to be -- I'd like to know why it isn't.
It's not that it's not possible that this is the case.
It's that it's not accepted as the only possibility.
Suggesting it as an alternative (though at this point,
one without any real evidence to support it, as Ken G
points out) is fine, but continuing to insist that it must
be true is the problem.
I don't understand why you see a problem. I'm not
suggesting any possibility or alternative -- I'm saying
that one putative group of possibilities out of a whole
host of possibilities can be ruled out because of its
physical impossibility. Namely, an infinite universe in
which the entire universe participated in the Big Bang.
That still leaves the possibilities of a finite universe in
which the entire universe participated in the Big Bang,
or of an infinite universe in which only a portion of the
universe participated in the Big Bang. That includes all
kinds of possibilities in which inflation occurs or doesn't
occur, in which eternal inflation produces infinitely many
separate universes, in which branes bang together, and
whatnot. But this one limited category of suggested
possibilities can be ruled out, because it isn't physically
possible.



This just seems to be a failing of your imagination.
Suggesting that the universe can't be this way because
you can't imagine it is never a good position to take.
Well, that isn't what I suggested.

It is a fact that I can't imagine how an infinite universe
could do the same thing everywhere simultaneously.
That is not an argument, and it is not a reason for you
to stop imagining how an infinite universe could do the
same thing everywhere simultaneously. But it is reason
for you to consider the question: Why can't Jeff imagine
how an infinite universe can do the same thing everywhere
simultaneously? It should be pretty clear that that isn't
a question I can answer, so it is a question you need to
think about if an answer is to be had. I'm trying to think of
how you can imagine an infinite universe doing the same
thing everywhere simultaneously, and I keep coming back
to the idea that if you really can imagine such a thing, you
should be able to explain it in a way that I can understand,
too. But so far that hasn't happened.



I think the burden would be on you to show that this
actually works to create a universe like we see
Yeah, give me another thirty or forty years.

Really, I'm depending on other people to figure it out.
That's why I post here.



(I don't think it would). And it relies on the assumption
that matter and antimatter repel each other gravitationally.
We've had discussions about that before, and there are some
very good theoretical reasons to think that's not the case.
Maybe. The best, in my opinion, is the problem that
there are no obvious anti-photons. Photons appear to
be their own antiparticle. Photons obviously respond to
gravity similarly to ordinary matter, so if there are are
no antiphotons, antimatter can't have antigravity. The
second-best reason is the Pound-Rebka experiment.
I don't think anything else comes close to those two.



ALPHA will probably give us a definitive answer in a
few years, but I wouldn't bet your whole cosmological
model on it.
I do. Gravitational lensing of antiphotons would work
as well, but I have made zero progress on that front.

-- Jeff, in Minneapolis
.

Jeff Root
2015-Oct-07, 04:08 PM
You seem to be assuming that some kind of coherent
signal is needed to get things to coordinate over
distances.
Yes. For some reason I didn't even mention causality.



I can think of other ways to get even expansion, such
as the energy substrate just taking the same amount
of time to cool down everywhere.
Cool down from what? Did something heat an entire
infinite volume of space to the same temperature
simultaneously? How did that happen?



Or, expansion as it occurs is simply an innate property
of spacetime reacting to energy, so everywhere that has
spacetime and energy is going to expand at the same
rates.
How did an infinite volume of space acquire the same
density of energy everywhere simultaneously so that
it could all react the same way at the same time?



The universe is often counter-intuitive, maybe more often
than it matches our intuitive views, so what we "feel" is
physically possible is simply a bad guide to what actually
happens.
You are asking me -- practically telling me -- to throw
out essentially all of known physics in order to allow the
possibility that every part of an infinite volume of space
could simultaneously begin expanding, even though there
is no evidence that such a thing ever happend.

The evidence we have is that the Universe is expanding,
and that it is really, really, really enormous. But there is
no reason *at all* to think that an infinite universe is all
participating in the same expansion. To me, it is clear
that such a thing isn't possible. I can't prove it, but I do
wonder why it isn't equally clear to you. Do you see
something that I don't? Do I see something you don't?
Or what?

I expect that it's "what".

-- Jeff, in Minneapolis

Cougar
2015-Oct-07, 04:10 PM
Nature abhors infinities, and so do I.

mkline55
2015-Oct-07, 04:11 PM
Maybe. The best, in my opinion, is the problem that
there are no obvious anti-photons. Photons appear to
be their own antiparticle. Photons obviously respond to
gravity similarly to ordinary matter, so if there are are
no antiphotons, antimatter can't have antigravity. The
second-best reason is the Pound-Rebka experiment.
I don't think anything else comes close to those two.

I don't follow that reasoning. As I understand it, photons do not have gravity of their own, but they do respond to the effects of gravity just like all other massive particles. If gravity is a distortion of spacetime, then 'antigravity' can only exist mathematically by moving backwards through time.

Jetlack
2015-Oct-07, 04:18 PM
Here is a related thought experiment.
Imagine a room with perfect white walls that scatter and reflect all the light.
Start in the dark and light a lamp, what happens?
Now expand the room to let's say one light second size, you are very small in comparison but light the same light.
What happens?

A room a light second in size is pretty big right (hundreds of thousands of km)? I would imagine a small lamp in such a white room would not light it up at all unless one is right next to that lamp.

Reality Check
2015-Oct-07, 08:45 PM
Is this true or not, and why?
Not in standard cosmology where the entire universe expands whether it is finite or infinite.
We do not: assume that the universe is finite, set a radius for that finite universe, calculate the rate at which that radius gets bigger -> thus expansion.



2. Can an infinite space experience internal expansion?

Yes - standard cosmology is that the metric (Friedmann–Lemaître–Robertson–Walker metric (https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson% E2%80%93Walker_metric)) which describes the geometry of the universe has a scale factor that can get bigger as time increases. A metric is basically the distance between points. If that distance is getting bigger then the universe is expanding.



3. What evidence is there for universal homogeny?
Standard cosmology starts with the assumptions of homogeneity and isotropy of space. When we look at the distribution of matter in the universe it measured to be homogeneous over length scales longer than about 300 million light-years, e.g.
Mandolesi, N.; Calzolari, P.; Cortiglioni, S.; Delpino, F.; Sironi, G.; Inzani, P.; Deamici, G.; Solheim, J. -E.; Berger, L.; Partridge, R. B.; Martenis, P. L.; Sangree, C. H.; Harvey, R. C. (1986). "Large-scale homogeneity of the Universe measured by the microwave background". Nature 319 (6056): 751. doi:10.1038/319751a0 (https://dx.doi.org/10.1038%2F319751a0).

Reality Check
2015-Oct-07, 08:50 PM
If that is not mainstream -- and I agree that it seems
not to be -- I'd like to know why it isn't.
Mainstream cosmology is based on the Friedmann–Lemaître–Robertson–Walker metric (https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson% E2%80%93Walker_metric) which describes the entire universe as either infinite or finite.

ShinAce
2015-Oct-07, 09:21 PM
As Yoda might say: "End well, this thread will not."

The original statement is ambiguous and non-sequitor.
"If the Universe is infinite, then only a finite portion of it was involved in the Big Bang."

First, the Universe does not need to be infinite to have had a Big Bang. It could be infinite. It could be finite. There doesn't seem to be any disagreement about this.

Second, the notion of 'a finite portion of it was involved' is the ambiguous part. Is this saying that there are parts of the universe we can't see that never expanded?

If I were to rephrase the statement as "The part of the Universe that we see is but a finite piece of a larger universe from before the Big Bang.", then I see no problem. If we are trying to assume what undoubtedly applies to the parts of the universe that we cannot see, and will never see, then I take issue.

We know that the part of the Universe we can see is finite. We also know that it has little to no curvature. It appears flat. When you have a piece of paper that is flat, and you cannot see any edges, then you would assume that the actual paper is bigger than what you see.

The flatness of the universe isn't the amazing part. This just means that the net energy of the universe we see is a big ZERO. The amazing part is that the temperature of the microwave background in one direction is the same as the temperature in the opposite direction.

Nobody knows, and nobody will ever know exactly how big the parts of the unseen universe really are. We also will never know for sure if expansion is a 'localized' thing or a 'universal' thing.

The real issue is that when you make a hypothesis like the original statement did, you need to test it. But you only have one single Universe and one chance to test it. There is no way to verify the answer or discredit it. It's philosophy, not science.

Noclevername
2015-Oct-07, 09:52 PM
Cool down from what? Did something heat an entire
infinite volume of space to the same temperature
simultaneously? How did that happen?

How did an infinite volume of space acquire the same
density of energy everywhere simultaneously so that
it could all react the same way at the same time?



But that's exactly what observation says happened, whether finite or infinite; that there was no propagation involved. The CMB is the same age everywhere, so it must have happened at the same time everywhere and not spread at lightspeed.

We can't say how it came to that state as that covers creation of the Universe and not BBT, and we still don't know that. But we do know it happened that way. All of everything was hot and dense, and then all of everything became less so. And again, that applies no matter the extent of the Universe.

Reality Check
2015-Oct-07, 09:52 PM
My interpretation of the "If the Universe is infinite, then only a finite portion of it was involved in the Big Bang." statement is that the Big Bang was an expanding finite part of an infinite universe (nothing about whether that infinite universe is expanding, static or contracting).

Reality Check
2015-Oct-07, 10:06 PM
It is a fact that I can't imagine how an infinite universe
could do the same thing everywhere simultaneously.
You do not have to just imagine this, Jeff Root.
If you look at the mathematics you will understand that if all of the points in the universe get further apart then the entire infinite or finite universe is doing the same thing everywhere simultaneously (expanding).


Photons appear to
be their own antiparticle. Photons obviously respond to
gravity similarly to ordinary matter, so if there are are
no antiphotons, antimatter can't have antigravity.
No antiphotons means that matter (photons) reacts to gravity like matter. The Pound–Rebka experiment is a test of GR. A conclusive test of the gravitational interaction of antimatter (https://en.wikipedia.org/wiki/Gravitational_interaction_of_antimatter)has not been done yet.

Cougar
2015-Oct-07, 10:25 PM
Very strangely, even though the CBR is incredibly smooth,
the slight variations in it are said to be greater than can
be explained without some mechanism to create them.

I hadn't heard that... except for perhaps a questioned anomalous variation or two, the oddity of which I don't find too compelling. But the original fear was that the CMB would turn out to be perfectly smooth. Then the astrophysicists would have a real problem explaining where all these galaxies and clusters came from! They were relieved to see variations, even if they were only on the order of 1 in 100,000.


More definitely, the very large-scale structure of galactic
filaments and walls is said to be more advanced than can
be explained by existing theory. There hasn't been enough
time since the Big Bang for the structure to be so evolved.

Jeff, where are you getting this? :D Are they teaching this in universities, or is this one team's finding? I understand that for a long time, early structure formation was an unsolved problem, and it has always been difficult to explain how soon we observe structures have formed, metallicity was enhanced, etc. I'm not a researcher, but I think considerable strides have been made.


My suggestion is
that mutual gravitational repulsion ("antigravity") between
ordinary matter and antimatter could provide the needed
extra push to form the filaments and walls rapidly. As well
as causing acceleration of the expansion. And expansion
in general.

I can see why people are sick of hearing this suggestion. :lol: The antimatter is not there.

Jens
2015-Oct-07, 10:27 PM
Believe it or not, an inverse-square force is regarded as an "infinite range" force, for reasons quite similar to the "Olber's paradox" in static cosmology. The finite age of the universe solves both problems.

I guess the concept of fractality wasn't known in Newton's time, but did Newton consider the possibility that the density of the universe could get lower at higher scales, so that gravity tends to zero at larger scales? It's the same issue with Olber's paradox.

WayneFrancis
2015-Oct-07, 11:26 PM
If the universe is infinite, then how could the rate of expansion be affected by gravity? Effectively, the pull of gravity should be equal in all directions, shouldn't it?

it is equal in all directions in a homogeneous finite closed universe too. The following is something that I've accepted but never wrapped my head around. That is how gravity works on the manifold of space time. Better minds on the topic then mine say it does so I accept it and try to learn more until the 'why' clicks in my brain.

Ken G
2015-Oct-08, 01:50 AM
OK, so if I have this right, the reason gravity doesn't pull in all directions equally is that it's a light speed phenomenon, and we're simply outrunning its effects?
In GR, gravity is not a pull, it is a geometric effect on spacetime. That geometric effect can slow, or accelerate, a uniform expansion, without pulling or pushing at all.

Ken G
2015-Oct-08, 01:54 AM
On the issue of Olber´s paradox. That also confuses me because even on the most clear night sky we can´t see dim or very distant stars even when we know they are there, at the centre of our visual focus. That's because our eyes have a "resolution element" that is much smaller than the angular size of those distant stars. So if you imagine a ray drawn from your eye outward, we only see the light along that ray coming from that star, not the light coming from a huge collection of stars along that same ray, stars whose angular sizes would fill up the resolution element of our eye along that ray. That's what Olbers does.
I´m not saying the universe is not finite, but is Olber´s paradox really any proof it is?Yes, Olbers' paradox refutes an infinite homogeneous universe that has not changed for an eternity of time.

Jeff Root
2015-Oct-08, 02:44 AM
I don't follow that reasoning. As I understand it,
photons do not have gravity of their own, but they
do respond to the effects of gravity just like all other
massive particles.
Anything that has energy is a source of gravity.
Photons have energy, of course. It just takes a lot
of photons to equal the energy of even a small mass,
according to everybody's favorite formula, e=mc2.

Even if that were not the case, in order for antimatter
and ordinary matter to repel each other gravitationally
(which is not directly germaine to the original question
of the thread, but does tie in with it), there would have
to be antiphotons which would be repelled by ordinary
gravity. I know that because ordinary photons are
attracted by ordinary gravity. So antiphotons would
have to be attracted by antigravity and repelled by
ordinary gravity. But no such thing has been observed.
The big question is: Should antiphotons have been
observed if they exist? I don't know. There may be
many situations in which antiphotons would be created
and easily observed and identified if they exist. In that
case, my hypothesis is completely torn down. On the
other hand, it might be possible to observe gravitational
lensing of antiphotons from distant antimatter galaxies,
or gravitational lensing of ordinary photons by antimatter
galaxies. If such observations were identified definitively,
they would be as good as observing hydrogen atoms
falling up in vacuum in Earth's gravity. But I have made
no progress in determining criteria by which to identify
observations of possible antigravitational lensing. All I
can say is that it appears it would create faint radial lines
around gravitating sources. That is in contrast to the
bright circumferential arcs that are seen with ordinary
gravitational lensing.

The Pound-Rebka experiment is a test of what happens
to the energy of a particle in a gravitational field. When
antimatter particles are used, it appears that the results
indicate that antimatter reacts to Earth's gravity the same
as ordinary matter does. I just still haven't studied the
experiment in enough depth to be convinced that it does
that. The burden there is certainly entirely on me.



If gravity is a distortion of spacetime, then 'antigravity'
can only exist mathematically by moving backwards
through time.
I'm positing that ordinary matter attracts ordinary matter
gravitationally, and antimatter attracts antimatter
gravitationally, but ordinary matter and antimatter repel
each other gravitationally.

Consider an embedding diagram as being a graph of
gravitational potential. Ordinary matter makes dimples
in the graph where the surface is pulled down. Antimatter
makes bumps where the surface is pushed up. Averaged
out over the entire Universe, the amount of dimples and
bumps would be exactly the same, resulting in an overall
flat geometry, which matches what is observed.

But again, although these ideas have connections with
the subject of this thread that are important to me, they
are not essential to the subject of the thread, which is the
question of whether the Big Bang could involve an infinite
universe.

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-08, 03:01 AM
Imagine a room with perfect white walls that scatter
and reflect all the light.
Start in the dark and light a lamp, what happens?
Now expand the room to let's say one light second size,
you are very small in comparison but light the same light.
What happens?
A room a light second in size is pretty big right (hundreds
of thousands of km)? I would imagine a small lamp in
such a white room would not light it up at all unless one
is right next to that lamp.
I had to think about it a bit, and was surprised to realize
that profloater actually made the point correctly.

I think that what he has in mind is that in both cases, all
the light from the lamp will be reflected back to you. The
same amount of light will reach you in both cases, from
the same solid angle. So the walls will be equally bright,
nomatter their distance.

But the photons reflected off of the walls hit the enlarged
walls at more widely-scattered points, so there is more
room in between those points for more light from added
lamps to reach you. Add a trillion lamps, and a trillion
times as much light will be reflected back at you.

-- Jeff, in Minneapolis

Ken G
2015-Oct-08, 03:02 AM
The universe is neither finite nor infinite. Those words apply to attributes of a model. Put differently, when we say "the universe is (in)finite", what we mean is "our best model of the universe is one with the attribute of being (in)finite." Our best models have a way of changing, so that's why this distinction is scientifically important.

Jens
2015-Oct-08, 04:19 AM
I cannot conceptualize that. You mean take the edges of the infinite plane and fold them together?

No, I simply mean to copy-and-paste the plane over and over again until it makes a cube. Or more realistically, take the plane and add volume above it and below it.

Jens
2015-Oct-08, 04:21 AM
On the issue of Olber´s paradox. That also confuses me because even on the most clear night sky we can´t see dim or very distant stars even when we know they are there, at the centre of our visual focus. We can only see as many stars as the effectiveness or our instruments permit. I´m not saying the universe is not finite, but is Olber´s paradox really any proof it is?

Well, the argument is this, and it is not refutable: if the universe has always existed, and is infinite in size, then at anywhere you look, there will be a star, and no matter how far away the star is, the light will have reached us. So the sky must be light (and hot!).

Noclevername
2015-Oct-08, 04:42 AM
Well, the argument is this, and it is not refutable: if the universe has always existed, and is infinite in size, then at anywhere you look, there will be a star, and no matter how far away the star is, the light will have reached us. So the sky must be light (and hot!).

The CMB is just that, photons from and to all directions equally, the frequency spread out beyond naked-eye detectability by expansion and redshift.

WayneFrancis
2015-Oct-08, 04:46 AM
On the issue of Olber´s paradox. That also confuses me because even on the most clear night sky we can´t see dim or very distant stars even when we know they are there, at the centre of our visual focus. We can only see as many stars as the effectiveness or our instruments permit. I´m not saying the universe is not finite, but is Olber´s paradox really any proof it is?

I'll try addressing this question.

There are 2 basic things that limit the stars we see.

First is what are the stars that have had enough time to get their light to us.
Since the universe is of a finite age that number is also finite.
Taking away any expansion it is just a sphere with a radius of the age of the universe.
Even if the universe is bigger the photons from stars further away haven't had enough time to reach us.
The second thing is the number of photons we receive.
A star produces a finite amount of photons. These photons are, generally, evenly distributed in all directions.
The number of photons you receive is a function of the distance to the star in question, the total area of your collector, and the amount of time you spend collecting.
Far enough away and you may only receive a photon every few minutes or hours. Maybe even days.

In our universe, again ignoring expansion, If you draw a line from you to any point in the sky one of 2 things happen. Your ray ends on a star or your ray doesn't because if you traced back to the beginning of the universe there wasn't a star at that location in 4 dimensional space time.
If it does land on a star then you have to work out how many photon you'd expect to receive from that star.

The fact is most of the sky will not end up on a star.
Look at this picture of the eXtreme Deep Field
21078
Even this patch of sky which was "black" isn't black everywhere, but it doesn't have a star at every point. Most of it is still empty space. Zooming in further and collecting light longer won't make it brighter because we are nearing the point where there was no stars. This is, again, because the further away we look the further back in time we look.


Now if the universe was infinitely old you could keep looking further away and back in time. If the universe is infinite in size it would mean, eventually, you would land on a star no mater where you shot your ray out. This also has the affect of upping the number of photons you receive from that end point. You be receiving essentially an infinite amount of photon in theory. Thus every point on the sky would land on a star.

Since we know that either the universe is not infinite in size or it is not infinite in age or both. Turns out that our universe has a practical finite age. We don't know if it is finite in size.

Jeff Root
2015-Oct-08, 04:52 AM
Cool down from what? Did something heat an entire
infinite volume of space to the same temperature
simultaneously? How did that happen?

How did an infinite volume of space acquire the same
density of energy everywhere simultaneously so that
it could all react the same way at the same time?
But that's exactly what observation says happened,
whether finite or infinite; that there was no propagation
involved. The CMB is the same age everywhere, so it
must have happened at the same time everywhere and
not spread at lightspeed.
Observations do NOT say that an an infinite volume was
affected in the same way, simultaneously or almost
simultaneously. That is what I'm saying is not possible,
and there is no evidence that any such thing happened.

What is observed can be explained in a way which does
not violate causality. You keep insisting that ideas which
violate causality cannot be ruled out. You support that
here by claiming that the observed expansion of the
Universe violates causality. But it doesn't. Or at least,
there is no need for it to violate causality. If everything
involved in the Big Bang was in causal contact at the time
it occurred, then causality isn't violated. But that limits
the Big Bang to a finite volume. if the Big Bang occurred
throughout an infinite volume, then causality is violated.

Okay, so maybe this is yet another assumption that I
failed to state explicitly: I consider causality sacrosanct.

The problem is that when I say this, people always drag
in quantum uncertainty, which really is irrelevant. I'm
not talking about something small here. I'm not talking
about something on the scale of quantum particles. I'm
talking about the biggest thing there is, or at least, the
biggest thing we know of: The entire Universe. If an
event occurs which involves everything in the entire
Universe, then it needs to conform to causality, or it
looks physically impossible.

If the temperature of the cosmic microwave background
radiation is almost identical in every direction, then that
is because the particles which released the light were all
heated by the same energy source, so naturally they had
the same temperature. They are causally connected.

Asserting that an infinite universe of finite age could
have the same temperature everywhere, or the same
composition everywhere, or the same expansion or the
same anything everywhere is asserting magic in place
of physics and causality.

-- Jeff, in Minneapolis

Noclevername
2015-Oct-08, 05:04 AM
What is observed can be explained in a way which does
not violate causality. You keep insisting that ideas which
violate causality cannot be ruled out. You support that
here by claiming that the observed expansion of the
Universe violates causality. But it doesn't. Or at least,
there is no need for it to violate causality. If everything
involved in the Big Bang was in causal contact at the time
it occurred, then causality isn't violated. But that limits
the Big Bang to a finite volume. if the Big Bang occurred
throughout an infinite volume, then causality is violated.

How? Causality is based on the propagation of events through spacetime. I don't understand your assertion at all.

Jeff Root
2015-Oct-08, 05:30 AM
You raised two really important questions that are too
big for me to attempt to respond to right now. (Though
they are again a sidetrack from the topic of the thread,
they are related, and I should answer them.) For now...




My suggestion is that mutual gravitational repulsion
("antigravity") between ordinary matter and antimatter
could provide the needed extra push to form the filaments
and walls rapidly. As well as causing acceleration of the
expansion. And expansion in general.
I can see why people are sick of hearing this suggestion.
:lol: The antimatter is not there.
Why do you say the antimatter is not there? Because
nobody has identified it as antimatter? What makes you
think anyone would be able to identify it as antimatter?
Because it would interact with ordinary matter, mutually
annihilating, and giving off huge amounts of light?
Where do you think the CMB came from? That's it!
After that, ordinary matter and antimatter would be
separated by their mutual gravitational repulsion, so
there would be no more large-scale annihilations.

-- Jeff, in Minneapolis

Jens
2015-Oct-08, 05:47 AM
Now if the universe was infinitely old you could keep looking further away and back in time. If the universe is infinite in size it would mean, eventually, you would land on a star no mater where you shot your ray out. This also has the affect of upping the number of photons you receive from that end point. You be receiving essentially an infinite amount of photon in theory. Thus every point on the sky would land on a star.

Since we know that either the universe is not infinite in size or it is not infinite in age or both. Turns out that our universe has a practical finite age. We don't know if it is finite in size.

It's important to add the third qualification, however.

Since we know that, we know that either the universe is not infinite in size or it is not infinite in age or it is not homogeneous.

Jeff Root
2015-Oct-08, 06:32 AM
What is observed can be explained in a way which does
not violate causality. You keep insisting that ideas which
violate causality cannot be ruled out. You support that
here by claiming that the observed expansion of the
Universe violates causality. But it doesn't. Or at least,
there is no need for it to violate causality. If everything
involved in the Big Bang was in causal contact at the time
it occurred, then causality isn't violated. But that limits
the Big Bang to a finite volume. if the Big Bang occurred
throughout an infinite volume, then causality is violated.
How? Causality is based on the propagation of events
through spacetime. I don't understand your assertion
at all.
How is causality violated by the same thing happening
throughout an infinite volume simultaneously? Is that
what you are asking?

You suggested that the Universe could have begun with
infinite volume, and expanded uniformly from there.
That means every part of the infinite volume came into
existance at the same time with the same properties.
That is a monsterously massive coincidence, infinitely
far beyond anything remotely plausible. Not *vastly* far
beyond anything remotely plausible -- *infinitely* far.
If that isn't obviously a violation of causality, then there
is no such thing as a violation of causality. I don't know
what else to say. The whole idea you are proposing is
just so absurdly preposterous that it seems completely
silly to argue it, yet several knowledgeable people are
doing so.

If you meant "how" something else, please spell out
the question in detail so I know what you are asking.

-- Jeff, in Minneapolis

Jetlack
2015-Oct-08, 06:34 AM
That's because our eyes have a "resolution element" that is much smaller than the angular size of those distant stars. So if you imagine a ray drawn from your eye outward, we only see the light along that ray coming from that star, not the light coming from a huge collection of stars along that same ray, stars whose angular sizes would fill up the resolution element of our eye along that ray. That's what Olbers does.Yes, Olbers' paradox refutes an infinite homogeneous universe that has not changed for an eternity of time.

Okay so it refutes an infinite universe with homogeneously positioned stars, not having changed for eternity. That´s quite conditional model of an infinite universe. I mean could the universe still be infinite but its content in flux or perpetually changing? I´m just asking, not challenging whether the universe is finite or not :-)

Ken G
2015-Oct-08, 06:37 AM
Okay so it refutes an infinite universe with homogeneously positioned stars, not having changed for eternity. That´s quite conditional model of an infinite universe.Yes it is, I'm not really sure how people like Newton and Einstein, who would have been able to figure that out, dealt with it, because they did seem to hold to an infinite unchanging universe, somehow.

I mean could the universe still be infinite but its content in flux or perpetually changing?Sure, that's the current standard model, the one where the universe is flat and obeys a "cosmological principle" (is the same everywhere). It's not necessary to believe the model is some actual truth, it's just that we have no reason to change from that model-- we wouldn't know how to change it that wouldn't be completely arbitrary. Some try to change it anyway, and we have models like "eternal inflation" (which involves finite bubbles popping off their own Big Bangs all the time), but those are indeed arbitrary-- they can have any distribution of bubbles you like, there are no observational constraints on such models. Some claim they deal with issues like the anthropic principle and the nature of noise in the CMB, but they still make assertions about vast regions of the universe we can never see. They push the limit of what we mean by a scientific model.

Jetlack
2015-Oct-08, 06:58 AM
I'll try addressing this question.

There are 2 basic things that limit the stars we see.

First is what are the stars that have had enough time to get their light to us.
Since the universe is of a finite age that number is also finite.
Taking away any expansion it is just a sphere with a radius of the age of the universe.
Even if the universe is bigger the photons from stars further away haven't had enough time to reach us.
The second thing is the number of photons we receive.
A star produces a finite amount of photons. These photons are, generally, evenly distributed in all directions.
The number of photons you receive is a function of the distance to the star in question, the total area of your collector, and the amount of time you spend collecting.
Far enough away and you may only receive a photon every few minutes or hours. Maybe even days.

In our universe, again ignoring expansion, If you draw a line from you to any point in the sky one of 2 things happen. Your ray ends on a star or your ray doesn't because if you traced back to the beginning of the universe there wasn't a star at that location in 4 dimensional space time.
If it does land on a star then you have to work out how many photon you'd expect to receive from that star.

The fact is most of the sky will not end up on a star.
Look at this picture of the eXtreme Deep Field
21078
Even this patch of sky which was "black" isn't black everywhere, but it doesn't have a star at every point. Most of it is still empty space. Zooming in further and collecting light longer won't make it brighter because we are nearing the point where there was no stars. This is, again, because the further away we look the further back in time we look.


Now if the universe was infinitely old you could keep looking further away and back in time. If the universe is infinite in size it would mean, eventually, you would land on a star no mater where you shot your ray out. This also has the affect of upping the number of photons you receive from that end point. You be receiving essentially an infinite amount of photon in theory. Thus every point on the sky would land on a star.

Since we know that either the universe is not infinite in size or it is not infinite in age or both. Turns out that our universe has a practical finite age. We don't know if it is finite in size.

Yes thanks for that explanation and i get it. Like i said earlier not challenging the actual finite age of the universe which obviously suggests it is not infinite, or at least the parts we observe. I guess what confuses me is that with better instruments/observation modern astronomers uncover new galaxies which we could not see before in our finite universe. The reason we couldn't see those earliest galaxies last year (example) was not because the light had not reached us, just that we lacked the ability to detect it because its so far away. I guess what i am confused about in Olber´s paradox "logic" shouldn't we be able to see all the stars in our finite universe now as well?

Jetlack
2015-Oct-08, 07:03 AM
Yes it is, I'm not really sure how people like Newton and Einstein, who would have been able to figure that out, dealt with it, because they did seem to hold to an infinite unchanging universe, somehow.


That´s interesting, I didn't realise Einstein thought that. I can sort of understand that idea in Newton´s time, and eternity sort of being a concept from the religious beliefs in those times.

Ken G
2015-Oct-08, 08:11 AM
Yeah, it was Einstein's original motivation for the cosmological constant. He was trying to create a model for a static, infinite, unchanging universe. He is said to have called it his greatest blunder, though of course he wasn't around to see the return of his cosmological constant!

Noclevername
2015-Oct-08, 08:14 AM
That means every part of the infinite volume came into
existance at the same time with the same properties.
That is a monsterously massive coincidence, infinitely
far beyond anything remotely plausible.

Yes, but to the best of my knowledge, it's exactly what happened, and the finite or infinite distances involved have nothing to do with it. It's what matches observation, and that trumps your personal sense of absurdity.

Noclevername
2015-Oct-08, 08:33 AM
Let me clarify: According to your definition, Jeff, a finite volume of space the size of the Observable Universe, all coming to existence at the exact same time, would also be a "violation of causality" because it too is simultaneous. So that really has no bearing on the size of the universe at all.

grapes
2015-Oct-08, 08:34 AM
How is causality violated by the same thing happening
throughout an infinite volume simultaneously? Is that
what you are asking?

You suggested that the Universe could have begun with
infinite volume, and expanded uniformly from there.
That means every part of the infinite volume came into
existance at the same time with the same properties.
That is a monsterously massive coincidence, infinitely
far beyond anything remotely plausible. Not *vastly* far
beyond anything remotely plausible -- *infinitely* far.
If that isn't obviously a violation of causality, then there
is no such thing as a violation of causality. I don't know
what else to say. The whole idea you are proposing is
just so absurdly preposterous that it seems completely
silly to argue it, yet several knowledgeable people are
doing so.

If you meant "how" something else, please spell out
the question in detail so I know what you are asking.

It's easy to produce a mathematical model of what you call infinitely far from plausible though.

Imagine a very small universe, finite but extremely small. Points on its surface start radiating outward on paths that cause their velocities to interact, but at the same time they are different based upon some initial parameter. At a finite time, the points will fill an infinite space, and will share a common relationship to each other that will guarantee the rest of the development is homogeneous.

I've just imagined a couple mathematical models that fit that--in lesser dimensions, but still, it makes it plausible.

So, I now have a plausible model of a finite universe that expands in a finite time into a homogeneous infinite universe, without violating causation.

Jeff Root
2015-Oct-08, 11:03 AM
That means every part of the infinite volume came into
existance at the same time with the same properties.
That is a monsterously massive coincidence, infinitely
far beyond anything remotely plausible.
Yes, but to the best of my knowledge, it's exactly what
happened, ...
There is no evidence that anything like that happened.



... and the finite or infinite distances involved have
nothing to do with it. It's what matches observation,
and that trumps your personal sense of absurdity.
Not only is there no evidence or observation of any
such thing, but it is contradicted by essentially every
fundamental principle of physics: Causality, action at
a distance, conservation of mass-energy, the laws of
thermodynamics, probably quantum uncertainty, too.
You are throwing out essentially all of physics in order
to hang on to this idea that a large or infinite volume
which is not in causal contact could all have the same
properties and behavior simultaneously.



Let me clarify: According to your definition, Jeff, a
finite volume of space the size of the Observable
Universe, all coming to existence at the exact same
time, would also be a "violation of causality" because
it too is simultaneous. So that really has no bearing
on the size of the universe at all.
Yes, I agree with that. I don't recall the idea of a large
but finite volume coming into existence simultaneously
being a subject of debate previously, because it can be
replaced by the idea of a compact, causally-connected
volume expanding over time. That's the main feature
of Inflation theory. There is no need in such a scenario
for the kind of causality-violating event required in your
infinite Universe scenario.

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-08, 11:13 AM
... Points on its surface start radiating outward on
paths that cause their velocities to interact, but at the
same time they are different based upon some initial
parameter.

That just reads like word salad. If it actually means
anything, it went over my head. Despite the fact that
all the words you used look like good, ordinary English.

-- Jeff, in Minneapolis

Noclevername
2015-Oct-08, 12:20 PM
There is no evidence that anything like that happened.


Um, yes, the CMB. It's all the same age everywhere.

ADDED: Okay, one of us or the other is misapplying the word "causality" here. I define causality as events that happen in spacetime preceding results, propagating at lightspeed. What definition do you use, and how does it relate to the beginning of spacetime?

mkline55
2015-Oct-08, 12:41 PM
Um, yes, the CMB. It's all the same age everywhere.

ADDED: Okay, one of us or the other is misapplying the word "causality" here. I define causality as events that happen in spacetime preceding results, propagating at lightspeed. What definition do you use, and how does it relate to the beginning of spacetime?

I think you are using the same meaning. I believe Jeff is arguing that in order for all of the universe or all of the visible universe to have undergone the same transformation simultaneously, there must have been some thing which caused that to occur simultaneously, even though after over 10 billion years, the fastest thing in the universe has only now reached us from the most distant objects we recognize. That thing must have acted on all of the universe or all of the visible universe at the same time. How is that possible? He is essentially asking, what was that thing? And if the answer is "it was hot and dense" hence much closer, then you have to violate principles or physics to get the model to match current observations.

Noclevername
2015-Oct-08, 12:45 PM
I think you are using the same meaning. I believe Jeff is arguing that in order for all of the universe or all of the visible universe to have undergone the same transformation simultaneously, there must have been some thing which caused that to occur simultaneously, even though after over 10 billion years, the fastest thing in the universe has only now reached us from the most distant objects we recognize. That thing must have acted on all of the universe or all of the visible universe at the same time. How is that possible? He is essentially asking, what was that thing? And if the answer is "it was hot and dense" hence much closer, then you have to violate principles or physics to get the model to match current observations.

Jeff seems to me to be saying that for some reason, a finite universe can do what we observe and an infinite universe can't. Which makes no sense to me.

But yes, we still don't understand the beginning of the Universe, that's why it's called a singularity. And why it's not covered by the BBT. I just don't see how it relates to finity or infinity.

mkline55
2015-Oct-08, 12:52 PM
No, I simply mean to copy-and-paste the plane over and over again until it makes a cube. Or more realistically, take the plane and add volume above it and below it.

While I understand your intent, I still don't follow the example. A cube is defined by its equal length edges and 90 degree angles. You cannot have angles without edges. An infinite plane has no edges. Also, stacking two-dimensional planes on top of one another yields a zero-depth set of two-dimensional planes.

Ken G
2015-Oct-08, 01:00 PM
There's also the question of whether or not expansion needs to be "caused." In the current Big Bang model, the expansion is not caused. So rather than object to it being infinite, you should simply object to it not being caused at all. But who said what happens has to be caused? That's just another model. Anyone who objects to "infinite things" based on some philosophical principle can also object to "caused things" based on a different philosophical principle. But science cannot assert "it has been shown that the universe is infinite" and it cannot assert "it has been shown that the universe was caused." Science just doesn't do those things, it only tests models.

Noclevername
2015-Oct-08, 01:03 PM
Is expansion simply a property of spacetime?

grapes
2015-Oct-08, 02:03 PM
... Points on its surface start radiating outward on
paths that cause their velocities to interact, but at the
same time they are different based upon some initial
parameter.

That just reads like word salad. If it actually means
anything, it went over my head. Despite the fact that
all the words you used look like good, ordinary English.

If you mean, you didn't understand it, but want an example, I'll try.

Start the one-d universe at time=-R, where it consists of a single point, and many points radiate out from it in spacetime. Their interactions depend upon their relationship to each other at a specific time (t-coordinate). Their y-coordinate is tan(θ)*sqr(R2-t2), where θ represents an initial angle at the time the point leaves the initial point, and -90°<θ<90°, so no point has an infinite velocity, and its velocity is incrementally greater/lesser than that of its neighbors.

At time t=0, all points in the universe have homogenized--that is their relative motion in spacetime is identical to their neighbors'

So, at time zero, any change in any point trajectory will only depend upon local conditions, but all conditions on the universe are identical. So, from time=0 on, if the universe expands at one point, it will expand in the same fashion at other points, with the same proportionality constant.

Any subsequent measurement will indicate the expansion started at time 0, even though the process actually started at time=-R

So here we have a mathematical example which starts with a finite universe, becomes an infinite universe in a finite time, and results in a homogenous universe, without needing anything more than local interaction.

And, it fits the "word salad" description. :)

Jeff Root
2015-Oct-08, 02:51 PM
grapes,

I read that. I'll come back later when I have time (or take
time I don't have) and read it again to see if I can make any
sense of it.

Thank you!

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-08, 03:50 PM
There is no evidence that anything like that happened.
Um, yes, the CMB. It's all the same age everywhere.
The fact that the CMB is the same age everywhere is not
evidence that something happened everywhere in a large
or infinite volume simultaneously. The CMB could get that
way without such an event, and undoubtedly did.



ADDED: Okay, one of us or the other is misapplying the
word "causality" here. I define causality as events that
happen in spacetime preceding results, propagating at
lightspeed. What definition do you use, and how does it
relate to the beginning of spacetime?
I don't think that's a particularly good definition, but I
don't find definitions very good in general. I just stick
with descriptions most of the time, and your "definition"
is actually a fairly OK description. I don't really disagree
with it. I see no need to qualify that the events happen
"in spacetime". Where else would they happen? I see
no reason to specify that they propagate at lightspeed.
Most cause-effect relations occur at much less than the
speed of light. I push the button on my radio to turn it
on, and it takes almost five seconds before I begin to
hear sound come out of the speakers, even though the
speakers are only a metre away from me. Clearly that
is a causal relationship: My pushing the button caused
the sound to come out of the speakers. Along with lots
and lots of other things that had to happen in addition
to my pushing the button, of course. Also it isn't clear
what is meant by "events" propagating". I'd say that
"effects propagating" makes more sense, although I'm
not sure it is a whole lot better. An event in one place,
at one time, is a cause of another event at another
place and a later time. That's a generic description of
an instance of causality. But a lousy definition.

Bottom line is, I don't think the problem is a difference
in what we mean by "causality".

Your acceptance of the idea of a single event occurring
simultaneously everywhere throughout an infinite volume
of space is so freaky weird to me that I have to think the
problem is in our differing views of physics. Do you find
something about my views freaky weird? If so, what?

-- Jeff, in Minneapolis

Jim
2015-Oct-08, 03:55 PM
After some discussion among the Mods - and more than a few reports from Members - this thread is not suitable for Q&A. It's being moved to Astronomy.

Everyone is cautioned that the debatable aspects of this topic can be debated, that questions about non-mainstream ideas can be asked, but this thread must not be used to advocate completely non-mainstream ideas.

ShinAce
2015-Oct-08, 05:42 PM
Is expansion simply a property of spacetime?

That's a likely possibility, yes. I'm going to take expansion as 'accelerated expansion'.

On the other hand, I can't reject a 'material' form of dark energy. We expect to experimentally 'find' dark matter. Perhaps one day it will be the same for dark energy.

My money is on 'intrinsic property of universal space-time'. Which is a fancy way of saying the current size(barring matter creation) of the universe determines the current value of dark energy. That must happen on the whole, and isn't a local thing like a magnetic field.

Jeff Root
2015-Oct-08, 06:02 PM
Jeff seems to me to be saying that for some reason, a
finite universe can do what we observe and an infinite
universe can't. Which makes no sense to me.
I am saying that nothing can happen simultaneously
in a large number of different places unless something
causes that to occur. Trees all around the world cannot
all rotate clockwise by 5 degrees at 9:27 UT unless
something causes them to do that. It could not happen
just by chance, or just because it is natural for that to
happen.

You suggested the idea of the Universe starting out
infinite in extent, which is an example of something
happening simultaneously in an infinite number of
places. It would be equally impossible if you had
suggested the Universe starting out finite, say one
light-minute in diameter. It would still be a case of
something happening simultaneously in many places
(everywhere throughout the volume) without anything
causing it to do so. But I didn't object to that case
because you didn't suggest it, and that was because
there was no need for it. You suggested the infinite
case because you were trying to justify the idea of an
infinite universe resulting from the Big Bang. So I
objected to the case you described, not to a case you
didn't describe.



But yes, we still don't understand the beginning of the
Universe, that's why it's called a singularity.
As somebody else here said, the idea of a singularity
has a definite definition in mathematics, which is the
definition Penrose and Hawking used when they
determined that the Universe began in a singularity.
That definition of singularity means, in this case, that
the Universe began at a point in time. According to
the FLRW metric calculated using general relativity
with a minimal set of basic assumptions, anyhow.

I agree that we still don't understand the beginning of
the Universe, but that doesn't make it a singularity, and
being a singularity doesn't imply not understanding it.

-- Jeff, in Minneapolis

ShinAce
2015-Oct-08, 06:42 PM
I am saying that nothing can happen simultaneously
in a large number of different places unless something
causes that to occur. Trees all around the world cannot
all rotate clockwise by 5 degrees at 9:27 UT unless
something causes them to do that. It could not happen
just by chance, or just because it is natural for that to
happen.



So if I have many photons which are entangled and widely separated, are you saying that their polarization states cannot collapse simultaneously? That's a case where a localized cause can have a non-local effect.

Jeff, it would appear you're holding onto locality. I'd also like to add that finite things can be measured. Infinite things cannot. Infinity is a concept, not a number.

mkline55
2015-Oct-08, 07:29 PM
So if I have many photons which are entangled and widely separated, are you saying that their polarization states cannot collapse simultaneously? That's a case where a localized cause can have a non-local effect.

Is there evidence that all of the early universe was entangled?

Noclevername
2015-Oct-08, 08:14 PM
I am saying that nothing can happen simultaneously
in a large number of different places unless something
causes that to occur.

OK, then. How would you classify observed universal phenomena like the same age of the CMB in all directions?


As somebody else here said, the idea of a singularity
has a definite definition in mathematics, which is the
definition Penrose and Hawking used when they
determined that the Universe began in a singularity.
That definition of singularity means, in this case, that
the Universe began at a point in time. According to
the FLRW metric calculated using general relativity
with a minimal set of basic assumptions, anyhow.

I agree that we still don't understand the beginning of
the Universe, but that doesn't make it a singularity, and
being a singularity doesn't imply not understanding it.


I've been given the wrong definition of singularity then. Now, at least, I know what is meant by it in this context.

ShinAce
2015-Oct-08, 08:23 PM
Is there evidence that all of the early universe was entangled?

Is there evidence that all of the early universe was NOT entangled? I wasn't trying to stress entanglement, I'm just giving an example where things can be widely separated to the point that cause and effect are now instantaneous. Causality is fundamental to the conversation. Yet we(physicists) tend to be very careful when explaining what causality it. The best description I can come up with is that it is the distinction between past and future.

If the holographic principle holds, then it would appear that our visible universe is in fact one big whole.

Reality Check
2015-Oct-08, 08:36 PM
Bottom line is, I don't think the problem is a difference
in what we mean by "causality".
The bottom line may be that the definition of causality that you are trying to use does not apply. The problem is the "propagation limited to the speed of light" bit. This is correct for events within space-time. It is not true for an expanding space-time. For example we can detect the light from galaxies that are travelling away from us at velocities greater than the speed of light.

Reality Check
2015-Oct-08, 08:46 PM
As somebody else here said, the idea of a singularity
has a definite definition in mathematics, which is the
definition Penrose and Hawking used when they
determined that the Universe began in a singularity.
That definition of singularity means, in this case, that
the Universe began at a point in time.
Can you give a citation to Penrose and Hawking deriving that the universe began in a singularity whose definition means that the universe began?
As far as I know the singularity in the Friedmann–Lemaître–Robertson–Walker metric (https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson% E2%80%93Walker_metric) was known about immediately (1920s and 1930s) and the meaning is that the solution breaks down at t = 0 (gravitational singularity (https://en.wikipedia.org/wiki/Gravitational_singularity)).
There are the Penrose–Hawking singularity theorems (https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singularity_theorems) that state that under certain conditions a GR solution will have at least one 1 (and maybe all) "non-spacelike geodesic that is only finitely extendible into the past".

Jens
2015-Oct-08, 10:36 PM
While I understand your intent, I still don't follow the example. A cube is defined by its equal length edges and 90 degree angles. You cannot have angles without edges. An infinite plane has no edges. Also, stacking two-dimensional planes on top of one another yields a zero-depth set of two-dimensional planes.

I understand your problem with my example. Let be change it then. Suppose you have an infinitely long 2*4 (an American piece of lumber). You will agree it has infinite volume. Now suppose you stack an infinite number on top of it.

Jens
2015-Oct-08, 10:56 PM
To Jeff, without quoting any specific post, I wanted to mention that things can happen more or less spontaneously (on average) because they are stochastic or the result of physical principles. For example, entropy happens simultaneously throughout the universe because it's a universal law, and virtual particle production takes place everywhere without any need for causality because it's a property of the universe.

WayneFrancis
2015-Oct-08, 11:53 PM
It's important to add the third qualification, however.

Since we know that, we know that either the universe is not infinite in size or it is not infinite in age or it is not homogeneous.

Thanks Jen...yes I'm assuming homogeneity I also assume that it is isotropic but that doesn't really apply ...well I can't think why it would. Thanks for pointing that out.

WayneFrancis
2015-Oct-08, 11:56 PM
How is causality violated by the same thing happening
throughout an infinite volume simultaneously? Is that
what you are asking?

You suggested that the Universe could have begun with
infinite volume, and expanded uniformly from there.
That means every part of the infinite volume came into
existance at the same time with the same properties.
That is a monsterously massive coincidence, infinitely
far beyond anything remotely plausible. Not *vastly* far
beyond anything remotely plausible -- *infinitely* far.
If that isn't obviously a violation of causality, then there
is no such thing as a violation of causality. I don't know
what else to say. The whole idea you are proposing is
just so absurdly preposterous that it seems completely
silly to argue it, yet several knowledgeable people are
doing so.

If you meant "how" something else, please spell out
the question in detail so I know what you are asking.

-- Jeff, in Minneapolis

Wouldn't causality in our universe be violated by our universe being created in the first place?
If our universe some how was created by a collision of 2 branes in a high level dimension the cause there is outside of our space time...so technically as far as our universe goes we'd have something happen without a cause.

WayneFrancis
2015-Oct-09, 12:06 AM
Okay so it refutes an infinite universe with homogeneously positioned stars, not having changed for eternity. That´s quite conditional model of an infinite universe. I mean could the universe still be infinite but its content in flux or perpetually changing? I´m just asking, not challenging whether the universe is finite or not :-)

I'm going to add to what has been said. This isn't the only evidence that our universe has a finite age. Forget expansion and running that backwards. An infinitely old universe should have either run out of fuel an infinite time ago or we should see some sign that new hydrogen is being created but we don't.

Many ideas can easily shot down when you remove assumptions and other evidence.

WayneFrancis
2015-Oct-09, 12:11 AM
Yes thanks for that explanation and i get it. Like i said earlier not challenging the actual finite age of the universe which obviously suggests it is not infinite, or at least the parts we observe. I guess what confuses me is that with better instruments/observation modern astronomers uncover new galaxies which we could not see before in our finite universe. The reason we couldn't see those earliest galaxies last year (example) was not because the light had not reached us, just that we lacked the ability to detect it because its so far away. I guess what i am confused about in Olber´s paradox "logic" shouldn't we be able to see all the stars in our finite universe now as well?

we are really at the limit at this point. Sure we can see fainter and fainter objects in the future with bigger and bigger telescopes but we won't be able to see objects much further away because we are coming to the point where we are observing, literally, the first stars. We just can not see further away because for us there is no light path that goes further away.

WayneFrancis
2015-Oct-09, 12:23 AM
There is no evidence that anything like that happened.


Not only is there no evidence or observation of any
such thing, but it is contradicted by essentially every
fundamental principle of physics: Causality, action at
a distance, conservation of mass-energy,


It is my understanding that conservation of mass-energy is just a local phenomena. That dark energy and the expanding universe kind of throw that to the wind at those scales.



the laws of
thermodynamics,
[/QUOTE

Umm again ... that assumes a closed system. We don't know where the energy of our system came from and I'm not sure if even something like Dark Energy fits with the idea of a closed system.

[QUOTE=Jeff Root;2317866]
probably quantum uncertainty, too.


Could you elaborate on that one?



You are throwing out essentially all of physics in order
to hang on to this idea that a large or infinite volume
which is not in causal contact could all have the same
properties and behavior simultaneously.


I'll point out we have very little idea of the physics of the early universe so how can we throw out that which we don't know. You seem to want to throw out any physics which we don't know which would include physics of the early universe that we KNOW we don't know and understand.



Yes, I agree with that. I don't recall the idea of a large
but finite volume coming into existence simultaneously
being a subject of debate previously, because it can be
replaced by the idea of a compact, causally-connected
volume expanding over time. That's the main feature
of Inflation theory. There is no need in such a scenario
for the kind of causality-violating event required in your
infinite Universe scenario.

-- Jeff, in Minneapolis
Just because there are some models that fit observation that doesn't mean other models can't also be correct. Like it is pointed out they are just models and the important bit is that they make useful predictions and for the domain of applicability they match current observations.

Essentially your argument is like Einstein not liking quantum mechanics because it went against all that he knew. The universe doesn't care what anyone knows.

ShinAce
2015-Oct-09, 03:37 AM
In regards to thermodynamics --> Since we know that inflation disconnected vast stretches of the early universe and that the current CMBR is in thermal equilibrium, we can treat the cosmic horizon as a wall where no net energy flows across. Therefore, a closed system approximates the universe extremely well.

In other words, the universe is a closed system where dQ=0.

Jetlack
2015-Oct-09, 06:48 AM
I'm going to add to what has been said. This isn't the only evidence that our universe has a finite age. Forget expansion and running that backwards. An infinitely old universe should have either run out of fuel an infinite time ago or we should see some sign that new hydrogen is being created but we don't.

Many ideas can easily shot down when you remove assumptions and other evidence.

absolutely, i agree there is plenty of better evidence the universe is finite.

Noclevername
2015-Oct-09, 07:42 AM
Yes, when I said infinite I meant in spatial extent, not length of time. I have always believed that there was a beginning to the Universe.

Jeff Root
2015-Oct-09, 07:43 AM
I still believe that, as I said to Noclevername in post #27,
there isn't much reasoning involved in my objection to the
idea that the Big Bang could involve or result in an infinite
volume. But there are quite a number of assmptions, and
it might possibly clarify something for someone if I were
to list them all together:

* The Big Bang occurred about 14 billion years ago.

* The Big Bang resulted in everything we see. There is no
direct evidence of how much larger the volume involved is
than what we see, but what we see is very, very big, and the
volume involved in the Big Bang is certainly even bigger.

* On scales much larger than galaxies, everything is getting
farther and farther apart.

* Known laws of physics applied at the start of the Universe.
Things like, it takes longer to travel a large distance at a given
speed than it takes to travel a shorter distance at that speed,
or it takes more energy to change the speed of a given mass
by a large amount than by a smaller amount, or that opposite
electric charges attract one another and like charges repel one
another, or that the frequency of a photon is a measure of its
ability to do work. The conditions at the start may have been
very different from conditions now, but the laws of physics
were the same.

* Known laws of physics are not adequate to describe what
happened in the Big Bang. New laws are required. They will
involve quantum mechanics.

* Causality holds on macroscopic scales. Events have causes.
They don't just happen. On quantum scales, it is fundamentally
impossible to see everything that is going on, so the principle
of causality can't be applied strictly. But we are talking about
an infinite Universe here. As macroscopic as you can get.

* Improbable things happen spontaneously, but the bigger,
more complex an event is, the less likely it is to occur without
any apparent cause. That is the same as saying that causality
holds on macroscopic scales. Socks do not spontaneously
disappear from or reappear in the Universe without something
causing them to do so.

The idea I'm arguing against is the instantaneous existence
of an infinite amount of stuff that all has the same properties,
spread throughout an infinite volume of space. The problem
is not that it is infinite, but that it is a very large amount of
stuff that instantaneously has the same properties in places
which have no causal connection. If it were only a handful of
quantum particles, I would have no objection, but the scenario
is that an infinite amount of stuff spontaneously appears, all
with the same properties, throughout all of infinite space.
That is infinitely beyond "improbable".

-- Jeff, in Minneapolis

grapes
2015-Oct-09, 10:17 AM
The idea I'm arguing against is the instantaneous existence
of an infinite amount of stuff that all has the same properties,
spread throughout an infinite volume of space. The problem
is not that it is infinite, but that it is a very large amount of
stuff that instantaneously has the same properties in places
which have no causal connection. If it were only a handful of
quantum particles, I would have no objection, but the scenario
is that an infinite amount of stuff spontaneously appears, all
with the same properties, throughout all of infinite space.
That is infinitely beyond "improbable".

I thought I provided a good example--which reduces the "improbability" by quite a lot, from infinitely beyond to just finitely improbable :)

mkline55
2015-Oct-09, 12:07 PM
I thought I provided a good example--which reduces the "improbability" by quite a lot, from infinitely beyond to just finitely improbable :)

You mean this one?
Imagine a very small universe, finite but extremely small. Points on its surface start radiating outward on paths that cause their velocities to interact, but at the same time they are different based upon some initial parameter. At a finite time, the points will fill an infinite space, and will share a common relationship to each other that will guarantee the rest of the development is homogeneous.

It has at least two causes. 1) a surface which places the points at their starting positions, 2) an event which starts them radiating outward.

I agree with Jeff that the current model demands an impossible set of circumstances, at least in terms of known physics. The "evidence" of that event is one interpretation of observations. My personal opinion, which is not enough to purchase a cup of coffee, is that the current model is either very much incomplete or just completely wrong. Nevertheless, it is the current model. It provides acceptable results within its limitations. In that light scientists must be willing to consider alternate models that provides equal or better results in order to remove those limitations. I don't have that model. So, I use what is available, like it or not.

Jeff Root
2015-Oct-09, 12:29 PM
I'm sorry, grapes, but both your posts #64 and #73 read
like doubletalk, intended to fool me into thinking you are
saying something real. I don't get anything real out of it.

Since you call it "a mathematical model" in post #64 and
start with a one-dimensional universe in post #73, you
may just be describing something akin to the Hilbert Hotel,
where I stayed once and was woken up in the middle of
the night and told I had to change rooms. The bellboy
wouldn't wait for me to help me with my bags. He had to
tell lots of other guests besides me that they had to move.
When I got to the new room, there was someone already
in it, and I had to wait for her to finish moving out before
I could move in. Housekeeping came and tried to clean up
at the same time I was moving in, and I never saw such a
disgusted or harried-looking housekeeping crew! The move
was nothing like instantaneous, as the bellboy claimed it
would be. I was so tired out from the move that I was late
to my morning meeting. Anyway, I later asked at the front
desk, and the manager said they were still moving in the
new guests that had arrived in the middle of the night, and
would probably still be moving them in when the Sun turns
into a red giant, so they weren't sure what they were going
to do. He also said the bellboy was out of cell phone range.
But at least they have plenty of rooms. No problem there.!

-- Jeff, in Minneapolis

publiusr
2015-Oct-09, 09:50 PM
Some of you may like this book:
https://en.wikipedia.org/wiki/White_Light_%28novel%29
I have always enjoyed Rucker's books.

Noclevername
2015-Oct-09, 09:54 PM
Some of you may like this book:
https://en.wikipedia.org/wiki/White_Light_%28novel%29
I have always enjoyed Rucker's books.

A story about a pothead who astral projects?


During an out-of-body experience, Felix loses his physical body and nearly falls victim to the Devil, who hunts the Earth for souls like his to take to Hell; Felix calls upon Jesus, who saves him. Jesus asks Felix to do him a favor: to take a restless ghost named Kathy to a place called "Cimön", and bring her to God/Absolute Infinite, which can be found there.

publiusr
2015-Oct-09, 09:56 PM
The idea was for hippies to go for the trip--but stay for the math.

Jens
2015-Oct-10, 04:47 AM
Yes, when I said infinite I meant in spatial extent, not length of time. I have always believed that there was a beginning to the Universe.

So do you think that at the moment of creation it was already infinitely large?

Noclevername
2015-Oct-10, 05:03 AM
So do you think that at the moment of creation it was already infinitely large?

I don't know. But I'm not willing to rule out the possibility.

grapes
2015-Oct-10, 07:48 AM
You mean this one?

No, Jeff said that one was word salad :)

My next post had some functions in it.


It has at least two causes. 1) a surface which places the points at their starting positions, 2) an event which starts them radiating outward.

Absolutely. My comments, and then my example, was only in response to Jeff's claim that what we know proves that an infinite universe could not have arisen in a finite time from a finite source, without violating causality. My example was not what happened, but only an example of a finite universe that grows to an infinite size in a finite time and results in a homogeneous infinite universe without violating causality. My example was mathematical and one-dimensional, but it refutes his claim.


I agree with Jeff that the current model demands an impossible set of circumstances, at least in terms of known physics.

I think everybody agrees with that. :)

grapes
2015-Oct-10, 07:50 AM
I'm sorry, grapes, but both your posts #64 and #73 read
like doubletalk, intended to fool me into thinking you are
saying something real. I don't get anything real out of it.

Function salad? :)


Since you call it "a mathematical model" in post #64 and
start with a one-dimensional universe in post #73, you
may just be describing something akin to the Hilbert Hotel,

Definitely not.

Shaula
2015-Oct-10, 09:23 AM
I agree with Jeff that the current model demands an impossible set of circumstances, at least in terms of known physics.
I think everybody agrees with that. :)
I wouldn't. The current model states that the observable universe expanded from a hot, dense initial state that was present a finite time ago. Which part of that do you regard as impossible?

I think it is important to remember than when you are speculating about creation events and so on you are well outside what the current models actually cover.

grapes
2015-Oct-10, 09:42 AM
I wouldn't. The current model states that the observable universe expanded from a hot, dense initial state that was present a finite time ago. Which part of that do you regard as impossible?

The hot, dense initial state

An impossible set of circumstances, at least in terms of known physics.


I think it is important to remember than when you are speculating about creation events and so on you are well outside what the current models actually cover.
Exactly what I meant by my comment. If I wasn't posting from my phone, I possibly would have said it the same way you do, except not as well. Thanks.

Jeff Root
2015-Oct-10, 09:56 AM
Bottom line is, I don't think the problem is a difference
in what we mean by "causality".
The bottom line may be that the definition of causality
that you are trying to use does not apply. The problem
is the "propagation limited to the speed of light" bit.
Please note that I have never expressed a speed-of-light
limitation on causality, in this thread or any other. I *did*
say in the post you quoted that "Most cause-effect relations
occur at much less than the speed of light", but I didn't
mean to imply any limit on speed. The speed of light is
clearly a limit in some situations, but I never applied it to
the cosmic expansion.

Since cause and effect are necessarily related by time
and possibly by space (they may be at the same location
in space), a causal relation is characterized by a maximum
speed. I am not specifying what that maximum speed is,
except to say that it varies from one situation to another.
However, "infinite" is not a speed. That's what I'm saying
can be ruled out in any question of whether there is a
causal connection between events.

-- Jeff, in Minneapolis

Noclevername
2015-Oct-10, 10:03 AM
Jeff, I asked before, but didn't get an answer: How would you classify observed universal phenomena like the same age of the CMB in all directions?

Shaula
2015-Oct-10, 10:04 AM
The hot, dense initial state. An impossible set of circumstances, at least in terms of known physics.
It is an initial state, one that can be described by known physical laws. I guess that is what I don't understand - what exactly is impossible about it? The initial state doesn't need an explanation for how it got there, doesn't need any history - it is just where the system starts and is akin to a postulate in the theory. We know how it evolved (mostly), we know how it behaved (mostly). I don't see anything impossible about it.

grapes
2015-Oct-10, 10:04 AM
Since cause and effect are necessarily related by time
and possibly by space (they may be at the same location
in space), a causal relation is characterized by a maximum
speed. I am not specifying what that maximum speed is,
except to say that it varies from one situation to another.
However, "infinite" is not a speed. That's what I'm saying
can be ruled out in any question of whether there is a
causal connection between events.

Yes, that's why I was careful, in my example, to not use infinite speed. The speeds are always finite, with finite local variation. But in the example, a finite local object becomes infinite in a finite amount of time, and becomes homogenous.

Noclevername
2015-Oct-10, 10:07 AM
Is there any evidence of propagation of information in the initial state, at any speed?

grapes
2015-Oct-10, 10:10 AM
It is an initial state, one that can be described by known physical laws. I guess that is what I don't understand - what exactly is impossible about it? The initial state doesn't need an explanation for how it got there, doesn't need any history - it is just where the system starts and is akin to a postulate in the theory. We know how it evolved (mostly), we know how it behaved (mostly). I don't see anything impossible about it.
There is nothing impossible about it. It occurred.

But in terms of known physics, we have no explanation for it. That's what Jeff is focussing on. He claimed that there is a proof that puts limits on the explanation. I think I have given an example that refutes his claim.

ETA: it goes back to the claim that was quoted in the OP of this thread:


1. From the Hubbles Law vs. Universal Acceleration (http://cosmoquest.org/forum/showthread.php?158709-Hubbles-Law-vs-Universal-Acceleration) thread:


If the Universe is infinite, then only a finite portion of it was involved in the Big Bang.


Is this true or not, and why?

I'm saying, it's not necessarily true.

Drummer62
2015-Oct-10, 10:17 AM
As Shaula said all our models become obsolete beyond the BB.
Jeff Root, you are applying concepts of causality and time to something where we don't know if they still apply or not.

You are basically speculating about a creation event.
As far as I am concerned, the fact that there is anything at all - and not nothing - is the biggest mystery of them all.
(Even if you assume that the sum-total of all energy is zero, the fact remains that that zero energy somehow resulted in there being something, for example our minds to speculate about such things).
You are speculating about something like a first cause.
That first cause is the biggest mystery. Even if you assume some kind of creator, the question remains: where did the creator come from?

Why would it be any more mysterious if that first cause created an infinite system compared to a finite system?
Both are equally mysterious as far as I am concerned.

Noclevername
2015-Oct-10, 10:21 AM
Why would it be any more mysterious if that first cause created an infinite system compared to a finite system?
Both are equally mysterious as far as I am concerned.

Agree 100%.

grapes
2015-Oct-10, 10:26 AM
That first cause is the biggest mystery. Even if you assume some kind of creator, the question remains: where did the creator come from?

Why would it be any more mysterious if that first cause created an infinite system compared to a finite system?
Both are equally mysterious as far as I am concerned.
Jeff answered this question before, in this thread, I think. Post #27, maybe?

Drummer62
2015-Oct-10, 10:46 AM
Jeff answered this question before, in this thread, I think. Post #27, maybe?

Hmm, I can't find an answer in post #27 or in any other post.
I don't think there is an answer.

I think concepts like causality and time (which are closely linked, as causality is meaningless without a concept of time) are more or less invented by our minds to make sense of our very limited observations.

If they are applied beyond their realm of applicability (and that is what I think is happening here) we end up with meaningless questions, or at least questions that are way beyond our minds to even begin to comprehend.

I know, they are very fascinating and captivating, but at the end of the day we just have to come to terms with the fact that we shall forever remain agnostics on those issues.

Shaula
2015-Oct-10, 11:13 AM
But in terms of known physics, we have no explanation for it. That's what Jeff is focussing on. He claimed that there is a proof that puts limits on the explanation. I think I have given an example that refutes his claim.
ETA: it goes back to the claim that was quoted in the OP of this thread:
OK, I think I see what you are saying. I guess I just see it as initial conditions - known physics doesn't have to explain that and it doesn't lead to anything being impossible in the current model. Essentially what you are saying is just that things that are outside the domain of applicability of the current model are not explained by the current model. It just sounded, to me, like you were saying something more general.

As for Jeff's claim - I am pretty sure no example or explanation anyone can give will ever convince him that he is wrong. He has shown in many threads on this topic that when it comes to physics he trusts his own 'common sense' beliefs and biases more than anything else. I see no value in trying (again) to convince him that science is not natural philosophy. That deceased equine has been beaten more times than the Cubs at the World Series.

Jeff Root
2015-Oct-10, 11:34 AM
Jeff, I asked before, but didn't get an answer:
How would you classify observed universal phenomena
like the same age of the CMB in all directions?
I didn't understand what you were asking, and still don't.

What do you mean by "classify"? What classifications
do you have in mind? If you just mean "describe", I
don't know what kind of description you are asking for.
If you are just asking what I think about the assertion
that the CMB is the same age in all directions, I agree
that it is the same age in all directions, and would look
that way to all observers, nomatter where they are.

At the current time of roughly 14 billion years after the
Big Bang, all observers see the light emitted by hot but
cooling plasma that-- at the time it was emitted-- was
about 42 million light-years from the point at which it is
observed. That light traveled a distance of 14 billion ly
through expanding space to reach the observers.

It is my contention that the hot but cooling plasma was
all the same composition, temperature, and age, and
expanding everywhere at the same rate because it had
a common origin with which it was causally-connected.
Even if two widely-separated particles were never in
causal contact with each other-- neither particle could
ever affect the other-- they were causally connected by
their common ancestors. The properties of the particles
were determined by particles and forces ancestral to
both of them. There is a chain of causality going all the
way back to the beginning of the Big Bang which links
everything participating in the cosmic expansion.

Observed present-day relative abundances of primordial
hydrogen, deuterium, helium, and lithium put some limits
on the composition, density, temperature, and expansion
rate of the expanding matter in the first few minutes after
the Big Bang. But conditions and events during the first
few seconds are unknown. The simplest extrapolations
backward in time predict higher density and temperature
as we look closer to time zero, resulting in a singularity
at time zero in which the density and temperature were
infinite. However, the matter-energy we are quite certain
existed a minute later must have come into existence at
some time, and that would have been at or around time
zero. So we don't know how close to time zero the simple
extrapolation works, or what the maximum density and
temperature actually were. We can be sure they weren't
infinite.

During this time close to time zero, all the known laws of
physics should apply, but because the conditions were so
different from any conditions now, some uknown physics
must have been even more important than known physics,
and it would describe how the matter-energy came into
existence, and what its properties would be.

Is the answer to your question anywhere in there?

-- Jeff, in Minneapolis

grapes
2015-Oct-10, 03:53 PM
Jeff answered this question before, in this thread, I think. Post #27, maybe?

Hmm, I can't find an answer in post #27 or in any other post.
I don't think there is an answer.

I meant, Jeff supplied his personal reasons


I think concepts like causality and time (which are closely linked, as causality is meaningless without a concept of time) are more or less invented by our minds to make sense of our very limited observations.

If they are applied beyond their realm of applicability (and that is what I think is happening here) we end up with meaningless questions, or at least questions that are way beyond our minds to even begin to comprehend.

Jeff can speculate, but what I did was use his own premises, and refute his conclusion.


I know, they are very fascinating and captivating, but at the end of the day we just have to come to terms with the fact that we shall forever remain agnostics on those issues.
Yes, he can have principles or assumptions that he holds, and draws conclusions from. I think I used his same principles to show that the conclusion was unwarranted.

Noclevername
2015-Oct-10, 04:11 PM
Coordination of BB events requires an instantaneous propagation of causality that is harder for me to believe than an infinite universe. It seems more likely that what we see happened without coordination or information transfer. But I'm not calling FTL itself impossible because expansion and inflation do just that, I'm just calling Jeff's "instant causality" a violation of known physics.

Yes, the starting conditions don't have to be the result of anything we'd recognize as "normal" physics, but I think it's important to point out bugs even if you know they are supposed to be features. ;)

Jeff Root
2015-Oct-10, 05:56 PM
Noclevername,

Why did you refer to my "instant causality"? I'm fairly
sure that you are the one who introduced the idea a couple
of years ago (or more) in opposition to my assertion that the
Big Bang must be finite, and it is the idea I've been battling
against. Being opposed to it makes it mine???

Please explain! I know I haven't responded to all of your
questions, but you've left several things unexplained, too.

I should have said something about expansion and inflation
being FTL-like, as you point out, in my posts #96 and #119.
That was a bad omission by me. Cosmic expansion and
inflation both essentially act faster than light. They carry
particles and information great distances apart in times far
shorter than would be possible without expanding space.
So I stipulate that the chain of causality can be extended
that way. But no mainstream description of expansion or
causality that I've ever seen suggests that they could be
instantaneous or occur with infinite speed, which is what
you suggested: A Universe of finite age which is infinite in
spatial extent from the beginning, with the same properties
everywhere.

I'm going to try to puzzle out grapes's "math" next.

-- Jeff, in Minneapolis

Noclevername
2015-Oct-10, 06:15 PM
Why did you refer to my "instant causality"? I'm fairly
sure that you are the one who introduced the idea a couple
of years ago (or more) in opposition to my assertion that the
Big Bang must be finite, and it is the idea I've been battling
against. Being opposed to it makes it mine???

Please explain! I know I haven't responded to all of your
questions, but you've left several things unexplained, too.


Not mine! I attributed it to you because you said you think the BB event's results were "causally connected" all over the universe. They are all the same age, so if they are causally connected, it would have to be instantaneous or nearly so. That's a different definition of causality than physics uses, BTW.

Anything in my posts you want me to explain, go ahead and ask, I'll answer to the best of my abilities.

grapes
2015-Oct-10, 07:03 PM
I'm going to try to puzzle out grapes's "math" next.

Maybe this will help


Start the one-d universe at time=-R, where it consists of a single point, and many points radiate out from it in spacetime. Their interactions depend upon their relationship to each other at a specific time (t-coordinate). Their y-coordinate is tan(θ)*sqr(R2-t2), where θ represents an initial angle at the time the point leaves the initial point, and -90°<θ<90°, so no point has an infinite velocity, and its velocity is incrementally greater/lesser than that of its neighbors.

At time t=0, all points in the universe have homogenized--that is their relative motion in spacetime is identical to their neighbors'

So, at time zero, any change in any point trajectory will only depend upon local conditions, but all conditions on the universe are identical. So, from time=0 on, if the universe expands at one point, it will expand in the same fashion at other points, with the same proportionality constant.

Any subsequent measurement will indicate the expansion started at time 0, even though the process actually started at time=-R

So here we have a mathematical example which starts with a finite universe, becomes an infinite universe in a finite time, and results in a homogenous universe, without needing anything more than local interaction.

And, it fits the "word salad" description. :)
All trajectories start at (-R,0), with time t=-R. So, in this illustration, the entire universe is a point at time=-R. Each element of the universe is distinguished by a parameter θ (180°<θ<180°), a characteristic acquired when the elements were coincident, so there is no need for a violation of causality. The path of each element, after time = -R, is locally close to its neighbors and may be influenced by local neighbors, but in all cases the path follows an ellipse whose vertical axis is length tan(θ), and the horizontal axis is length R. For no element is the velocity infinite, and each velocity is close to the velocity of the neighboring velocities.

At time t=0, then, the elements all have identical and horizontal velocities.

And there is no finite limit to how far they have spread.

At time t=0, the conditions of each point and their relationship to their local neighbors is the same for the entire universe. So, at t=0, the conditions might be right for the start of a "big bang" which would occur throughout the universe without the need for causality violation.

As I said, this example just moves it from "impossible" to "highly improbable", I'm not advocating any sort of mechanism. Of course, once something does occur, it may still be highly improbable--but it can no longer be considered impossible.

ETA: Weird, the angle range should be -90°<θ<90° and not 180°<θ<180° (which is, kinda narrow)

Jeff Root
2015-Oct-10, 08:46 PM
I explained what I mean by "causally connected" this
morning in post #119:

"It is my contention that the hot but cooling plasma was
all the same composition, temperature, and age, and
expanding everywhere at the same rate because it had
a common origin with which it was causally-connected.
Even if two widely-separated particles were never in
causal contact with each other-- neither particle could
ever affect the other-- they were causally connected by
their common ancestors. The properties of the particles
were determined by particles and forces ancestral to
both of them. There is a chain of causality going all the
way back to the beginning of the Big Bang which links
everything participating in the cosmic expansion."

You and I have probably never been anywhere near each
other, yet we undoubtedly are physically very similar, built
of similar molecules, cells, tissues, organs, with two legs,
two arms, one head, two eyes, two ears, and so forth.
We are so alike because we are causally-connected to our
common ancestors who gave both of us their genes, which
are now our genes. That is how we are causally connected.
Without that causal connection, we would not be so alike.

My hands are both warm from washing them in hot water.
They have similarly elevated temperatures not because heat
transfered from one hand to the other, but because heat
transferred from the water to both hands. The heat had one
origin, and it affected both hands, making their temperatures
similar. The elevated temperatures of my hands are causally
connected by the hot water.

Nothing instantaneous about these causal connections.

I had forgotten that we had this difference in views about
what is meant by "causally connected", so you were probably
right a few posts back that a difference in what we mean by
"causality" is a problem-- although it might be the word
"connected" that is actually the problem.

I'd appreciate it if you could explain what you meant by
"classify" when you asked, "How would you classify observed
universal phenomena like the same age of the CMB in all
directions?"

There was a sentence or paragraph at the end of one of
my recent posts that I wanted you to respond to, but I've
forgotten which post. Maybe we've progressed beyond it
so that it is no longer so important.

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-10, 08:50 PM
grapes,

Another post I'm going to have to quote in my reply to you!

My God! It's full of quotes!

-- Jeff, in Minneapolis

ShinAce
2015-Oct-10, 09:43 PM
What's the difference between 'causally connected' and 'correlated'?

Noclevername
2015-Oct-10, 09:48 PM
I'd appreciate it if you could explain what you meant by
"classify" when you asked, "How would you classify observed
universal phenomena like the same age of the CMB in all
directions?"


Maybe classify wasn't the right word, but I think your explanation covers it well. It fits with my posts #63 and 69, the condition of the early universe appears to have been just the way that spacetime began. I still think you're incorrect about whether infinite space is a possibility, but at least now I know why I'm disagreeing with it!

Jeff Root
2015-Oct-11, 01:33 AM
That sounds good.

I'm writing a reply to grapes, and the process is three
steps forward, two steps back. Or two steps forward, three
back... I may finally have some idea what he's saying.

-- Jeff, in Minneapolis

ShinAce
2015-Oct-11, 01:44 AM
What's the difference between 'causally connected' and 'correlated'?

That question is for you, Jeff.

Jeff Root
2015-Oct-11, 09:18 AM
ShinAce,

I didn't see the post until just now. I don't think it was
on my screen when I read Noclevername's post.

Things are "correlated" when they are found to occur
together in some way. Correlation often hints at the
existence of a causal connection, but doesn't say what
causes what. A correlation would be what is observed,
and a causal connection would be an explaination of
the correlation.

I observe a correlation in that when one of my hands gets
warm, the other hand often also gets warm. I also find a
correlation in that when I put a hand into hot water, the
hand gets warm. If I put both hands into the hot water,
they both get warm. I explain these correlations by saying
that there is a causal relation between the heat in the water
and the heat in my hands: The heat in the water causes the
heat in my hands. So the heat in my left hand is causally
connected to the heat in my right hand via the heat in the
water.

Still slogging through a reply to grapes.

-- Jeff, in Minneapolis

Reality Check
2015-Oct-11, 11:04 PM
The hot, dense initial state

An impossible set of circumstances, at least in terms of known physics.

Sorry, grapes, but it is known physics that is projected backward to produce the hot, dense initial state of the Universe. It is the most probable set of circumstances given what we know about the universe.

Reality Check
2015-Oct-11, 11:07 PM
Please note that I have never expressed a speed-of-light
limitation on causality, in this thread or any other. I *did*
say in the post you quoted that "Most cause-effect relations
occur at much less than the speed of light", but I didn't
mean to imply any limit on speed.
Then there is no problem - the expansion of the universe does not invalidate any cause-effect relations since you allow them to happen at any finite velocities and no one is claiming that the universe is expanding at an infinite velocity.

Jeff Root
2015-Oct-12, 05:46 AM
The hot, dense initial state

An impossible set of circumstances, at least in terms of
known physics.
Sorry, grapes, but it is known physics that is
projected backward to produce the hot, dense initial
state of the Universe. It is the most probable set of
circumstances given what we know about the universe.
Extrapolate backward far enough and the conditions
are too hot and too dense to be described by known
physics. New physics is required. The new physics
is expected to find that it was not as hot or as dense
as the extrapolation predicts.

I believe that is what grapes is saying there.

This thread is about the possibility or impossibility of
an infinite universe resulting from the Big Bang. I say
that it is an impossibility. One suggested solution to
this problem (which I think was first suggested here
on BAUT/CosmoQuest by Noclevername) is that
the Universe could begin already infinite in its first
instant of existence, hot and dense throughout an
infinite volume. I say that that is also impossible.

Although this thread began with the question of the
possibility or impossibility of an infinite universe
resulting from the Big Bang, it is more fundamentally
about whether known laws of physics apply to the
Universe as a whole, whether causality applies, and
what causality means when applied to the Big Bang.
The question of an infinite universe resulting from
the Big Bang combines these issues into one neat
and colorful package.

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-12, 05:57 AM
Please note that I have never expressed a speed-of-light
limitation on causality, in this thread or any other. I *did*
say in the post you quoted that "Most cause-effect relations
occur at much less than the speed of light", but I didn't
mean to imply any limit on speed.
Then there is no problem - the expansion of the universe
does not invalidate any cause-effect relations since you
allow them to happen at any finite velocities and no one
is claiming that the universe is expanding at an infinite
velocity.
Read the OP. The question of this thread is whether an
infinite universe could result from the Big Bang. That
requires the universe to become infinite in finite time,
which means infinite speed. If the universe is infinite
from the very beginning, that is infinite speed.

-- Jeff, in Minneapolis

Noclevername
2015-Oct-12, 06:01 AM
If the universe is infinite
from the very beginning, that is infinite speed.


It really isn't. That the starting conditions of the Universe require propagation of something at a "speed", is solely your conclusion.

grapes
2015-Oct-12, 10:01 AM
The hot, dense initial state

An impossible set of circumstances, at least in terms of known physics

Sorry, grapes, but it is known physics that is projected backward to produce the hot, dense initial state of the Universe. It is the most probable set of circumstances given what we know about the universe.
No problem. I completely agree.

However, in the context of this thread, we are talking about the physical process involved in the origin of the hot, dense state. That's the "impossible in terms of known physics" part.

Jeff has made a claim that there is a certain constraint we can infer about that process, and I'm thinking I've come up with a way to convince him otherwise :)

Jeff Root
2015-Oct-12, 11:21 AM
If the universe is infinite from the very beginning,
that is infinite speed.
It really isn't. That the starting conditions of the
Universe require propagation of something at a
"speed", is solely your conclusion.
How do you explain these "starting conditions" being
the same everywhere simultaneously throughout an
infinite volume? At exactly the same instant, all of an
infinite volume suddenly contains exactly the same kind
of matter, at exactly the same temperature, expanding
at exactly the same rate. How did that come about?
How did conditions get to be the same everywhere?
Why did it happen everywhere at the same time?

The idea is absurd.

And completely unnecessary because there is no
evidence that such a thing ever happened. Because
there is no evidence that the volume involved in the
Big Bang is infinite in extent or that the "starting
conditions" involved a large or infinite volume.

-- Jeff, in Minneapolis

Noclevername
2015-Oct-12, 02:28 PM
How do you, Jeff, explain these starting conditions being the same everywhere simultaneously throughout a finite volume? At exactly the same instant, all of a finite volume suddenly contains exactly the same kind of matter, at exactly the same temperature, expanding at exactly the same rate. How did that come about? How did conditions get to be the same everywhere? Why did it happen everywhere at the same time?

Noclevername
2015-Oct-12, 02:40 PM
What I'm saying is, Jeff is asking questions about what "happens" before t=0, the start of time. And that's not answerable.

Jeff Root
2015-Oct-12, 03:07 PM
How do you, Jeff, explain these starting conditions
being the same everywhere simultaneously throughout
a finite volume? At exactly the same instant, all of a
finite volume suddenly contains exactly the same kind
of matter, at exactly the same temperature, expanding
at exactly the same rate. How did that come about?
How did conditions get to be the same everywhere?
Why did it happen everywhere at the same time?
I never said or suggested that any such thing would
happen. I have said all along -- many times -- that
everything which came out of the Big Bang had the
same properties because it was causally connected.
It was all in causal contact at the very beginning, and
expanded away from that contact.

One popular assertion about the Big Bang is that it may
have started with a single quantum fluctuation (vacuum
fluctuation) in which virtual particles somehow became
physical particles, and the process cascaded, growing to
ludicrous size. I have no idea why it would do that, but
it is a very common description of how the Big Bang may
have begun, and it is all that is required in my scenario.
The properties of all the physical particles created in the
Big Bang come from the properties of the quantum
fluctuation which started it. Everything involved in the
Big Bang and the ensuing cosmic expansion has a single,
common origin.

So there was no "simultaneously throughout a volume"
to explain.

-- Jeff, in Minneapolis

Noclevername
2015-Oct-12, 03:11 PM
So there was no "simultaneously throughout a volume"
to explain.

Then what difference does it make if it was finite or infinite?

I guess I just don't understand your scenario, probably because you are using the term "causally connected" in a way very different from the way it's normally used in physics.

Jeff Root
2015-Oct-12, 03:19 PM
What I'm saying is, Jeff is asking questions about
what "happens" before t=0, the start of time. And
that's not answerable.
You may not know how the conditions you propose
get to be that way, but you need to admit that the
conditions have to get that way somehow. Even if
it is by someone just saying "Let there be light,"
and there is light.

Whether this involves a time before t=0 or not
depends on how you define t=0. And whether time
before t=0 is a problem for you to talk about depends
only on whether there *was* a time before t=0.
If there was, you should be able to talk about it just
as easily as we can talk about about what happened
shortly after t=0. We can't directly observe anything
that happened at either time, but we can figure out
what could and could not have been going on at those
times from what we do observe.

-- Jeff, in Minneapolis

Noclevername
2015-Oct-12, 03:27 PM
You may not know how the conditions you propose
get to be that way, but you need to admit that the
conditions have to get that way somehow. Even if
it is by someone just saying "Let there be light,"
and there is light.

Whether this involves a time before t=0 or not
depends on how you define t=0. And whether time
before t=0 is a problem for you to talk about depends
only on whether there *was* a time before t=0.
If there was, you should be able to talk about it just
as easily as we can talk about about what happened
shortly after t=0. We can't directly observe anything
that happened at either time, but we can figure out
what could and could not have been going on at those
times from what we do observe.

-- Jeff, in Minneapolis

Even I know that what you're asserting is very, very wrong. It's like saying that we should be able to 'easily" know what's happening in other universes. I'll let some of the scientist types here explain why. But you should know that what you're claiming is definitely not consistent with a mainstream model and is not supported by any evidence.

grapes
2015-Oct-12, 03:38 PM
Even I know that what you're asserting is very, very wrong. It's like saying that we should be able to 'easily" know what's happening in other universes. I'll let some of the scientist types here explain why. But you should know that what you're claiming is definitely not consistent with a mainstream model and is not supported by any evidence.
I think that's a given, in the OP. What Jeff asked (via a re-post of his comments) was why certain possibilities are still considered possible. I believe I've provided an answer to Jeff's question, but he's still mulling it over.

Jeff Root
2015-Oct-12, 03:58 PM
So there was no "simultaneously throughout a volume"
to explain.
Then what difference does it make if it was finite
or infinite?
It is the same difference as in the following scenarios.
Although both of these are finite, the first scenario has
exactly the problem that I find with your scenario.

Imagine that one day you see that everything has my
signature on it. Every scrap of paper you look at, every
wall, every brick, every leaf on every tree, every blade
of grass, every cat, every dog, every rock, every grain
of sand has "Jeffrey S. Root" written on it. Everyone
around the world says they see exactly the same thing.
Astronomers report that they see my signature on the
Moon and the moons of Jupiter, and JPL says Curiosity
has found my signature on every rock it has recently
imaged.

That is your scenario. I ask you how you explain my
signature getting on everything simultaneously like that.

Alternatively, imagine that one day I write my signature
on a rock, and later someone sees it and copies it onto
another rock, and a wall, and a sidewalk, and over time
doing that catches on and people farther and farther
away are writing my signature on all sorts of things.

That is my scenario. You are asking me what difference
it makes whether my signature appears the way it does
in my scenario or the way it does in your scenario.

Which figuratively blows my mind.



I guess I just don't understand your scenario, probably
because you are using the term "causally connected" in
a way very different from the way it's normally used in
physics.
I don't think that's it at all.

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-12, 04:08 PM
I think that's a given, in the OP. What Jeff asked
(via a re-post of his comments) was why certain
possibilities are still considered possible. I believe
I've provided an answer to Jeff's question, but he's
still mulling it over.
Yes. My math aversion is, unfortunately, a major
factor slowing me down, but I also have some other
very serious business to attend to, which I should be
doing instead of posting here. But I have a big chunk
of a reply to your posts #64 and #73. I haven't yet
seen what modifications I need to make to address
post #124 except that you have a different range for
the angle θ. Five minutes writing, 55 minutes editing.

-- Jeff, in Minneapolis

grapes
2015-Oct-12, 06:59 PM
Acckk, dunno how that happened. The angle range should not have changed. It should be -90°<θ<90° and not 180°<θ<180°, which looks kinda narrow to me

Noclevername
2015-Oct-12, 11:18 PM
Every scrap of paper you look at, every
wall, every brick, every leaf on every tree, every blade
of grass, every cat, every dog, every rock, every grain
of sand has "Jeffrey S. Root" written on it.

If you have really convinced yourself that that's a viable comparison at all, then I really can't say anything further. There's just a fundamental disconnect.

Reality Check
2015-Oct-12, 11:22 PM
Extrapolate backward far enough and the conditions
are too hot and too dense to be described by known
physics.
Extrapolate backward far enough using known physics and we get to a singularity where known physics breaks down.
New physics would be required to remove the singularity, e.g. quantum gravity.

Reality Check
2015-Oct-12, 11:27 PM
The question of this thread is whether an infinite universe could result from the Big Bang.
Then this thread is moot because it is mainstream cosmology that the Big Bang occurring in an infinite universe produces an infinite universe. If the universe is infinite from the very beginning, it expands because the distance between points gets bigger.
Your "infinite speed" comment sounds as if you are thinking of a boundary at "infinity" that is expanding.
I do not know of any mainstream cosmology that starts with a finite universe and produces an infinite universe. But maybe you have a citation, Jeff Root?

ETA: The OP has questions which I answered some days ago (7 October 2015). (http://cosmoquest.org/forum/showthread.php?158721-Expanding-infinities&p=2317743#post2317743)

Noclevername
2015-Oct-13, 12:01 AM
ETA: The OP has questions which I answered some days ago (7 October 2015). (http://cosmoquest.org/forum/showthread.php?158721-Expanding-infinities&p=2317743#post2317743)

Thank you for that, RC. I let the discussion get sidetracked, but it's important to point out that Jeff's original notions have already been answered, he just rejected the answers.

Jeff Root
2015-Oct-13, 12:02 AM
Noclevername,

You can say what you think is wrong with the comparison.

You undoubtedly view the scenario I described as ridiculous.
So do I. In what way or ways do you see it not being a good
analogy to the instantaneously infinite universe idea you
suggested? The biggest difference I see is that I didn't say
the signatures go on forever -- I only had them go as far as
we can currently see. Whether they extend to other stars or
galaxies isn't specified. That difference is of no importance,
though. It isn't the infiniteness of your scenario that makes
it impossible, it is the violation of causality, which is the
salient feature of the instantaneous signatures scenario.
It would be physically possible for my signature to be on all
those things. On some it would have to be very small, but
the smallest things I listed were blades of grass and grains
of sand. Those are big enough to print on using several
different techniques. So you can't complain that the
scenario is physically impossible.

The scenario of instantaneous signatures violates causality
in the same way your scenario of an instantaneously infinite
universe violates causality, and that is what the analogy is
intended to analogize. I think your scenario is freaky weird,
so my analogy is freaky weird. If you think my analogy is
freaky weird, then you probably understand it correctly.

So what do you think is wrong with it?

-- Jeff, in Minneapolis

Noclevername
2015-Oct-13, 12:09 AM
As I said, Jeff, I'm done trying to convince you of anything. You clearly have your mind made up, and there's no way to reason someone out of an emotional belief.

Jeff Root
2015-Oct-13, 12:18 AM
Your "infinite speed" comment sounds as if you are
thinking of a boundary at "infinity" that is expanding.
No.



I do not know of any mainstream cosmology that starts
with a finite universe and produces an infinite universe.
Neither do I.



But maybe you have a citation, Jeff Root?
No, I do not. Why would I? It is the idea I have been
arguing against.

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-13, 12:20 AM
Noclevername,

Don't try to convince me of anything. Just explain what you
think is wrong with the analogy.

-- Jeff, in Minneapolis

Noclevername
2015-Oct-13, 12:29 AM
Noclevername,

Don't try to convince me of anything. Just explain what you
think is wrong with the analogy.

-- Jeff, in Minneapolis

You took an arbitrary, complex factor like a signature, and compared it to something completely unlike it. The earliest know conditions were a primal mass of energy that's literally as chaotic and formless as physics can determine. There's no "signature" or specific pattern involved. And yes, you are still using "causally connected" in a non-physics way.

Noclevername
2015-Oct-13, 12:33 AM
Let me try one last time. Suppose you have a red sun set. The next day you look at pictures of the last evening and see that everything is red tinted. You therefore conclude that someone with a paintbrush must have stained everything red. If someone says the light was red, you ask "But how did that get red stain on everything? I can't envision that much red stain getting on everything, there's no brush that big."

Reality Check
2015-Oct-13, 01:08 AM
It is the idea I have been
arguing against.
So you have been arguing against your already debunked idea in the OP?
Or the idea that a finite universe produces an infinite universe?
Or some other idea?

Jeff Root
2015-Oct-13, 02:21 AM
You took an arbitrary, complex factor like a signature,
and compared it to something completely unlike it.
The earliest known conditions were a primal mass of
energy that's literally as chaotic and formless as
physics can determine. There's no "signature" or
specific pattern involved.
So your main objection to the analogy is that my
signature is significantly more complex than the initial
conditions of the Universe. The main reason you
consider my signature to be significantly more complex
than the initial conditions of the Universe is that those
conditions were extremely simple.

Is that correct?

If so, can you explain why the difference in complexity
is a significant defect in the analogy? In some analogies,
a large difference in complexity between the thing being
analogized and its analogue wouldn't matter. In this
case, you feel that it does matter very much. Can you
explain why it matters?

-- Jeff, in Minneapolis

.

Jeff Root
2015-Oct-13, 02:54 AM
Reality Check,

Here is the statement I made which Noclevername
quoted from in the OP of this thread:



... my personal view is that the spatial extent of
the Universe that was involved in the Big Bang and
is now participating in the resulting cosmic expansion
is necessarily finite. If the Universe is infinite, then
only a finite portion of it was involved in the Big Bang.
The two principle views opposing it are that the Big Bang
could have expanded to become infinite, or that the Big
Bang occurred everywhere simultaneously throughout an
already infinite volume. The latter scenario is what
Noclevername has been promoting in opposition to
my assertion, and is what I have primarily been arguing
against.

I have explained the assumptions which lead to my view
throughout this thread.

-- Jeff, in Minneapolis

Noclevername
2015-Oct-13, 03:32 AM
Can you
explain why it matters?


Seriously?

It matters, of course, because we are not talking about the state of the universe after some present-day event, but as a result of its creation. Why would you expect the first thing that ever happens to be complex? The rules we go by now were not even developed yet. It makes no sense to me to compare apples to oranges, then ask why they are not both apples.

I'm not sure I can explain it any better than that. Ask one of the other posters.

Reality Check
2015-Oct-13, 03:57 AM
Reality Check,

Here is the statement I made which Noclevername
quoted from in the OP of this thread:...
Jeff Root,

We have "my personal view is that the spatial extent of the Universe that was involved in the Big Bang and is now participating in the resulting cosmic expansion is necessarily finite." which means nothing much without evidence. There is seems no evidence that the universe is "necessarily" finite. Standard cosmology works with an infinite universe or a finite universe. Observations can give us an upper limit to the size of the universe and not rule out an infinite universe.

Then the invalid idea of "If the Universe is infinite, then only a finite portion of it was involved in the Big Bang" because the Big Bang applies to the entire universe, i.e.

If the universe is infinite then by definition all of this infinite universe was involved in the Big Bang. Ditto for a finite universe.

WayneFrancis
2015-Oct-13, 07:47 AM
I'll reiterate my position again. We don't know what happened at T=0. We don't know how or even if the Universe came into being. Within our universe IF there was a point of creation the "cause" by definition is not causal within our universe. Asking what, within our universe, caused the creation of our universe makes ZERO sense. Thus complaining about causality at the point of creation would also make zero sense. We can, I guess, imagine a cause at some higher order dimension or something like that. But the "physics" of that higher order dimension need not follow the physics of our universe.

So given the example, and I am also not saying this is the way it happened, where the universe does start at a single point and expands to infinity within a finite amount of time could explain why all of space, even if it is infinite, shares some basic attributes.

So 1 event could give rise to an infinite universe with very similar if not exactly the same properties. Such as the speed of light, the gravitational constant, space-time having given function for what dark energy is, blah blah blah.

The argument that something that can not be defined by the laws of physics of this universe, the creation of the universe, has to be subject to those same laws just doesn't make sense to me.

Jeff Root
2015-Oct-13, 08:02 AM
Noclevername,

Since you didn't explicitly respond to the question,
I take it that you agree that your main objection to
the analogy is that my signature is significantly more
complex than the initial conditions of the Universe,
and that the main reason you consider my signature
to be significantly more complex than the initial
conditions of the Universe is that those conditions
were extremely simple.




Can you explain why [the difference in complexity
between initial conditions of the Big Bang and what
I used as its analogue, my signature] matters?
Seriously?
Yes. I'm completely serious.



It matters, of course, because we are not talking about
the state of the universe after some present-day event,
but as a result of its creation.
You say "of course", but it isn't at all obvious how
the fact that we aren't talking about the state of the
universe after some present-day event, but as a result
of its creation, implies that the difference in complexity
matters to the quality of the analogy.



Why would you expect the first thing that ever happens
to be complex?
I would not expect the first thing that ever happens to
be complex. I would expect it to be extremely simple.



The rules we go by now were not even developed yet.
What rules are you referring to? The "laws of physics"?
You are asserting that the laws of physics did not exist
at the time of the Big Bang? You saw when I listed my
assumptions that I assume that laws of physics which
hold now also applied at the time of the Big Bang. You
didn't complain about it then. Did you just realize that
you disagree with that assumption? Or have I totally
misunderstood what "rules" you mean?

I'ill repeat that while I assume laws of physics were the
same at the Big Bang as they are now, conditions were
very different, causing some laws to be more important
and others to be less important. Unknown physics could
be more important than known physics at the start of
the Big Bang.

So, was this statement about "rules" intended to help
explain why you feel that the difference in complexity
between my signature and conditions at the start of
the Big Bang matters in the analogy? If so, I don't
understand how it does that.



It makes no sense to me to compare apples to oranges,
then ask why they are not both apples.
So you consider the difference in complexity to be so
large that the situation in the analogy is nothing like
the situation it was meant to be an analogy of. I'll
ask again:

Given that we both expect the initial conditions of the
Big Bang to be extremely simple, and we both agree
that my signature is a somewhat complex thing, please
explain why the difference in complexity is a significant
defect in the analogy. As I said, in some analogies, a
large difference in complexity between the thing being
analogized and its analogue wouldn't matter. In this
case, you feel that it does matter very much. Can you
explain why it matters? What effect do you think the
difference in complexity has on the analogy?

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-13, 08:18 AM
I'm just picking out one assertion you made:



If the universe is infinite then by definition all of this
infinite universe was involved in the Big Bang.
What definition? Show me.

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-13, 09:02 AM
I'll reiterate my position again. We don't know what
happened at T=0. We don't know how or even if the
Universe came into being. Within our universe IF there
was a point of creation the "cause" by definition is not
causal within our universe.
Really? "By definition?" What definition is that?

As I said in post #141:

One popular assertion about the Big Bang is that it may
have started with a single quantum fluctuation (vacuum
fluctuation) in which virtual particles somehow became
physical particles, and the process cascaded and grew.

Would that quantum fluctuation have been outside the
Universe? It seems to me that it would have been the
very first thing *inside* the Universe. But you claim
that any cause of the Big Bang would be outside of the
Universe by definition. I'd like to see that definition.



Asking what, within our universe, caused the creation
of our universe makes ZERO sense.
The actual question of this thread is whether the Big
Bang could have been large or infinite in extent at its
very beginning, or become infinite in finite time, not
what caused the creation of the Universe.



The argument that something that can not be defined
by the laws of physics of this universe, the creation of
the universe, has to be subject to those same laws
just doesn't make sense to me.
What I have said several times, in this thread and in
previous threads, is that I assume that known laws of
physics which apply now also applied at the beginning
of the Big Bang, but that unknown laws of physics are
required to accurately describe that beginning.

Do you have any reason to think that laws of physics
which apply now did not apply at the beginning?

-- Jeff, in Minneapolis

Noclevername
2015-Oct-13, 09:47 AM
You are asserting that the laws of physics did not exist
at the time of the Big Bang? You saw when I listed my
assumptions that I assume that laws of physics which
hold now also applied at the time of the Big Bang.

Enough other posters have already explained the problems with your assumption that I didn't feel it necessary to repeat them.


What effect do you think the
difference in complexity has on the analogy?


As I said, I think the comparison of two totally unlike things is not a valid analogy for anything.

Jeff, I'm done arguing with you. Feel free to direct any further questions to others.

grapes
2015-Oct-13, 12:51 PM
So given the example, and I am also not saying this is the way it happened, where the universe does start at a single point and expands to infinity within a finite amount of time could explain why all of space, even if it is infinite, shares some basic attributes.

So 1 event could give rise to an infinite universe with very similar if not exactly the same properties. Such as the speed of light, the gravitational constant, space-time having given function for what dark energy is, blah blah blah.

Yes, that was my example, but Jeff has chosen to not remark on it at all, except to say he doesn't understand it, and is preparing a response. :)

mkline55
2015-Oct-13, 02:22 PM
If you mean, you didn't understand it, but want an example, I'll try.

Start the one-d universe at time=-R, where it consists of a single point, and many points radiate out from it in spacetime. Their interactions depend upon their relationship to each other at a specific time (t-coordinate). Their y-coordinate is tan(θ)*sqr(R2-t2), where θ represents an initial angle at the time the point leaves the initial point, and -90°<θ<90°, so no point has an infinite velocity, and its velocity is incrementally greater/lesser than that of its neighbors.

At time t=0, all points in the universe have homogenized--that is their relative motion in spacetime is identical to their neighbors'

So, at time zero, any change in any point trajectory will only depend upon local conditions, but all conditions on the universe are identical. So, from time=0 on, if the universe expands at one point, it will expand in the same fashion at other points, with the same proportionality constant.

Any subsequent measurement will indicate the expansion started at time 0, even though the process actually started at time=-R

So here we have a mathematical example which starts with a finite universe, becomes an infinite universe in a finite time, and results in a homogenous universe, without needing anything more than local interaction.

And, it fits the "word salad" description. :)

I don't mind taking a stab at this example.
Your example states it is a one-d universe consisting of a single point and many points radiating out from it. So the 'radiating' must be along a straight line. The θ is nothing more than one direction or the other along that line. The distance along that line must be what you mean by the Y coordinate.
Then you have a time t=0 where all points have homogenized. What was the cause? Why are they equally spaced? What would cause ALL points to have exactly the same spacing at the same moment if they are not somehow controlled/managed/causally connected? And additionally, why do they all start moving at exactly the same time and at exactly the same velocity relative to their nearest neighbors? What initiates the motion?

Grey
2015-Oct-13, 04:45 PM
I've been away for a bit, but I'll try to catch up. :)


I do not know of any mainstream cosmology that starts with a finite universe and produces an infinite universe.There actually are inflationary models like this. I don't understand the math well (general relativity is complex!), but it is possible to construct models where the universe starts out finite and reaches infinite scale in a finite time (without reaching infinite speed; I know that sounds contradictory, but again, general relativity is complex). Grapes has actually shown a toy universe model that works like that, but there are real inflationary models that do essentially the same thing. Now, they aren't necessarily the most popular models, but I believe that some of them are perfectly consistent with our observations to date. I think we've actually discussed this here on the board in a prior conversation about this same subject.

And actually, inflation in general is a workaround to explain more or less the same issue that Jeff has raised: that the whole observable universe appears to be causally connected, even though it shouldn't be without adding something like inflation in. So we definitely have to add something in to explain the apparent causal connectedness of the observable universe, and there are possibilities that involve an infinite universe that work at least as well as those that involve a finite universe.

Jeff, your claims that it's simply not possible to have something of infinite size, or to have causal connections travel at infinite speed, seem to entirely be arguments based on incredulity. Why can't information travel at infinite speed? As noted above, infinite speed is not strictly necessary to reach a universe of infinite size, but even if it were, I can't think of a reason why this is obviously forbidden. That seems to be the same kind of insistence that locality holds that got Einstein into trouble with the EPR paradox. We know that particles can and do show correlations that violate locality, so it seems pretty foolish to insist that locality has to hold. You've tried to brush that under the rug by claiming that quantum theory only applies to small things, and the universe is really big, but something else that we know is that the earliest universe can't be accurately described without taking quantum theory fully into account.

mkline55
2015-Oct-13, 05:17 PM
There actually are inflationary models like this. I don't understand the math well (general relativity is complex!), but it is possible to construct models where the universe starts out finite and reaches infinite scale in a finite time (without reaching infinite speed; I know that sounds contradictory, but again, general relativity is complex). Grapes has actually shown a toy universe model that works like that, but there are real inflationary models that do essentially the same thing. Now, they aren't necessarily the most popular models, but I believe that some of them are perfectly consistent with our observations to date.

Since you don't understand the math, perhaps someone could put some actual finite numbers to that model? At what starting size? What density? At what moment in time does it become infinite?

grapes
2015-Oct-13, 05:31 PM
I don't mind taking a stab at this example.
Your example states it is a one-d universe consisting of a single point and many points radiating out from it. So the 'radiating' must be along a straight line. The θ is nothing more than one direction or the other along that line. The distance along that line must be what you mean by the Y coordinate.
Then you have a time t=0 where all points have homogenized. What was the cause? Why are they equally spaced? What would cause ALL points to have exactly the same spacing at the same moment if they are not somehow controlled/managed/causally connected? And additionally, why do they all start moving at exactly the same time and at exactly the same velocity relative to their nearest neighbors? What initiates the motion?
The cause/timing is controlled by their initial relationship, plus they are influenced by their neighbors, so there is an initial connection. They follow the formula from time -R to 0


I've been away for a bit, but I'll try to catch up. :)

There actually are inflationary models like this. I don't understand the math well (general relativity is complex!), but it is possible to construct models where the universe starts out finite and reaches infinite scale in a finite time (without reaching infinite speed; I know that sounds contradictory, but again, general relativity is complex). Grapes has actually shown a toy universe model that works like that, but there are real inflationary models that do essentially the same thing.

Yes, I did not want to develop a full and viable spacetime model, I just wanted to set it up so that it satisfied Jeff's conditions, and refuted his conclusion, with the simplest model I could think of.


Now, they aren't necessarily the most popular models, but I believe that some of them are perfectly consistent with our observations to date. I think we've actually discussed this here on the board in a prior conversation about this same subject.

And actually, inflation in general is a workaround to explain more or less the same issue that Jeff has raised: that the whole observable universe appears to be causally connected, even though it shouldn't be without adding something like inflation in. So we definitely have to add something in to explain the apparent causal connectedness of the observable universe, and there are possibilities that involve an infinite universe that work at least as well as those that involve a finite universe.

Jeff, your claims that it's simply not possible to have something of infinite size, or to have causal connections travel at infinite speed, seem to entirely be arguments based on incredulity. Why can't information travel at infinite speed? As noted above, infinite speed is not strictly necessary to reach a universe of infinite size, but even if it were, I can't think of a reason why this is obviously forbidden. That seems to be the same kind of insistence that locality holds that got Einstein into trouble with the EPR paradox. We know that particles can and do show correlations that violate locality, so it seems pretty foolish to insist that locality has to hold. You've tried to brush that under the rug by claiming that quantum theory only applies to small things, and the universe is really big, but something else that we know is that the earliest universe can't be accurately described without taking quantum theory fully into account.

grapes
2015-Oct-13, 05:34 PM
Since you don't understand the math, perhaps someone could put some actual finite numbers to that model? At what starting size? What density? At what moment in time does it become infinite?
Since it is my "model" I can answer those questions. Not sure it's going to help, since I am *not* trying to model actual space. My model is one-dimensional after all.

The starting size is zero (a point), the density is infinite, it "becomes" infinite at time t=0

Jeff Root
2015-Oct-13, 07:04 PM
mkline55 asked pretty much the same questions as I'm
asking, but more compactly. Here's what I have:



Imagine a very small universe, finite but extremely
small. Points on its surface start radiating outward
on paths that cause their velocities to interact, but at
the same time they are different based upon some
initial parameter. At a finite time, the points will fill
an infinite space, and will share a common relationship
to each other that will guarantee the rest of the
development is homogeneous.
...
So, I now have a plausible model of a finite universe
that expands in a finite time into a homogeneous
infinite universe, without violating causation.


Start the one-d universe at time=-R, where it consists
of a single point, and many points radiate out from it
in spacetime. Their interactions depend upon their
relationship to each other at a specific time (t-coordinate).
Their y-coordinate is tan(θ)*sqr(R2-t2),
where θ represents an initial angle at the time the point
leaves the initial point, and -90°<θ<90°, so no point has
an infinite velocity, and its velocity is incrementally
greater/lesser than that of its neighbors.

At time t=0, all points in the universe have homogenized--
that is their relative motion in spacetime is identical to
their neighbors'

So, at time zero, any change in any point trajectory will
only depend upon local conditions, but all conditions on
the universe are identical. So, from time=0 on, if the
universe expands at one point, it will expand in the same
fashion at other points, with the same proportionality
constant.

Any subsequent measurement will indicate the expansion
started at time 0, even though the process actually started
at time=-R

So here we have a mathematical example which starts
with a finite universe, becomes an infinite universe in a
finite time, and results in a homogenous universe,
without needing anything more than local interaction.




Since you call it "a mathematical model" in post #64 and
start with a one-dimensional universe in post #73, you
may just be describing something akin to the Hilbert Hotel,
Definitely not.
Okay, so that's one thing it isn't.

My basic impression is that you've given a description of
a geometric construction in abstract mathematics, rather
than a description of a physical process using physics.
If the geometric construction has no particular physical
meaning, it would appear to be irrelevant.



Imagine a very small universe, ...
Is this thing a "universe" in the same sense that the
Universe is? I see nothing about it that would lead me
to want to call it a "universe".



Points on its surface start radiating outward ...
It has a "surface"?

I don't understand how "points" can start radiating,
unless they are giving off some kind of radiation.
But that doesn't seem to be what you mean.



... on paths that cause their velocities to interact, ...
I don't see how velocities of points can interact, or
how the paths of the points would cause or otherwise
affect the interactions.



... but at the same time they are different ...
The points are different from one another? The
paths of the points are different from one another?
The velocities of the points are different from one
another? The interactions of the velocities are
different from one another?

Your wording "but at the same time" makes it sound
like it is somewhat surprising that "they are different".
Is it in fact surprising? Or why did you say that?



... based upon some initial parameter. At a finite time,
the points will fill an infinite space, ...
They will? Why will they do that? Why will they fill
the space, and why will the space they fill be infinite?

Looks like you address this with a function in post #73.



Start the one-d universe at time=-R, ...
So the "universe" has one dimension of space and
one of time?

After this rattled around in my brain for a while, I
concluded that you selected "R" as the variable name
(or is it a constant?) to represent "Root time": The
interval between germination of this universe and its
bursting forth above ground. Is that correct? Hmmm.
Below it looks like it may be for "radial" or "radiant".



... where it consists of a single point, and many points
radiate out from it in spacetime.
I guess that implies the "radiating" points are moving.
Yes, I see below that they are.

If this "universe" does have just one spatial dimension,
then all of the "radiation" is in one direction, along the
same geometric ray?



Their interactions depend upon their relationship to
each other at a specific time (t-coordinate).
Again, I don't see how they can interact, especially
since the "points" are radiating away from the origin
point (at -R), they should all get progressively farther
apart from one another, whether they are moving in
a 2-D plane or just a 1-D ray.

Is the "relationship" just the distance between the
points on the 1-D ray? So their "interactions" are at
a distance?



Their y-coordinate is tan(θ)*sqr(R2-t2),
where θ represents an initial angle at the time the point
leaves the initial point, and -90°<θ<90°, so no point has
an infinite velocity, and its velocity is incrementally
greater/lesser than that of its neighbors.
So y is the distance of a "point" from the origin along
the spatial axis-- the line-- and t is the time since it left
the origin, represented by distance on the time axis?

Is the origin at 0 on the y axis and -R on the time axis?

And θ is-- in effect-- the speed of the moving point?

If the trigonometric circle has its customary orientation,
time increases to the right and distance from the origin
on the y axis increases upwards?

I see that R acts like the hypoteneuse of a triangle on a
graph of distance versus time, so the choice of "R" may
be for "radial", but I don't see its purpose here. From
the way you introduced it I thought it would be an offset
from the origin-- or an offset *of* the origin.

Well, I seem to be stuck. I tried to use Wolfram Alpha to
plot the function. It understood the entire formula perfectly
and also the range for θ. It could plot just y=tan(θ), but
when I tried to multiply that by sqr(R^2 - t^2), it didn't
know what to do. I didn't know how to define the variables,
and I expect that an animation is required, not a static plot.

So basically I don't follow the math, but I strongly suspect
that it is just math, not a description of something that has
a relevant physical interpretation.

Can you describe or graph that function so I can see it?
And explain why you think it demonstrates what you say
it demonstrates?

-- Jeff, in Minneapolis

Reality Check
2015-Oct-13, 07:54 PM
What definition?
A point to consider, Jeff Root: A personal opinion about a subject should be based on learning a lot about that subject. So you should have already read the science that follows.
The metric used in the Big Bang theory applies to the entire universe, not part of it. It does not have any boundaries so the universe is not split up into parts This metric is called the
Friedmann–Lemaître–Robertson–Walker metric (https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson% E2%80%93Walker_metric)

The Friedmann–Lemaître–Robertson–Walker (FLRW) metric is an exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic expanding or contracting universe that may be simply connected or multiply connected.[1][2][3]
The Big Bang happened for an entire universe in mainstream cosmology because the theory is based on the FLRW metric.

Another invalid part of a big bang happening in a region of the universe is that the Big Bang is not considered a volume expanding into anything: What is the Universe expanding into? (http://www.astro.ucla.edu/~wright/cosmology_faq.html#XIN) (refers to the balloon analogy but also applies here)

Everything that we measure is within the Universe, and we see no edge or boundary or center of expansion. Thus the Universe is not expanding into anything that we can see, and this is not a profitable thing to think about.

mkline55
2015-Oct-13, 08:20 PM
Since it is my "model" I can answer those questions. Not sure it's going to help, since I am *not* trying to model actual space. My model is one-dimensional after all.

The starting size is zero (a point), the density is infinite, it "becomes" infinite at time t=0

If the time it took to expand from zero size to infinite size is finite, then shouldn't it be possible to put a value on -R? If you use the current rate of increasing separation to determine that, what physical properties are you assuming remain valid all the way back to -R? Are any new physical requirements introduced, such as having an infinite number of particles with the same location in spacetime?

Jens
2015-Oct-13, 11:30 PM
Let me try one last time. Suppose you have a red sun set. The next day you look at pictures of the last evening and see that everything is red tinted. You therefore conclude that someone with a paintbrush must have stained everything red. If someone says the light was red, you ask "But how did that get red stain on everything? I can't envision that much red stain getting on everything, there's no brush that big."

But it's not infinite; it's a local phenomenon, right? I think what Jeff (and I) are wondering is how this can happen on a hypothetically infinite universe. Do you see the difference?

WayneFrancis
2015-Oct-14, 12:48 AM
Really? "By definition?" What definition is that?


The definition that asking for a cause of our universe within the physical laws of our universe either A) doesn't make sense or B) some things don't require causes thus you shouldn't complain about them not being causal.
You seem to want to bait and switch that the creation of our universe needed a cause. That every event within our universe has some cause within our universe. So their for the creation of the universe should also have a cause. But I point out that "cause" would be exterior to our universe so there is no reason to think it would be limited to only a finite region of our universe since by definition the cause outside our universe worked on the entire volume of our universe



As I said in post #141:

One popular assertion about the Big Bang is that it may
have started with a single quantum fluctuation (vacuum
fluctuation) in which virtual particles somehow became
physical particles, and the process cascaded and grew.



But that is a quantum fluctuation in a existing false vacuum. It doesn't explain were that false vacuum came from and it isn't the only idea. I don't disagree with you on what you say could have happened and if it did I agree that the material we see in our universe was originated from a common cause within our universe. But that isn't the only idea that matches the observations.



Would that quantum fluctuation have been outside the
Universe? It seems to me that it would have been the
very first thing *inside* the Universe. But you claim
that any cause of the Big Bang would be outside of the
Universe by definition. I'd like to see that definition.


No Jeff I don't say any cause of the Big Bang would be outside of the universe. I say the creation of the universe would be outside of the universe. I say IF the cause of The Big Bang is external to our universe then it COULD work on a volume that is infinite in expanse. I am not saying it has to be like that. I'm saying it could be like that or, more than likely, could be something very different. Sorry if I didn't make that clear the multitude of other times we've had this exact same discussion. Please don't continue with that straw man fallacy.




The actual question of this thread is whether the Big
Bang could have been large or infinite in extent at its
very beginning, or become infinite in finite time, not
what caused the creation of the Universe.


Things we don't know.

How big is the universe at T=0.
The complete state of the universe at T=0.
What caused the Big Bang.


Was the universe homogeneous at T=0? We don't know. We know it seems to be very homogeneous on a scale much larger then the material now contained within our Hubble volume.
Was the cause of the big bang from an event within our universe or from some higher order bulk? We don't know. I'm not even sure that I'll concede that if it originated from within our universe that it couldn't have been infinite in extent but it doesn't matter I'll concede that because we are still faced with the issue that if it was triggered from some higher order dimension then there is no reason for us to limit it to only working on a finite region within our universe.



What I have said several times, in this thread and in
previous threads, is that I assume that known laws of
physics which apply now also applied at the beginning
of the Big Bang, but that unknown laws of physics are
required to accurately describe that beginning.


The known laws of physic...in our universe which, again by definition, doesn't cover the creation of our universe. Just like an embryo's development doesn't explain where the DNA originally came from. Our known laws of physics don't even work at T=0 so how do you push that back to a creation event? How do you even push that back to T=0 for that matter?



Do you have any reason to think that laws of physics
which apply now did not apply at the beginning?

-- Jeff, in Minneapolis

I have every reason to believe our understanding of physics are not sufficient to explain how our universe started to inflate let alone came into being. Thus I say I don't know where you say stuff like "it is beyond impossible"

Noclevername
2015-Oct-14, 12:51 AM
But it's not infinite; it's a local phenomenon, right? I think what Jeff (and I) are wondering is how this can happen on a hypothetically infinite universe. Do you see the difference?

I was using that as a metaphor, not a direct comparison. The point I was trying to make was that he missed the point of the red sunlight, and instead continued to blame red pigments for everything, even though the sunlight being red was explained to him repeatedly.

I was frustrated and tired when I wrote that. Sorry if it wasn't very clear.

Jens
2015-Oct-14, 01:11 AM
So given the example, and I am also not saying this is the way it happened, where the universe does start at a single point and expands to infinity within a finite amount of time could explain why all of space, even if it is infinite, shares some basic attributes.



I'm not sure this is what Jeff is asking about exactly, but I have a problem with the idea that the universe could start as a single point and expand to infinity within a finite amount of time. I am aware that mathematically, it's possible for a function to approach infinity in a finite time, but I think that in that case, for example if the sum goes to infinity as T approaches 1, then it is meaningless to talk of the function at say T=1.5. So if the universe started at a point and then approached infinity at a certain point, how would it make sense for us to be living after that point?

Now what I find more problematic or perhaps interesting is that the universe could have come into existence, at T=0, instantaneously within an already infinite volume, and then that somehow that volume is expanding like a Hilbert's hotel. I do see an issue with causality, and I think I'm using causality in the normal sense, but as I mentioned earlier, there can be processes that take place not because of causality but because of physical law, a good example being entropy, which takes place everywhere simultaneously not because of causality but because it's a physical principle. So if the universe was created simultaneously within an infinite volume by a process that is in fact a physical principle, then I suppose I could understand that.

Noclevername
2015-Oct-14, 01:24 AM
In fact, that metaphor sucks. Please disregard.

grapes
2015-Oct-14, 08:30 AM
If the time it took to expand from zero size to infinite size is finite, then shouldn't it be possible to put a value on -R? If you use the current rate of increasing separation to determine that, what physical properties are you assuming remain valid all the way back to -R? Are any new physical requirements introduced, such as having an infinite number of particles with the same location in spacetime?
The example was devised so that the current rate of expansion extrapolates back to t=0. We really have no constraints on t<0, or even t=0

Jeff was insisting that we did, my example was created to refute that. As Grey points out, it is only a toy model, it's a one-dimensional model that addressed Jeff's objections.

grapes
2015-Oct-14, 08:36 AM
I'm not sure this is what Jeff is asking about exactly, but I have a problem with the idea that the universe could start as a single point and expand to infinity within a finite amount of time. I am aware that mathematically, it's possible for a function to approach infinity in a finite time, but I think that in that case, for example if the sum goes to infinity as T approaches 1, then it is meaningless to talk of the function at say T=1.5. So if the universe started at a point and then approached infinity at a certain point, how would it make sense for us to be living after that point?

That was the purpose of my example.

It starts at a finite time -R before the big bang at t=0, starts from a point, expands to an infinite homogenous universe at t=0, and we're here at t=15 billion years, give or take 5 billion years

grapes
2015-Oct-14, 09:15 AM
It looks like you've answered most of the questions, in your internal dialogue, up to here. If there are any I missed, let me know.




Their y-coordinate is tan(θ)*sqr(R2-t2),
where θ represents an initial angle at the time the point
leaves the initial point, and -90°<θ<90°, so no point has
an infinite velocity, and its velocity is incrementally
greater/lesser than that of its neighbors.

So y is the distance of a "point" from the origin along
the spatial axis-- the line-- and t is the time since it left
the origin, represented by distance on the time axis?

Is the origin at 0 on the y axis and -R on the time axis?

The origin (t,y) is at (0,0)

That's our time t=0


And θ is-- in effect-- the speed of the moving point?

It's a parameter associated with each point that I use to determine its position at each time t between -R and 0


If the trigonometric circle has its customary orientation,
time increases to the right and distance from the origin
on the y axis increases upwards?

Distance increases downward as well, y values can be negative


I see that R acts like the hypoteneuse of a triangle on a
graph of distance versus time, so the choice of "R" may
be for "radial", but I don't see its purpose here. From
the way you introduced it I thought it would be an offset
from the origin-- or an offset *of* the origin.

It is an offset from the origin (0,0)


Well, I seem to be stuck. I tried to use Wolfram Alpha to
plot the function. It understood the entire formula perfectly
and also the range for θ. It could plot just y=tan(θ), but
when I tried to multiply that by sqr(R^2 - t^2), it didn't
know what to do. I didn't know how to define the variables,
and I expect that an animation is required, not a static plot.

It is describing the paths of all the points, one for each θ.

Instead, let R be 1, whatever units, and θ be -pi/3

tan(-pi/3)*sqr(1^2-t^2) for -1<t<0

Wolfram Alpha will plot that, and shows it as a quarter of an ellipse. That is the path of the point assiciated with θ=-π/3. That will be similar to the path for every other value of θ


So basically I don't follow the math, but I strongly suspect
that it is just math, not a description of something that has
a relevant physical interpretation.

It's one-dimensional!


Can you describe or graph that function so I can see it?

I hope the above helps.


And explain why you think it demonstrates what you say
it demonstrates?

The example starts off at time t=-R with a singularity, and its points follow finite (but unbounded) trajectories. At time t=0 their trajectories have homogenized throughout the entire universe.

It's a "toy" illustration of how a finite (in extent) universe can expand to an infinite (in extent) homogenous universe, in a finite time. In the question of the OP, you seem to be unable to imagine that possibility. I'm just trying to give you a handle on it.

ETA: I'm not saying that it worked that way. It's only a model that satisfies your conditions, but results in something that you were claiming would not be possible. I only came up with the math functions after you objected to my original general description of what could've happened. I think my original description was enough to refute your claim, but you said you didn't understand it, and wanted more of an illustration.

Jens
2015-Oct-14, 09:54 AM
That was the purpose of my example.

It starts at a finite time -R before the big bang at t=0, starts from a point, expands to an infinite homogenous universe at t=0, and we're here at t=15 billion years, give or take 5 billion years

II went back and read your example. What I'm not sure is, will that really lead to an infinitely large universe at T=0, so that there would be points from the initial point that will be infinitely distant from each other?

grapes
2015-Oct-14, 10:24 AM
II went back and read your example. What I'm not sure is, will that really lead to an infinitely large universe at T=0, so that there would be points from the initial point that will be infinitely distant from each other?
No two points are ever infinitely distant from each other. That's the nature of the y-axis

But the distance from one point to other points is unbounded. Each point of the y-axis is associated with some value of θ (-90°<θ<90°)

mkline55
2015-Oct-14, 11:36 AM
No two points are ever infinitely distant from each other.

How then can you claim it is infinite in size and at the same time claim that -R is a finite number?

Jeff Root
2015-Oct-14, 11:37 AM
grapes,

I agree with almost everything you said except for the one
most important thing-- the conclusion that the mathematical
operation in any way demonstrates that an infinite universe
could result from a finite Big Bang. I could just as well say
that the radiating points in your example represent points on
a meatball, demonstrating that meatballs could become
infinitly large in finite time. Or I could draw a triangle and
say that the triangle represents a universe, which shows
that the actual Universe could be triangular.

Ignoring this huge objection for a moment ... How would
you interpret the meaning of your function if you leave out
the (R^2 - t^2) multiplier, and just keep the very simple
y=tan(θ) ? That omits any reference to time, so I'm not
sure it has any significance in this context. But if it does,
then it would appear to give an infinite result in finite time,
without the R adjustment.

If it were not, as I say, just a math function rather than
physics, that alone would contradict my assertion. So I'm
not sure the R factor-- which so far I'm not grokking fully--
helps to make your point. But if it converts your function
from pure math into physics (which would surprise me),
then it may be crucial.

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-14, 11:46 AM
No two points are ever infinitely distant from each
other. That's the nature of the y-axis

But the distance from one point to other points is
unbounded. Each point of the y-axis is associated
with some value of θ (-90°<θ<90°)
I think you are saying that for any given distance on
the y-axis, there will always be points beyond it.
That's close enough to infinite for most practical
dolphins.

-- Jeff, in Minneapolis

mkline55
2015-Oct-14, 11:48 AM
I think the issue is that the expansion model points to an EVENT. It does not describe the event, but it puts constraints on the event which would violate current physical principles. Other than the model itself, there is absolutely no evidence that physical principles can be violated in the manner necessary to produce the EVENT. Is it really necessary to throw out current principles and invent entirely new imaginary and completely untestable physics principles to explain something that is really just the result of a model?

Jeff Root
2015-Oct-14, 12:19 PM
Simplifying the idea to the extreme:

Why not just posit an infinite ray, call it a "universe", and
say that it contradicts my assertion that an infinite universe
could not come about in finite time?

-- Jeff, in Minneapolis

mkline55
2015-Oct-14, 12:36 PM
Simplifying the idea to the extreme:

Why not just posit an infinite ray, call it a "universe", and
say that it contradicts my assertion that an infinite universe
could not come about in finite time?

-- Jeff, in Minneapolis

I think the only 'finite' part of that model is the time it takes to write the math.

Jeff Root
2015-Oct-14, 12:45 PM
So basically I don't follow the math, but I strongly suspect
that it is just math, not a description of something that has
a relevant physical interpretation.
It's one-dimensional!
Yes, but if that is a problem, I think it is a problem for
your assertion that it contradicts my assertion.

Can a one-dimensional model tell us various facts about
relationships between things in three-dimensional space,
how the relationships evolve over time, and how the
global properties of the space evolve? I think so.

Can a mathematical entity-- basically a geometric figure--
tell us what must be possible (or what is not impossible)
in an actual physical universe if it makes no reference to
matter, energy, or mass? I don't think so.

-- Jeff, in Minneapolis

Jens
2015-Oct-14, 01:03 PM
No two points are ever infinitely distant from each other. That's the nature of the y-axis

But the distance from one point to other points is unbounded. Each point of the y-axis is associated with some value of θ (-90°<θ<90°)

There might be a disconnect here, so just to check. When I say "infinite in volume", I am talking of a situation where things go on and on, so that if you travel in any direction you will be able to go on forever and space is always there. So I am talking about a line that is infinite in length. I'm not talking about the fact that, for example, there is an infinite number of real numbers between 0 and 1. Is that the same infinity that you are talking about?

Jeff Root
2015-Oct-14, 01:06 PM
I'm not sure I recognized until just now that I'm asserting
two things to be impossibilities: A volume of space which is
highly homogeneous despite lack of causal contact between
its parts, and anything finite becoming infinite in finite time.

Various suggested scenarios violate one or the other or both
impossibilities. A universe which is instantaneously created
as infinite violates both.

This sounds weasely, but I could be wrong about one while
being right about the other.

The scenario I constructed with my signature as an analogue
of homogeneous spacetime really only illustrated the first
impossibility, not the second. That could make it appear to
be irrelevant if one is trying to interpret it as demonstrating
the second impossibility.

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-14, 01:11 PM
Jens,

As I indicated in post #190, I think the infinite ray (or line)
is infinite because there are always more points beyond a
given point, which is quite a different thing from there being
an infinite number of points between two points.

-- Jeff, in Minneapolis

mkline55
2015-Oct-14, 01:16 PM
I think the model Grapes has offered is fairly in agreement with the expanding universe model. It has the same mathematical issues. Using the argument that there is always another more distant point, then for any value of t greater than zero, the model is infinite in size. Hence, except when the model has a size of zero, wherein all points are at the same location, the model never has a finite size. This is mathematically consistent. It is also physically impossible based on the current understanding of physical laws.

Grey
2015-Oct-14, 04:22 PM
I think the point that grapes is trying to make is that Jeff is saying that an infinite universe is just a logical impossibility: it can't possibly exist because it's a contradiction in terms. So he came up with an example that shows that there are definitely models where a finite universe becomes infinite in finite time, so that gets around Jeff's "logical" argument by showing that at least one of his premises is false.*

But if you don't like grapes's model, here's a more complicated model, based on the Friedmann equations, which really are what we use to talk about cosmology. If you assume a universe that's essentially composed of matter and dark energy (like ours), you can end up with a version that looks like this:

{H}^{2} \equiv {(\frac{\dot{a}}{a})}^{2} = {{H}_{0}}^{2}[\frac{{\Omega}_{m}}{{a}^{3}} + (1 - {\Omega}_{m}){a}^{-3(1+w)}]

Where {\Omega}_{m} is the fraction of matter in the universe, about 0.3, so that dark energy accounts for most of the energy density of the universe. The parameter w describes how that dark energy behaves, and is often considered to be -1 for "standard" dark energy. But if w is less than -1, then you get a different evolution of the scale factor (a), which diverges in finite time, just like in grapes's toy universe. Mostly, I've seen this used when discussing a possible "Big Rip" scenario in the future, but it also works for describing an inflationary period in the past. To be sure, it's not the most common one, but it is possible.

Jens, I understand your concern about how we could describe the universe after the time when distances diverge. There would have to be a sudden change to a non-diverging equation of state. That may sound a little contrived, but that's exactly how inflationary models work, even if they don't involve this kind of divergence: there's a change of state in the universe that triggers a period of extremely rapid expansion, but this expansion eventually causes another change in state which halts that rapid expansion, leaving us a universe expanding at a more sedate pace.

Inflation was an idea to solve the horizon problem: the universe appears more homogeneous than we think it should be if it had always been expanding in the leisurely manner it is now, and a period of extremely rapid expansion can explain that (and there are some fairly sound theoretical reasons why such a period of rapid expansion is reasonable, and a number of additional observational clues aside from just homogeneity to suggest that it really did happen). Jeff's objection to an infinite universe is essentially a larger version of the same horizon problem: if the universe is infinite, then how could it possible have become homogeneous in a finite time? Inflation can work here, too, provided that you pick a version that diverges in finite time. I'm certainly not saying that it must be that way, only that at this point, it's a possible model that is entirely consistent with our observations.



* There are other premises that may be false, too. For example, the claim that there can't be correlation between distant events unless there's a local causal link; we know this one is false, because quantum entanglement exhibits precisely such nonlocal correlations. Similarly, WayneFrancis brings up the perfectly valid notion that some process outside our universe could have created or affected the entirety of an infinite universe "at once" (if it's a process taking place outside of our universe, talking about time in a sensible way isn't clear to have an obvious meaning). Again, none of these are arguments that a model of the universe that is infinite is definitely better than a finite model, only that Jeff's a priori claim that an infinite model is ridiculous and should be immediately discarded isn't valid.

mkline55
2015-Oct-14, 05:35 PM
Again, none of these are arguments that a model of the universe that is infinite is definitely better than a finite model, only that Jeff's a priori claim that an infinite model is ridiculous and should be immediately discarded isn't valid.[/SIZE]

So to accept the current mainstream model we must immediately embrace magic? Branes? Strings? Multiverses? 10- or 12-dimensions? Never-seen physics changing state to observed physics? It is certainly possible to add as many imaginary variables to the system as needed to come up with all kinds of imaginary explanations, but how is any explanation which requires the acceptance of unproven and inherently not provable pseudo-physics any better than simply accepting magic as the premise from the start?

Would it be fair to say that based on currently accepted proven physical principles, there is no way to explain the causal nature of the universe as we observe it today? Is there any existing explanation which does not violate current proven physics and does not introduce magic?

Grey
2015-Oct-14, 06:41 PM
So to accept the current mainstream model we must immediately embrace magic? Branes? Strings? Multiverses? 10- or 12-dimensions? Never-seen physics changing state to observed physics?Of course not. To accept the mainstream cosmological model, you'd have to accept that we don't know whether an infinite universe or a finite (but unbounded, we think) universe fits our observations better. Currently, both are consistent with what we can see, although there's at least some pretty solid indirect evidence that it's at least much bigger than the part we can directly observe (technically, there could presumably be a solid wall or nothing at all just beyond the edge of what we can directly observe). What we can tell is that the universe seems to have been in a very hot, very dense state roughly 13.8 billion years ago and has expanded and cooled since then, but we acknowledge that our current understanding of physics doesn't give us enough information to answer questions like "where did that hot, dense early universe come from in the first place?" We can speculate about possibilities, discuss what might or might not be consistent with our observations up until now, and try to think of new observations or tests that might give us more information about those initial conditions. We don't dismiss either a finite or an infinite model of the universe as "absurd" based on our preconceived notions, and we try our best to constrain further the models that work, within the limitations of the observations that are available.

Ken G
2015-Oct-14, 07:19 PM
What's more, accepting magic is more or less what fundamentally new scientific models are all about. The only difference between the magic of science, and the magic of Harry Potter, is that scientific magic has evidence to support it. The first thing any physicist will do, when creating some completely new thinking-outside-the-box model, is throw in some magic. The more the better, actually. But it has to work-- it has to be the simplest and most unified way to explain the observations. So if the magic is an expanding universe, then poof, it's in the model. If the magic is an 11 dimensional string, then poof, that's in the model. It's whatever magic works, that's the first rule of science.

This means that if the magic that works is a universe that is infinite and flat, then that goes in the model. If there is some evidence that the universe is not infinite, or not flat, then that goes in the model. If we need cause-and-effect to be in our model, then poof, that magic goes in there too. But we never put anything in a model until we need it. What's so remarkable about the Big Bang model is that it is the first example of a model that explicitly puts in conditions that suggest the model will break down at some point in the past. So if one puts in a gravity model, one expects that to break down at some point in the Big Bang model. If one puts in a concept of cause and effect, one expects that to break down too. That's the really amazing thing about the Big Bang-- it's a model that does not trace back to any fundamental laws of physics, other than the rule that "this law too will break down." Some regard that as a big flaw in the Big Bang approach, but I'm not so sure it isn't its greatest innovation, given what an uneasy time any physics theory has with the issue of "first causes."

Now, to be sure, that is not a satisfactory state of affairs to many scientists. They don't regard the Big Bang as the final answer-- they seek a way to talk about the origin of the universe in terms of laws that don't break down. But that's just what scientists try to do-- there's no guarantee they will succeed, because there is no guarantee the goal is even possible. Maybe our universe is set up such that all laws really do break down, perhaps its origin is not ruled by laws at all. But science will not stop looking for such laws, so we try to see if we can still use cause and effect (as Jeff is trying to do), and we try to see if we can talk about something "before the Bang" (like brane collisions, or cosmic bounces, or whatever). But it isn't science until we show an observation that we can explain better this way. Hence, it isn't science to say the universe has to be finite because an infinite universe "couldn't be caused", and it isn't science to say there has to be a multiverse because the physical parameters "couldn't be fine tuned", those are all philosophical arguments rather than scientific ones. A scientific argument always looks like this: observation A is better explained by theory B than by any other theory. Unless the argument can be framed like that, it's just not science.

mkline55
2015-Oct-14, 07:35 PM
It may be just my opinion, but I believe the replacement to BBT will not have a first cause problem and will not demand the breaking down all of known physics.

Reality Check
2015-Oct-14, 09:01 PM
I'm not sure I recognized until just now that I'm asserting
two things to be impossibilities: A volume of space which is
highly homogeneous despite lack of causal contact between
its parts, and anything finite becoming infinite in finite time.

Various suggested scenarios violate one or the other or both
impossibilities. A universe which is instantaneously created
as infinite violates both.

That assertion looks wrong, Jeff Root.
* I do not know what your personal definition of "causal contact" is but that word "contact" is suggestive of assuming your conclusion, i.e. you assume that an infinite universe cannot have every part in contact and thus violates "causal contact".
The usage of causality in science (https://en.wikipedia.org/wiki/Causality_(physics)) is usually just that an effect has a cause. So there is no problem with an infinite universe having a cause.

* No mainstream cosmology states that the universe is infinite and began infinite from a finite extent in finite time.
Mathematically this seems possible. Take a infinite series that has a finite limit and expand that volume accordingly. How about doubling the size in one second, doubling again in 0.5 seconds, doubling again in 0.25 seconds, etc. The infinite series of 1 seconds + 0.5 seconds + 0.25 seconds + ... has a finite sum of 2. Start at t=0. Wait for 2 seconds. How big is that volume?

Jens
2015-Oct-15, 12:24 AM
* No mainstream cosmology states that the universe is infinite and began infinite from a finite extent in finite time.
Mathematically this seems possible. Take a infinite series that has a finite limit and expand that volume accordingly. How about doubling the size in one second, doubling again in 0.5 seconds, doubling again in 0.25 seconds, etc. The infinite series of 1 seconds + 0.5 seconds + 0.25 seconds + ... has a finite sum of 2. Start at t=0. Wait for 2 seconds. How big is that volume?

That's actually the issue that I have. I understand that mathematically that's possible, that if the universe expanded in that way, it could indeed become infinite in extent within a finite time. The problem though is, what happens at t=2.5 seconds?

Reality Check
2015-Oct-15, 01:15 AM
The universe (as in our universe) did not expand that way. But a hypothetical "volume of space" could. For t > 2 that volume will not be expanding and will remain infinite.

Jens
2015-Oct-15, 01:42 AM
The universe (as in our universe) did not expand that way. But a hypothetical "volume of space" could. For t > 2 that volume will not be expanding and will remain infinite.

I have two questions about the statement. First is about the hypothetical volume of space. If the function of the expansion is really that it doubles in size at a rate that goes from 1 to 0.5 to 0.25, etc., is it really fair to say that it will stop expanding at 2? Isn't it rather that the function becomes undefined above 2?

But I guess the more important question is about the first part, where you say the universe did not expand that way. If it didn't, then how could it become infinite in extent within a finite about of time, i.e. 13.7 billion years?

Reality Check
2015-Oct-15, 02:30 AM
If the function of the expansion is really that it doubles in size at a rate that goes from 1 to 0.5 to 0.25, etc., is it really fair to say that it will stop expanding at 2?
It is fair. At t = 2 seconds the volume is not being doubled and thus it is not expanding.


But I guess the more important question is about the first part, where you say the universe did not expand that way. If it didn't, then how could it become infinite in extent within a finite about of time, i.e. 13.7 billion years?
Because the universe (our universe) in mainstream cosmology either

started as either infinite and is still infinite or
started as finite and is still infinite finite.

And importantly: we do not know whether the universe is finite or infinite!

A finite universe becoming infinite in extent is a hypothetical toy model (not our universe) that popped up in the thread. But that toy model can become infinite in extent within a finite about of time such as 13.7 billion years. Just multiply the above series appropriately.

WayneFrancis
2015-Oct-15, 03:30 AM
I'm not sure this is what Jeff is asking about exactly, but I have a problem with the idea that the universe could start as a single point and expand to infinity within a finite amount of time. I am aware that mathematically, it's possible for a function to approach infinity in a finite time, but I think that in that case, for example if the sum goes to infinity as T approaches 1, then it is meaningless to talk of the function at say T=1.5. So if the universe started at a point and then approached infinity at a certain point, how would it make sense for us to be living after that point?

Now what I find more problematic or perhaps interesting is that the universe could have come into existence, at T=0, instantaneously within an already infinite volume, and then that somehow that volume is expanding like a Hilbert's hotel. I do see an issue with causality, and I think I'm using causality in the normal sense, but as I mentioned earlier, there can be processes that take place not because of causality but because of physical law, a good example being entropy, which takes place everywhere simultaneously not because of causality but because it's a physical principle. So if the universe was created simultaneously within an infinite volume by a process that is in fact a physical principle, then I suppose I could understand that.

I've got my reservations about that to. If you look through past posts of mine I used to say "if the universe started finite then it would still be finite, if it started as infinite then it would stay infinite". I'd point out going from zero volume to a infinite volume instantly was just as much of a problem for me as going from a zero volume to an infinite volume.

But others with magnitudes more understanding in the maths and GR said you could go from a finite volume to a infinite volume in a finite amount of time. So not understanding it I error on the side of a argument from authority rather than an argument from my ignorance.

If the universe came into being infinite in extent then we've already have a cause, outside of our universe, that produced that infinite volume at the same time. Essentially they all have the same cause even though every single point at the start of the universe are out of causal contact.

End of the day I, and many others here, say "we don't know". Jeff says "We don't know but it can't be x because x doesn't match what we know" He's using the no black swan and argument from ignorance fallacies.

We might not understand how but if the universe was created already in thermal equilibrium it doesn't matter what the volume was at T=0. If the reason inflation started is something that can work on a volume of space >0m3 then it has worked over a distance that would not be in causal contact in that instance. Thus the distinction of how large of a volume it can affect instantly becomes arbitrary.

I realize there are a lot of "ifs" but since we don't really know what created our universe that "if" is a trumps my conditional "ifs"

WayneFrancis
2015-Oct-15, 03:55 AM
So to accept the current mainstream model we must immediately embrace magic? Branes? Strings? Multiverses? 10- or 12-dimensions? Never-seen physics changing state to observed physics? It is certainly possible to add as many imaginary variables to the system as needed to come up with all kinds of imaginary explanations, but how is any explanation which requires the acceptance of unproven and inherently not provable pseudo-physics any better than simply accepting magic as the premise from the start?

Would it be fair to say that based on currently accepted proven physical principles, there is no way to explain the causal nature of the universe as we observe it today? Is there any existing explanation which does not violate current proven physics and does not introduce magic?

No. We don't need to embrace magic. We say "We don't know" we can tack on "observations are consistent with this model. This model states this." Our universe being created in itself would fall under your "embrace magic" because we don't know. But we know our universe is here. Coming up with cosmological models that are observation is what we do. Models, by definition, aren't exactly what happens with reality. They are just accurate for making predictions within their domain of applicability.

As others pointed out I'm not sure we can say make grand sweeping claims about causality. We have examples where causality seems to break and we can't explain it. Jeff seems to say, if I understand him right, that it they must be a proper causal link we just don't understand. But that is just and argument from ignorance. It might feel better for people at a gut level to dismiss stuff they don't understand but it doesn't make it true.

WayneFrancis
2015-Oct-15, 04:01 AM
It may be just my opinion, but I believe the replacement to BBT will not have a first cause problem and will not demand the breaking down all of known physics.

Yup, that is your gut feel. Mine too. But that doesn't mean that is the way it is. Even if we come up with a model that explains it like that it doesn't mean that is the way it happened. But that doesn't really matter. What does is that the model is useful in making predictions.

Ken G
2015-Oct-15, 04:46 AM
Exactly-- we can have gut feelings, but we don't adjudicate science that way. They do help us decide where to put our energies, or where to lay our bets, but that's all outside of actual scientific knowledge. I just want to point out that it is a very common attitude to imagine that a theory that requires all its own laws to break down at some point must be terribly flawed, and that's really not necessarily true. It might be a sublime attribute of a science theory, given the fundamental limitations in the kinds of things we can actually test.

grapes
2015-Oct-15, 06:35 AM
No two points are ever infinitely distant from each other. That's the nature of the y-axis

But the distance from one point to other points is unbounded. Each point of the y-axis is associated with some value of θ (-90°<θ<90°)

There might be a disconnect here, so just to check. When I say "infinite in volume", I am talking of a situation where things go on and on, so that if you travel in any direction you will be able to go on forever and space is always there. So I am talking about a line that is infinite in length. I'm not talking about the fact that, for example, there is an infinite number of real numbers between 0 and 1. Is that the same infinity that you are talking about?
I'm talking about distance.

grapes
2015-Oct-15, 06:37 AM
No two points are ever infinitely distant from each other.

How then can you claim it is infinite in size and at the same time claim that -R is a finite number?
The units of R are time, the size is space

grapes
2015-Oct-15, 06:51 AM
I think the point that grapes is trying to make is that Jeff is saying that an infinite universe is just a logical impossibility: it can't possibly exist because it's a contradiction in terms. So he came up with an example that shows that there are definitely models where a finite universe becomes infinite in finite time, so that gets around Jeff's "logical" argument by showing that at least one of his premises is false.*

Yes

My first response was just a general description of what could happen. The math I provided later was only so it could be checked, no need to accept "argument from authority"


But if you don't like grapes's model, here's a more complicated model, based on the Friedmann equations, which really are what we use to talk about cosmology. If you assume a universe that's essentially composed of matter and dark energy (like ours), you can end up with a version that looks like this:

{H}^{2} \equiv {(\frac{\dot{a}}{a})}^{2} = {{H}_{0}}^{2}[\frac{{\Omega}_{m}}{{a}^{3}} + (1 - {\Omega}_{m}){a}^{-3(1+w)}]

Jeff Root
2015-Oct-15, 12:16 PM
grapes,

I hope you'll answer my questions in post #189 and later,
including confirming (or correcting!) my attempts in posts
#190 and #197 to clarify what you were saying to Jens.
You did reply to him, but just saying that you're talking
about distance only answers part of the question.

I still don't understand the formula for your 1-D "universe".




At a finite time, the points will fill an infinite space, ...
They will? Why will they do that? Why will they fill
the space, and why will the space they fill be infinite?

... especially since the "points" are radiating away
from the origin point (at -R), they should all get
progressively farther apart from one another ...
You didn't respond to that. But you also said:



At time t=0, all points in the universe have homogenized--
that is their relative motion in spacetime is identical to
their neighbors'
Does that mean the points are spread more-or-less
evenly across the infinite length of the ray, or does
their density decrease with distance from the origin,
as I guessed? Does the homogenization apply only
to their speeds, not their density?

-- Jeff, in Minneapolis
.

grapes
2015-Oct-15, 12:28 PM
Does that mean the points are spread more-or-less
evenly across the infinite length of the ray, or does
their density decrease with distance from the origin,
as I guessed? Does the homogenization apply only
to their speeds, not their density?

Every point of the y-axis is associated with some θ. So, the "density" of that universe is the same as the density of the (infinite real number) y-axis.

At time t=0, in my example, every point on the y-axis has a zero velocity relative to the origin (or, every other point). That's why I call it homogenous. No point has any characteristic that can distinguish it from any other point at time t=0. At any other time less than 0, that would not be true: every point had a unique velocity relative to the origin (except at time t=-R, when all points were at the same place)

I chose t=0 to be where I ended my example, because it is supposed to represent our time t=0, the origin. I used -R so that I could emphasize that it was before 0, and that it was unknown. There's nothing special about R, you can let it be 1 if you want to graph the path at Wolfram Alpha.

ETA: the suggestion then is that the universe would progress past time t=0, perhaps because it is in a hot, dense state, but the expansion would be uniform across the universe because there would be no difference in conditions.

Grey
2015-Oct-15, 01:50 PM
It may be just my opinion, but I believe the replacement to BBT will not have a first cause problem...Any universe that's not eternal pretty much has a first cause problem, doesn't it? Actually, even an eternal universe has first cause problem if it doesn't remain essentially unchanged. It just changes the question from "how did the universe get here in the first place?" to "after spending an eternity in a certain state, why did the universe suddenly start changing?" There aren't many models of the universe that won't run into this. The first one that comes to mind is the old "expanding steady state" model, where we agree that the universe is expanding, but we posit that new hydrogen forms out of nothing at a very slow rate, eventually forming new starts and galaxies in the empty space created by expansion, so that the universe as a whole remains essentially the same over time. But of course that model has a hard time explaining things like the cosmic microwave background, and we have no evidence of spontaneous hydrogen creation. If you've got a better alternative, or even a vague suggestion of some kind of theory that would be compatible with all the available evidence but without a "first cause problem", feel free to present it.


...and will not demand the breaking down all of known physics.You say this like you think our knowing the limits of quantum theory and general relativity is a bad thing, but actually, I'd say it's a positive development. All theories are only approximations, and they will all break down and no longer be applicable in some circumstances. Originally, we probably didn't realize that; Newton may have thought his laws of motion were perfect and unchanging. Later discoveries led us to realize that they are just very close to correct, as long as we keep relative speeds and gravitational forces low, and don't try to use them to describe really small things (or some big things where quantum effects become important, like neutron stars, for example).

So we came up with new theories that can handle those situations, that reduce to Newtonian mechanics in the limit. The fact that we realize that general relativity and quantum mechanics are likewise only approximations means that we understand the limits of our theories better, and the fact that we have a pretty good idea about what conditions are necessary for them to break down is just that much better. In particular, that gives us ideas about the experiments or observations we need to make to find out if and how we need to modify those theories. The circumstances under which we know they break down are quite extreme; it would be pretty ridiculous to pretend that we understand exactly what happens in those circumstances, even though we've never been able to observe them directly (precisely because they are so extreme), because we've extrapolated from what we have observed.

Ken G
2015-Oct-15, 01:53 PM
I think one thing that can be said confidently is that if the expansion of the universe needs to be "caused", in the usual way we put "causation" in our models, then the cosmological principle cannot be expected to extend to infinity. But there are two reasons why that argument is somewhat irrelevant to models of the universe:
1) there is no reason to expect any model of the universe to go farther than our intentions for the model. In other words, there is never a reason to expect a model of the universe to tell you things about the universe that extrapolate the data you used to test the model into a completely new, and potentially fundamentally different sphere. You had reasons for making the model in the first place, and the model addresses those reasons, and that's all. Even terms like "finite" and "infinite" are just aspects of a model, they don't have meaning for anything else.
2) cause and effect is also a model, so we apply it to the universe when it agrees with data we have, and we extend it to domains we have not yet tested because that's what models are used for. But whenever we extrapolate a model, we must always allow for the possibility that the model has a limited domain of applicability. So just like we don't expect general relativity to work to describe the first Planck time of our universe, we also have no reason to expect causality concepts to apply there either. It's fine to build a model that does extend these models, but only with the intention of testing them-- never with the intention of using them to assert truths that have not been tested.

Frankly, I think the size of the universe is never going to be something we can test. It's just not going to fall within the purvey of what we are capable of knowing, because even if we do detect a nonzero curvature, we know it will be very small, so we have no way of knowing it remains consistent beyond what we can observe. So if we never do detect any finite curvature, or any fractal structure in the universe at large, then we are never going to have a simpler model than a flat infinite model universe. That will leave each individual completely free to believe the universe is really infinite, on the grounds that causality is one of the things that breaks down in that first Planck time, or that it is really finite, if they feel causality is a higher principle of some kind (which is essentially a religious viewpoint), or they can even think there as no first Planck time, the universe evolved from some other state where the scale parameter was affected by other things not currently in our models.

This last view is probably the most common, because it is generally suspected that we will someday have a theory that unifies quantum mechanics and gravity, and when applied to the early universe, that theory might explain how quantum fluctuations can break the cosmological principle. Then you'll have tiny "pockets of inflation" popping off all over. This is often called "eternal inflation." It hasn't been tested, and I don't know if it ever will be testable, but it is a standard kind of mindset that scientists can embrace in the comfort and privacy of their own opinion. Ironically, that picture essentially ducks the question of what is the size of the universe. It agrees that the universe of our own Big Bang would indeed be finite, as Jeff is saying, but it also allows for an infinite universe of other such Big Bangs. So we still don't know if the whole universe is infinite, or just Really Huge. Is there even a scientific difference?

So what I'm saying is, even if one holds that our own Big Bang has a finite spatial extent, one is only saying that there is a length scale associated with it. This is perfectly normal-- we have a length scale associated with solar systems, on which we see totally different kinds of behaviors than the length scale of a galaxy, on which we see totally different behaviors than on the length scale of the Big Bang, which might be a totally different length scale than the scale of the universe of bubbles popping off eternal inflation. What we are seeing is that when you look on some length scale, you see behavior that is endemic to that length scale, but each length scale is embedded in a larger one, and we see different behavior. So is that a finite or an infinite universe? Most people live in the "finite universe" of their own house and workplace, and without astronomy, wouldn't care about anything more than that anyway. I say what "the universe is" depends on how you look at it, what you want to know about it, and on what length scale you are able to see, and the answer changes every time you change one of those factors.

Grey
2015-Oct-15, 02:23 PM
It is fair. At t = 2 seconds the volume is not being doubled and thus it is not expanding.Careful. Remember that an infinite universe can still be expanding (this would be hard to observe in an empty universe, but is easily observable if there are a bunch of markers to watch getting farther apart, like galaxies).


Because the universe (our universe) in mainstream cosmology either

started as either infinite and is still infinite or
started as finite and is still infinite.

And importantly: we do not know whether the universe is finite or infinite!I assume you mean "started as finite and is still finite", but still, I have a slight quibble. It's true that most cosmological models I've seen fit in these two, but I have seen a handful of inflationary models tossed around that diverge in finite time. I don't think that they're popular, but I'd still characterize them as mainstream attempts to come up with workable models of inflation, even if they're just exploring all the possibilities. (Cosmologists do this all the time, tweaking various parameters just to see what happens; for example, even before we discovered that the universe really does seem to have a nonzero cosmological constant, cosmologists tested out model universes that included a cosmological constant just to see how they would behave.)



I have two questions about the statement. First is about the hypothetical volume of space. If the function of the expansion is really that it doubles in size at a rate that goes from 1 to 0.5 to 0.25, etc., is it really fair to say that it will stop expanding at 2? Isn't it rather that the function becomes undefined above 2?Jens, for models that diverge in finite time, they just keep diverging. :) In a "Big Rip" scenario, the net effect is that every particle becomes completely isolated, with every other particle beyond the particle horizon, and nothing ever interacts with anything else again. Obviously, that's not our universe (at least not yet). For an inflationary period that diverged in finite time, the inflationary period would have to end at the limit where it diverges. That's not quite as weird or coincidental as it sounds; any model of inflation has to start suddenly, and then end just as suddenly; my (limited) understanding of the divergent inflationary models that I've seen is that the divergence itself is what triggers the end of inflation.

Again, I wouldn't characterize that as a dominant mainstream view by any means, but it is a sufficiently workable model that it shows there's at least one way to end up with an infinite universe that started from essentially zero size. Remember, my argument here is not to try to claim that the universe definitely is infinite*, only that it remains an open question, and that Jeff's argument (that an infinite Big Bang is logically impossible) has several flaws.



* By the way, I keep phrasing it like that, but I'm definitely with Ken G on this one. What I mean when I say "the universe is infinite" or "the universe is finite" is something more like "cosmological models of the universe that are infinite (or finite) do a better job of matching our experimental results and observations, and helping us understand the structure and behavior of the universe, than those that are not". But that's way too long to say every time. And as it currently stands, there is no clear difference between finite and infinite models, which is why the question remains open, and it may indeed always be so. The fact that it's not obvious which one is better is itself an intriguing cosmological observation, and is part of what led to the suggestion of inflationary models in the first place.

Ken G
2015-Oct-15, 02:23 PM
Any universe that's not eternal pretty much has a first cause problem, doesn't it? Actually, even an eternal universe has first cause problem if it doesn't remain essentially unchanged. It just changes the question from "how did the universe get here in the first place?" to "after spending an eternity in a certain state, why did the universe suddenly start changing?" Yes, I always find it a bit strange, this abhorrence for a "first cause" problem. I admit that science has its hands a bit tied there, and no one likes to have their hands tied, but recognizing when your hands are tied is better than pretending you don't have hands at all!

I've never really understood why people like Newton and Einstein and Hoyle all preferred steady-state models, universes that extend infinitely forward and backward in time without undergoing any fundamental change. I can see that it eliminates the first cause problem, but how is that an improvement? At least if you have a first cause that science cannot address, you know what you are sweeping under the rug. If you have a steady state universe, you can pretend you haven't swept anything under the rug, but of course you have-- you've swept under the rug the entire question of why there is a universe at all! That's better than a "first cause" problem? I say what's better is to recognize that there are some questions science is just not good at answering, and questions like "what made the universe" are among those. Whether you frame that question as "what was the first cause" or "why has there always been a universe" doesn't escape that. To me, the only difference between those questions is our choice of time parameter-- if I reparametrize time by a new quantity that goes to negative infinity when our common time parameter goes to zero, I can turn both those questions into the same question.

You say this like you think our knowing the limits of quantum theory and general relativity is a bad thing, but actually, I'd say it's a positive development. All theories are only approximations, and they will all break down and no longer be applicable in some circumstances. Yes, I agree completely. We've never had a theory that worked in all situations and all scales, so it seems unrealistic to think science deals in such theories at all. Given this, we either have theories that break down and don't tell us when, or we have theories that break down and do tell us when. Clearly, the second type is the better one, yet is generally viewed as some kind of weakness! It's like if a person is competing at chess, and they know they have a very good memory for seeing the board after a lot of moves, so can painstakingly and slowly analyze any situation, but they are terrible at speed chess because they have no intuitive feel for the board, then is it a weakness if they admit to this situation, or is it a strength? At least they can seek competitive environments that do not have a limited time in which to move, and in that way excel. It is the person who refuses to accept that they stink at speed chess that is in the position of weakness, for they force themselves to believe they should win in any timekeeping environment, and they just won't.

Grey
2015-Oct-15, 02:41 PM
I just want to point out that it is a very common attitude to imagine that a theory that requires all its own laws to break down at some point must be terribly flawed, and that's really not necessarily true. It might be a sublime attribute of a science theory, given the fundamental limitations in the kinds of things we can actually test.I absolutely agree with this. We know that any theory is going to be an approximate model that breaks down in some circumstances. A theory that tells us where it does so is more helpful than one that doesn't, because it lets us know in advance when we should not rely on it, and because it points us where we need to look to find something new. The second theory still fails somewhere, we can be pretty sure of that, we just don't necessarily know where. The reason we keep doing high energy particle experiments (for example), is partly because we're hoping to confirm predictions from theory, but also largely because we're hoping to find something unexpected that our theory didn't predict, and then we'll have to rework the theory to account for it. That's the fun part.


Edited to add: I see that we both said something like this, and then we both agreed with each other. ;)

grapes
2015-Oct-15, 03:55 PM
I've never really understood why people like Newton and Einstein and Hoyle all preferred steady-state models, universes that extend infinitely forward and backward in time without undergoing any fundamental change. I can see that it eliminates the first cause problem, but how is that an improvement? At least if you have a first cause that science cannot address, you know what you are sweeping under the rug. If you have a steady state universe, you can pretend you haven't swept anything under the rug, but of course you have-- you've swept under the rug the entire question of why there is a universe at all! That's better than a "first cause" problem? I say what's better is to recognize that there are some questions science is just not good at answering, and questions like "what made the universe" are among those. Whether you frame that question as "what was the first cause" or "why has there always been a universe" doesn't escape that.

In this paragraph, you go from accusing Newton and Einstein and Hoyle of "eliminating" the first cause problem, to arguing that it really doesn't eliminate the first cause problem. Of course, they were aware of this, they were no dummies.

The reason Newton and Einstein preferred steady-state models was because they saw no data/evidence otherwise. Einstein even called it his biggest blunder, because his famous equations actually pointed towards a non-steady-state, but he had no data to fit to it. You, of all people, should appreciate that! :)

They were well aware of the "first cause problem" and weren't trying to sweep it under the rug. They just didn't have any answers for it. We still don't.

grapes
2015-Oct-15, 04:02 PM
I think one thing that can be said confidently is that if the expansion of the universe needs to be "caused", in the usual way we put "causation" in our models, then the cosmological principle cannot be expected to extend to infinity.

I think that's Jeff's claim in a nutshell.

I agree that we'll never be able to gather data from all parts of an "infinite" universe, but I think my "toy" model is an example that shows that an infinite universe can be homogenous, that is, the cosmological principle holds without violating causation.

Jeff Root
2015-Oct-15, 05:10 PM
I think one thing that can be said confidently is that
if the expansion of the universe needs to be "caused",
in the usual way we put "causation" in our models,
then the cosmological principle cannot be expected
to extend to infinity.
I think that's Jeff's claim in a nutshell.
Yes. Very closely.

In post #196 yesterday, I said that I just realized I am
claiming *two* impossibilities: A volume of space which is
highly homogeneous despite lack of causal contact between
its parts, and anything finite becoming infinite in finite time.
I've generally combined them into one without distinguishing
between them, and that's how Ken's statement expresses it.

Ken's statement and comments by Wayne and others refer
to an initial cause. I haven't claimed anything about an initial
cause. The causality I'm concerned about is the apparent
causality which made all of the observable Universe the same--
homogeneity in my statement and the cosmological principle
in Ken's. That *could* be an initial cause, but it could just as
well be an intermediate step, as in inflation, including eternal
inflation.

My claim is that inflation can solve the causality problem for
a finite universe, but not for an infinite universe. A universe
which instantaneously has a significant size-- such as a
universe that is infinite from the very start-- has the problem
with causality.

So it is possible that there are two separate problems I have
been failing to distinguish, or two phenomenae that I'm failing
to see have the same origin.



I think my "toy" model is an example that shows that an
infinite universe can be homogenous, that is, the cosmological
principle holds without violating causation.
Without some kind of evidence to the contrary, I assume
the model is just a mathematical relation that says nothing
about anything physical. As I said before: imagine a ray of
infinite length and call it a "universe". Doesn't that work
just as well as your model?

-- Jeff, in Minneapolis

Reality Check
2015-Oct-15, 09:49 PM
Careful. Remember that an infinite universe can still be expanding (this would be hard to observe in an empty universe, but is easily observable if there are a bunch of markers to watch getting farther apart, like galaxies).

Hopefully I was careful - this toy model of a volume of expanding space is not our universe as I emphasized or even mainstream cosmology. It is designed to expand to an infinite extent in a finite time.



I assume you mean "started as finite and is still finite", but still, I have a slight quibble.
I corrected that. I am concentrating on the standard Big Bang model so there will be many quibbles :).

grapes
2015-Oct-15, 11:23 PM
I think my "toy" model is an example that shows that an
infinite universe can be homogenous, that is, the cosmological
principle holds without violating causation.

Without some kind of evidence to the contrary, I assume
the model is just a mathematical relation that says nothing
about anything physical. As I said before: imagine a ray of
infinite length and call it a "universe". Doesn't that work
just as well as your model?

Work for what? Do you think an infinite ray, by itself, refutes your (two) claims?

I don't either.

But my model does refute your (two) claims.

Reality Check
2015-Oct-15, 11:56 PM
My claim is that inflation can solve the causality problem for
a finite universe, but not for an infinite universe. A universe
which instantaneously has a significant size-- such as a
universe that is infinite from the very start-- has the problem
with causality.
The first problem with your claim is that you have not shown that any "causality problem" exists, e.g. by citing the scientific literature on it, Jeff Root. All we have is your opinion that this "causality problem" exists. This looks like more of a claim that you cannot imagine that an infinite universe can come into existence.
The second problem with your claim is a universe "which instantaneously has a significant size" without a definition of "significant " applies both to a finite universe and an infinite universe. Basically you are claiming that any universe has this "causality problem" :p!

A cause makes an infinite universe pop into existence - no causality problem there. Ditto for a finite universe.
An infinite universe has always existed (e.g. brane cosmology) - no causality problem there. Ditto for a finite universe.

Jeff Root
2015-Oct-16, 01:08 AM
Work for what? Do you think an infinite ray, by itself,
refutes your (two) claims?

I don't either.

But my model does refute your (two) claims.
Why does your model refute my claims while the infinite
ray does not? The part of your model which might refute
my claims is the fact that the result is infinite. An infinite
ray. The motion of the points along that ray is a nice
touch, but doesn't contribute to the argument.

-- Jeff, in Minneapolis

grapes
2015-Oct-16, 03:24 AM
Why does your model refute my claims while the infinite
ray does not? The part of your model which might refute
my claims is the fact that the result is infinite. An infinite
ray. The motion of the points along that ray is a nice
touch, but doesn't contribute to the argument.

From the OP:



If the Universe is infinite, then only a finite portion of it was involved in the Big Bang.

and, more recently:


In post #196 yesterday, I said that I just realized I am
claiming *two* impossibilities: A volume of space which is
highly homogeneous despite lack of causal contact between
its parts, and anything finite becoming infinite in finite time.
I've generally combined them into one without distinguishing
between them, and that's how Ken's statement expresses it.

So, your ray is infinite, like my example universe, but your example lacks any sort of mechanism to make it homogenous in a finite amount of time, whereas I provide such a mechanism. Also, mine starts from something finite, and transforms into something infinite.

Jens
2015-Oct-16, 04:45 AM
Any universe that's not eternal pretty much has a first cause problem, doesn't it? Actually, even an eternal universe has first cause problem if it doesn't remain essentially unchanged. It just changes the question from "how did the universe get here in the first place?" to "after spending an eternity in a certain state, why did the universe suddenly start changing?" There aren't many models of the universe that won't run into this. The first one that comes to mind is the old "expanding steady state" model, where we agree that the universe is expanding, but we posit that new hydrogen forms out of nothing at a very slow rate, eventually forming new starts and galaxies in the empty space created by expansion, so that the universe as a whole remains essentially the same over time.

Yes, I would agree. An eternal universe would either have to be cyclic or evolving in such a way that it can evolve eternally, as if a fractal pattern is constantly emerging and going to smaller and smaller scales.

mkline55
2015-Oct-16, 11:51 AM
So, your ray is infinite, like my example universe, but your example lacks any sort of mechanism to make it homogenous in a finite amount of time, whereas I provide such a mechanism. Also, mine starts from something finite, and transforms into something infinite.

As I read it your model starts with an infinity - an infinite number of points infinitely dense.

mkline55
2015-Oct-16, 12:01 PM
If you've got a better alternative, or even a vague suggestion of some kind of theory that would be compatible with all the available evidence but without a "first cause problem", feel free to present it.

I wouldn't present any such model here, but I do appreciate the input I've received from regulars at this site. I would point out that the "all the available evidence" phrase is part of the issue that must be overcome. If you start with "all the interpretations of the available evidence" then you can understand that perhaps some "evidence" might be subject to reevaluation. For example, when someone claims they found something that matches expectations, be careful that you are not prejudicing that interpretation of the evidence by looking for anything that comes close to what you want. I'm reminded of the Ghost Hunters series.

mkline55
2015-Oct-16, 12:11 PM
The first problem with your claim is that you have not shown that any "causality problem" exists . . .

I think the first problem is that the model does not show that a causality can be tossed out the window.

mkline55
2015-Oct-16, 12:16 PM
Exactly-- we can have gut feelings, but we don't adjudicate science that way. They do help us decide where to put our energies, or where to lay our bets, but that's all outside of actual scientific knowledge. I just want to point out that it is a very common attitude to imagine that a theory that requires all its own laws to break down at some point must be terribly flawed, and that's really not necessarily true. It might be a sublime attribute of a science theory, given the fundamental limitations in the kinds of things we can actually test.

Every ATM poster should use that. "The best part of my wild theory is that it does parts of it do not even work. How sublime!" Don't take that to mean that BBT is a wild theory.

Jeff Root
2015-Oct-16, 01:01 PM
So, your ray is infinite, like my example universe, but
your example lacks any sort of mechanism to make it
homogenous in a finite amount of time, whereas I
provide such a mechanism.
The ray produced by simply positing its existence is as
homogeneous as the ray produced by your function.



Also, mine starts from something finite, and transforms
into something infinite.
The ray produced by simply positing its existence becomes
infinite instantaneously. That's true of yours, isn't it?

The only real difference I see between them is that your
function has a somewhat more complex description.

-- Jeff, in Minneapolis

grapes
2015-Oct-16, 01:14 PM
So, your ray is infinite, like my example universe, but your example lacks any sort of mechanism to make it homogenous in a finite amount of time, whereas I provide such a mechanism. Also, mine starts from something finite, and transforms into something infinite.

As I read it your model starts with an infinity - an infinite number of points infinitely dense.
Yes, of course.

The distinction between infinite density and infinite in extent has been made several times in this thread. The y-axis is infinitely dense, in that sense, as is any finite-extent interval.

Infinity, in the context of Jeff's questions, is in regards to extent, I believe.

grapes
2015-Oct-16, 01:37 PM
The ray produced by simply positing its existence is as
homogeneous as the ray produced by your function.

Is just being homogeneous OK?

I thought the "causal contact" part was your sticking point.


In post #196 yesterday, I said that I just realized I am
claiming *two* impossibilities: A volume of space which is
highly homogeneous despite lack of causal contact between
its parts, and anything finite becoming infinite in finite time.
I've generally combined them into one without distinguishing
between them, and that's how Ken's statement expresses it.




The ray produced by simply positing its existence becomes
infinite instantaneously. That's true of yours, isn't it?

The only real difference I see between them is that your
function has a somewhat more complex description.

-- Jeff, in Minneapolis

Grey
2015-Oct-16, 01:40 PM
Every ATM poster should use that. "The best part of my wild theory is that it does parts of it do not even work. How sublime!" Don't take that to mean that BBT is a wild theory.You seem to be deliberately misinterpreting this. It should be, rather, "the best part of my theory is that it tells you what conditions it works under and what conditions it doesn't". And if the conditions under which it doesn't work are very widespread, then it's obviously not a very useful theory. But if the conditions under which it doesn't work are very extreme, very seldom occur, and are very difficult to observe, it's pretty unsurprising that you'd want to acknowledge that your theory isn't tested in those circumstances, and probably doesn't hold.

So, yes, we're quite sure that we'd have to find a quantum theory of gravity to be able to accurately described things when the energy density is on the order of the Planck density, which is many orders of magnitude higher than we've ever been able to directly observe. Why is that surprising? In the same way, trying to extrapolate Newtonian mechanics, which works perfectly fine at speeds of a few meter per second, to speeds of a hundred million meters per second, you run into some big problems and have to come up with a new theory. I genuinely don't get why you find this so surprising, or think it represents a problem with a theory.

Grey
2015-Oct-16, 01:52 PM
I wouldn't present any such model here, but I do appreciate the input I've received from regulars at this site. I would point out that the "all the available evidence" phrase is part of the issue that must be overcome. If you start with "all the interpretations of the available evidence" then you can understand that perhaps some "evidence" might be subject to reevaluation. For example, when someone claims they found something that matches expectations, be careful that you are not prejudicing that interpretation of the evidence by looking for anything that comes close to what you want. I'm reminded of the Ghost Hunters series.I'm just not able to come up with a model that doesn't have a "first cause problem" as you've described it, and can still match even the broadest theoretical evidence that led us to a big bang model in the first place. As I've said, it seems to me that any universe that isn't eternal and largely unchanging over long timescales will have the same kind of first cause issue that you think is a problem. If you've got something in mind that can even account for increasing redshift correlated with distance, the microwave background, and at least address issues like the relative abundance of elements and formation of large scale structure, I'd be genuinely curious to hear about it, because I can't come up with one. Are you imagining something like an eternal steady state universe, and somehow coming up with an alternate explanation for the CMB?

Of course, you don't have to present such an idea if you don't want to. But then it seems like you're complaining that the current cosmological theory has what you consider a flaw, and you want a theory without that flaw, but you aren't willing or able to propose any alternative that would meet that criteria. It's fine for you to feel that way, but I guess don't expect to persuade anyone to your way of thinking with that attitude.

Jeff Root
2015-Oct-16, 01:58 PM
Infinity, in the context of Jeff's questions, is in regards
to extent, I believe.
I think that's right.

But you mentioned "density" in post #174 when describing
your function:


Since it is my "model" I can answer those questions.
Not sure it's going to help, since I am *not* trying to
model actual space. My model is one-dimensional after all.

The starting size is zero (a point), the density is infinite,
it "becomes" infinite at time t=0
Density of what?

-- Jeff, in Minneapolis

grapes
2015-Oct-16, 02:13 PM
Density of what?

I was answering mkline55's question about what the density was at the initial, staring point. Since all the point timelines start there, it has infinite density.

Jeff Root
2015-Oct-16, 02:27 PM
Is just being homogeneous OK?

I thought the "causal contact" part was your sticking point.
It is.

But the cause of homogeneity in your function is not
different from the cause of homogeneity in any posited
ray. The resulting rays have the same homogeneity,
the same density of points for the same mathematical
reason. They are that way because of the properties
lines are assumed to have in Euclidean geometry.
In a system in which the concept of "line" is defined
(usually in terms of "points") rather than assumed as
a primitive concept, the homogeneity and density are
by definition.

Your function doesn't demonstrate causality of the ray's
homogeneity any more than does simply postulating it
as existing.

That is because the function describes a mathematical
object, not a physical object.

And having just one spatial dimension is not what makes
your function only mathematical rather than physical--
The limitation is imposed by having no connection to
matter or energy. The way in which the ray is generated
is not limited by *any* physics. It is just mathematics
and geometry.

I made up an aphorism two or three decades ago: Time
is a factor in everything that happens, but it is never the
only factor. I think that describes the problem here.

-- Jeff, in Minneapolis

Jeff Root
2015-Oct-16, 02:34 PM
I was answering mkline55's question about what the
density was at the initial, staring point. Since all the
point timelines start there, it has infinite density.
If you mean infinite mass density, nothing about the
function indicates or even suggests that. Not even a hint.

It might mean density of points, but the density of points
is always infinite, everywhere.

-- Jeff, in Minneapolis

mkline55
2015-Oct-16, 03:29 PM
I'm just not able to come up with a model that doesn't have a "first cause problem" as you've described it, and can still match even the broadest theoretical evidence that led us to a big bang model in the first place. As I've said, it seems to me that any universe that isn't eternal and largely unchanging over long timescales will have the same kind of first cause issue that you think is a problem. If you've got something in mind that can even account for increasing redshift correlated with distance, the microwave background, and at least address issues like the relative abundance of elements and formation of large scale structure, I'd be genuinely curious to hear about it, because I can't come up with one. Are you imagining something like an eternal steady state universe, and somehow coming up with an alternate explanation for the CMB?

Of course, you don't have to present such an idea if you don't want to. But then it seems like you're complaining that the current cosmological theory has what you consider a flaw, and you want a theory without that flaw, but you aren't willing or able to propose any alternative that would meet that criteria. It's fine for you to feel that way, but I guess don't expect to persuade anyone to your way of thinking with that attitude.

The first cause problem is created by the model itself. The model points to something which requires characteristics that violate nearly every physical principle. To simply say that origin is outside the model is just an acceptance that the model might have a problem, but that for practical purposes we just don't care.

You know perfectly well that if I proposed a new model it would go straight to ATM. And it should. The model I am working on has so far met your short list of requirements as well as many more, but it is far more than 30 days from completion.

grapes
2015-Oct-16, 03:54 PM
It is.

But the cause of homogeneity in your function is not
different from the cause of homogeneity in any posited
ray.

No, not true.

Not only does the "toy" universe have that homogeneity, but each point has a history of relative velocity, and that relative velocity becomes homogenous at t=0

It evolves from a finite local system to a system that does not have homogeneity, and then to an infinite (in extent) one that does have.


The resulting rays have the same homogeneity,
the same density of points for the same mathematical
reason. They are that way because of the properties
lines are assumed to have in Euclidean geometry.
In a system in which the concept of "line" is defined
(usually in terms of "points") rather than assumed as
a primitive concept, the homogeneity and density are
by definition.

Your function doesn't demonstrate causality of the ray's
homogeneity any more than does simply postulating it
as existing.

That's why they evolve from a local finite neighborhood. That was the whole reason for creating the example. So, it is different from simply saying "it's a ray"


That is because the function describes a mathematical
object, not a physical object.

And having just one spatial dimension is not what makes
your function only mathematical rather than physical--
The limitation is imposed by having no connection to
matter or energy. The way in which the ray is generated
is not limited by *any* physics. It is just mathematics
and geometry.

The example was only one of spacetime. We can add mass and energy in there but you're probably going to get something that looks like what grey posted.

I was just trying to simplify it to illustrate the concepts.


I made up an aphorism two or three decades ago: Time
is a factor in everything that happens, but it is never the
only factor. I think that describes the problem here.

You can add another factor: gray matter. :)

grapes
2015-Oct-16, 03:56 PM
If you mean infinite mass density, nothing about the
function indicates or even suggests that. Not even a hint.

Right, it was an example about spacetime


It might mean density of points, but the density of points
is always infinite, everywhere.

I'm not sure how you'd characterize the density at 1, just 1, but regardless, I was just answering a question, not emphasizing any particular attribute

Grey
2015-Oct-16, 04:17 PM
The first cause problem is created by the model itself.I've said that I think any non-eternal universe has the same first cause problem, and even most of the eternal universes have something similar. Can you tell me the flaw in my reasoning?


The model points to something which requires characteristics that violate nearly every physical principle. To simply say that origin is outside the model is just an acceptance that the model might have a problem, but that for practical purposes we just don't care. Well, the simplest explanation for the redshift distance relation really does seem to be just that the universe is expanding. An expanding universe should get cooler and less dense over time, so looking backward, it should have been hotter and denser in the past. Eventually, if you get far enough in the past, you'll get to a point where it's sufficiently hot and dense that we won't have direct observations of that kind of energy density, and extrapolating from lower energy densities is likely to run into problems. That's going to happen with most models where the universe is expanding (the exceptions are the ones like the expanding steady state model, where matter is spontaneously created to keep the density roughly constant over time; I've already mentioned some of the problems with models like that), and non-expanding models have their own problems (the first one is just finding an alternate explanation for the appearance of expansion).


You know perfectly well that if I proposed a new model it would go straight to ATM. And it should. The model I am working on has so far met your short list of requirements as well as many more, but it is far more than 30 days from completion.Fair enough, take your time (although I should note that it only has to go to ATM if you advocate for it, not just if you suggest it as a hypothetical alternative; grapes doesn't have to take his toy universe model to ATM, because he's not trying to claim that it's a real model of the universe, only that it's an example that shows a flaw in Jeff's claims about causality). I confess that I'm not going to just take your word for it that it meets those requirements, though, but I'll take your hypothesis more seriously if I ever get to see it.

mkline55
2015-Oct-16, 05:15 PM
I've said that I think any non-eternal universe has the same first cause problem, and even most of the eternal universes have something similar. Can you tell me the flaw in my reasoning?

Not in a manner you will likely find acceptable, but I will try again. Any non-eternal model will have a starting point. The expansion model, by its nature, requires something which is impossible based on observed physics. Aside from a model which requires impossible physics, we have zero - nada - zilch actual physical evidence that physical properties can be violated. The solution has been to draw a line in the sand and say that the model does not apply beyond that point. Where to draw that line is up to whoever likes to draw lines. But anyone with a ruler and a sheet of paper can show where the model is headed. This is being accepted as "good science", and someone even posted that it was the highlight of the theory.


Well, the simplest explanation for the redshift distance relation really does seem to be just that the universe is expanding. An expanding universe should get cooler and less dense over time, so looking backward, it should have been hotter and denser in the past. Eventually, if you get far enough in the past, you'll get to a point where it's sufficiently hot and dense that we won't have direct observations of that kind of energy density, and extrapolating from lower energy densities is likely to run into problems. That's going to happen with most models where the universe is expanding (the exceptions are the ones like the expanding steady state model, where matter is spontaneously created to keep the density roughly constant over time; I've already mentioned some of the problems with models like that), and non-expanding models have their own problems (the first one is just finding an alternate explanation for the appearance of expansion).

I agree that the universe from our viewpoint appears to be and/or appears to have been expanding (redshift), cooling (in the sense of fewer particle collisions), and had some form of residual energy (CMB), and BBT is one simple explanation.

Grey
2015-Oct-16, 06:14 PM
Not in a manner you will likely find acceptable, but I will try again. Any non-eternal model will have a starting point. The expansion model, by its nature, requires something which is impossible based on observed physics.I disagree. The expansion model extrapolates back and says that the energy density in the early universe looks to have been very high, and at a certain point would be high enough that we know that neither quantum theory nor general relativity alone can accurately describe how things would behave (and we don't know how to combine the two, partly because we don't have observations at such a high energy density to check any proposed theory against). But neither general relativity nor quantum theory actually prohibits having an energy density that high. I don't think anyone has suggested that there's some specific limit to energy density, and if they did, I don't know what observation they would base it on. So the big bang model does not require you to accept something "impossible based on observed physics". It requires you to accept something that seems like it should be perfectly possible, but that we know we can't describe accurately.