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ToBOrNotToB
2015-Jun-16, 08:09 PM
Hello.

Sorry for my lack of experience in formulating astronomy questions. Im just a guy who watched alot of documentaries on youtube about the universe lately. I heard the big bang theory, then the inflation theory, the dark matter, the dark energy, even the dark flow. Problem is more documentaries I view, more some questions bug my head harder. So I was looking on google for some forum about the universe where I can get some clarifications. Thank you in advance to whoever will bother to answer my noobish questions. Ok so much for an introduction. Now the questions.

I keep hearing this thing that the universe is 13.4 billion years old (correct me if I'm wrong). Now, this I belive it's based on the most far star/galaxy, space object observed, right? And this I don't understand. When it's refered that it's the most far object observed its a bit confusing because we live in a 3D space. So ... you know, if you look north, west, east, south you would see various space objects. Its not clear to me if this 13.4 billion years old refers to a certain object in a certain direction or as in all around us?

There are more questions. but I would apreciate some clarifications to the one mentioned above first of all. Is just, the way people not mentioning any direction to that 13,4 billion years old makes me think we are the center of the universe after all. Or what?

Thank you for all your answers

Noclevername
2015-Jun-16, 11:53 PM
I keep hearing this thing that the universe is 13.4 billion years old (correct me if I'm wrong). Now, this I belive it's based on the most far star/galaxy, space object observed, right? And this I don't understand. When it's refered that it's the most far object observed its a bit confusing because we live in a 3D space. So ... you know, if you look north, west, east, south you would see various space objects. Its not clear to me if this 13.4 billion years old refers to a certain object in a certain direction or as in all around us?

There are more questions. but I would apreciate some clarifications to the one mentioned above first of all. Is just, the way people not mentioning any direction to that 13,4 billion years old makes me think we are the center of the universe after all. Or what?

Hello, and welcome.

No, the age of the universe is not based on the distance to the furthest galaxy-- the universe does not have a discernable edge. The limit of our observation (the observable universe (https://en.wikipedia.org/wiki/Observable_universe)) is about 46 billion light years, but not because the universe ends at that distance, rather because the remainder of it is just too far away for light to reach us. There could be a lot of galaxies beyond that "horizon", but we'd never know it.

Our estimate of the age is based on a number of observations found all over space uniformly, such as the temperature of the Cosmic Microwave Background. For some basic information, I recommend the Wikipedia article on universal age (https://en.wikipedia.org/wiki/Age_of_the_universe).

Cougar
2015-Jun-16, 11:59 PM
I keep hearing this thing that the universe is 13.4 billion years old (correct me if I'm wrong). Now, this I belive it's based on the most far star/galaxy, space object observed, right?

As Noclev said, that's not right (plus, it's closer to 13.8). Actually there are about 7 different independent measurements that rather converge on the current figure. The most important, though, I imagine, is the rate of universal expansion, which, when run backwards, yields a good estimate.

John Mendenhall
2015-Jun-17, 01:51 AM
Yup. At large scales the universe looks the same in all directions. It is one of the basic and observable foundations of modern cosmology.

ToBOrNotToB
2015-Jun-17, 08:57 AM
Hello, and welcome.

No, the age of the universe is not based on the distance to the furthest galaxy-- the universe does not have a discernable edge. The limit of our observation (the observable universe (https://en.wikipedia.org/wiki/Observable_universe)) is about 46 billion light years, but not because the universe ends at that distance, rather because the remainder of it is just too far away for light to reach us. There could be a lot of galaxies beyond that "horizon", but we'd never know it.

Our estimate of the age is based on a number of observations found all over space uniformly, such as the temperature of the Cosmic Microwave Background. For some basic information, I recommend the Wikipedia article on universal age (https://en.wikipedia.org/wiki/Age_of_the_universe).

Thank you for your kind answers. Sorry for my missunderstanding. That's why I like asking questions. Watching videos on youtube arent always clarifing, but sometimes confusing.

You know there is this thing about the universe that, once again, I do not understand, or maybe I missunderstand completly. Now that I got your answer it hopefully helps me formulate this next question. Ok, here it is:

Suppose I'm looking at the furthest end of the observable universe. And I see there someone, or should I say I see someone's light. I see him as he was 46 billion light years ago. Right? Now, assuming, he is still alive today. He can see earth if he looks towards the earth? (Of course this is imposible IRL considering the fact noone lives that long, and earth its too small. But just hipotetically what if earth was the size of an observable object and the guy from the furthest end of the obs. univ. would still be alive?) Would he see earth?

Noclevername
2015-Jun-17, 09:42 AM
To clarify, when I said that parts of the Universe are "too far away for light to reach us", that may have come out misleading. There's nothing wrong with the light that comes from them, it can go on indefinitely. But the initial expansion of the universe included a period of rapid inflation, faster than what we now call the speed of light (space can expand faster than light can move through space).

So from our point of view, the light from distant galaxies is permanently out of reach, moving too fast, just as our light could never reach them. It's like a fighter jet, outracing its own sonic boom.

This also explains why the visible universe is larger than 13.8 BLY in diameter.

ToBOrNotToB
2015-Jun-17, 10:12 AM
To clarify, when I said that parts of the Universe are "too far away for light to reach us", that may have come out misleading. There's nothing wrong with the light that comes from them, it can go on indefinitely. But the initial expansion of the universe included a period of rapid inflation, faster than what we now call the speed of light (space can expand faster than light can move through space).

So from our point of view, the light from distant galaxies is permanently out of reach, moving too fast, just as our light could never reach them. It's like a fighter jet, outracing its own sonic boom.

This also explains why the visible universe is larger than 13.8 BLY in diameter.

Hm, interesting, space can expand faster then light speed. Maybe the universe might never ment to be an "universe" after all, but rather space filled with clusters, galaxies, gas clouds etc? Sorry ... I'm just yabbling nonsense.

grapes
2015-Jun-17, 10:17 AM
Suppose I'm looking at the furthest end of the observable universe. And I see there someone, or should I say I see someone's light. I see him as he was 46 billion light years ago. Right? Now, assuming, he is still alive today. He can see earth if he looks towards the earth? (Of course this is imposible IRL considering the fact noone lives that long, and earth its too small. But just hipotetically what if earth was the size of an observable object and the guy from the furthest end of the obs. univ. would still be alive?) Would he see earth?
The article at the link that Noclevername provided says that the most distant object observed (2011) is 30 billion light years away, the light leaving 13 billion years ago, less than a billion years after the Big Bang

The Hubble constant is 68 kilometers per second per megaparsec. Since the speed of light is 300,000 km/s, something 300000/68 megaparsecs away would have been going faster than the speed of light away from us and we'd never be able to see it. (300000/68) megaparesecs is (300000/68) x 3.3 million lightyears, which is (300/68)x3.3 billion light years, or, 14.6 billion light years. So, close.

Noclevername
2015-Jun-17, 05:03 PM
Hm, interesting, space can expand faster then light speed. Maybe the universe might never ment to be an "universe" after all, but rather space filled with clusters, galaxies, gas clouds etc? Sorry ... I'm just yabbling nonsense.

If the universe is not space filled with clusters, galaxies, gas clouds etc, then what is it?

Noclevername
2015-Jun-17, 05:07 PM
Thank you for your kind answers. Sorry for my missunderstanding. That's why I like asking questions. Watching videos on youtube arent always clarifing, but sometimes confusing.

You know there is this thing about the universe that, once again, I do not understand, or maybe I missunderstand completly. Now that I got your answer it hopefully helps me formulate this next question. Ok, here it is:

Suppose I'm looking at the furthest end of the observable universe. And I see there someone, or should I say I see someone's light. I see him as he was 46 billion light years ago. Right? Now, assuming, he is still alive today. He can see earth if he looks towards the earth? (Of course this is imposible IRL considering the fact noone lives that long, and earth its too small. But just hipotetically what if earth was the size of an observable object and the guy from the furthest end of the obs. univ. would still be alive?) Would he see earth?

If he had the universe's best telescope and gravitational lensing just happened to line up just perfectly, he'd see the place where Earth would eventually develop, using light from billions of years ago.

ADDED: Not 46 billion years ago, 13.8. Remember, we can't see anything older than that.

ToBOrNotToB
2015-Jun-17, 06:28 PM
If the universe is not space filled with clusters, galaxies, gas clouds etc, then what is it?

Sorry, bad statement from me. Let me try to rephrase my nonsense. I ment to say that maybe "clusters, galaxies, gas clouds ...etc" were not ment to be part of a whole entity called "universe" and they are not all coming from the same source, as from the big bang. Is just, if we have things in the universe that are moving, expanding... etc, at a speed faster then light itself, then nothing makes sense anymore.

I saw this documentary called "Is everything we know about this universe wrong?". It kinda annoyed me because they were saying most of the universe is made out of stuff we can't see, can't touch, can't feel, can't detect in other words. You know, stuff like dark matter and dark energy. So now when I red your reply, thank you for it btw, in which you stated there are things in the universe faster then the speed of light and we might never see the the furthest end of the universe... well this ideea just popped into my head. That maybe those things I mentioned earlier, clusters, galaxies, gas clouds... etc, aren't all from the same sourse and they were never ment to be a whole entity. Sorry I don't know how to explain better, is just a stupid ideea I know.

ToBOrNotToB
2015-Jun-17, 06:40 PM
If he had the universe's best telescope and gravitational lensing just happened to line up just perfectly, he'd see the place where Earth would eventually develop, using light from billions of years ago.

ADDED: Not 46 billion years ago, 13.8. Remember, we can't see anything older than that.

And if it was nothing, not even light, billions of years ago in the place where earth is now? He would see a void, as in he wouldnt even know earth exists?

Noclevername
2015-Jun-17, 09:07 PM
Sorry, bad statement from me. Let me try to rephrase my nonsense. I ment to say that maybe "clusters, galaxies, gas clouds ...etc" were not ment to be part of a whole entity called "universe" and they are not all coming from the same source, as from the big bang. Is just, if we have things in the universe that are moving, expanding... etc, at a speed faster then light itself, then nothing makes sense anymore.



But the trouble with your assumption is, moving and expanding space are two different things. Moving means going through space, and is limited to lightspeed. Expansion of space means that what light moves through, is growing.

Think of it like this; Jack goes to climb down the magic beanstalk. The beanstalk is growing faster than he can climb down safely. Does he ever reach the ground without falling?


I saw this documentary called "Is everything we know about this universe wrong?". It kinda annoyed me because they were saying most of the universe is made out of stuff we can't see, can't touch, can't feel, can't detect in other words. You know, stuff like dark matter and dark energy. So now when I red your reply, thank you for it btw, in which you stated there are things in the universe faster then the speed of light and we might never see the the furthest end of the universe... well this ideea just popped into my head. That maybe those things I mentioned earlier, clusters, galaxies, gas clouds... etc, aren't all from the same sourse and they were never ment to be a whole entity. Sorry I don't know how to explain better, is just a stupid ideea I know.

We can detect the effects of dark matter and dark energy. They are part of the same universe as matter and other forms of energy. Otherwise they could not interact with matter and energy in the ways that we observe.

And popular shows about cosmology are always "dumbed down" or simplified versions of real science; something is always lost in translation. Using them as a primary source of ideas is not recommended. Even Wikipedia is better.

Noclevername
2015-Jun-17, 09:10 PM
And if it was nothing, not even light, billions of years ago in the place where earth is now? He would see a void, as in he wouldnt even know earth exists?

He would see the primordial galaxy that our Sun would one day form in. A diffuse cloud of gas, mostly hydrogen. Prior to that, nothing visible except the energy that is now the Cosmic Microwave Background.

Noclevername
2015-Jun-17, 10:57 PM
And if it was nothing, not even light, billions of years ago in the place where earth is now? He would see a void, as in he wouldnt even know earth exists?

A quick nitpick; the Cosmic Microwave Background (https://en.wikipedia.org/wiki/Cosmic_microwave_background) means light was everywhere. The existing energy packed together so tightly, gave off tremendously intense radiation and light. The CMB is what is left after the high wavelengths of that radiation get spread out and cooled (red-shifted) by expanding space.

Cougar
2015-Jun-18, 02:13 AM
The existing energy packed together so tightly, gave off tremendously intense radiation and light.

IIUIC, most of the photons in the universe came from the matter/anti-matter annihilation that occurred sometime within the first second.

Cougar
2015-Jun-18, 02:26 AM
Suppose I'm looking at the furthest end of the observable universe. And I see there someone... as he was 46 billion light years ago. Right?

The furthest back in time we can see is about 13.7 billion years ago. But there are no galaxies or stars there. There's only a super-smooth gas of hydrogen and helium atoms, a tiny bit of other light atoms, and a lot of photons. That light is the CMB (https://en.wikipedia.org/wiki/Cosmic_microwave_background). From there, gravity has been very busy creating the structures we see throughout the (later) universe.

Noclevername
2015-Jun-18, 03:16 AM
IIUIC, most of the photons in the universe came from the matter/anti-matter annihilation that occurred sometime within the first second.

Right. :doh:

ToBOrNotToB
2015-Jun-18, 09:34 PM
IIUIC, most of the photons in the universe came from the matter/anti-matter annihilation that occurred sometime within the first second.

Why is everyone saying all happened in less then a second? Why was this first second so important that people say so many happened in such a short time? I keep hearing everywhere alot of stugg hapened in the first second. As if someone blinked and everything appeared

malaidas
2015-Jun-19, 12:00 AM
I think something needs to be made clear here that hasn't been. If 2 objects moved away from each other at 20mph they would observe a relative speed of the other at 40mph but neither is travelling at that speed The same goes for the expansion of the universe. Each point moving away from each other at a certain speed, but the net effect over distance is that speed is greater than the speed of light despite at no point expansion violating the speed limit

pzkpfw
2015-Jun-19, 12:14 AM
I think something needs to be made clear here that hasn't been. If 2 objects moved away from each other at 20mph they would observe a relative speed of the other at 40mph but neither is travelling at that speed The same goes for the expansion of the universe. Each point moving away from each other at a certain speed, but the net effect over distance is that speed is greater than the speed of light despite at no point expansion violating the speed limit

I don't think that's quite right. Comparison of objects in causal contact doesn't apply to the expansion of the Universe. Very distant objects can be "violating the speed limit".

https://en.wikipedia.org/wiki/Metric_expansion_of_space


...
While special relativity constrains objects in the Universe from moving faster than light with respect to each other when they are in a local, dynamical relationship, it places no theoretical constraint on the relative motion between two objects that are globally separated and out of causal contact
...
For example, galaxies that are more than approximately 4.5 gigaparsecs (14.7 billion light-years) away from us are expanding away from us faster than light.
...


This isn't the same scenario as, for example, the (up to) 2 x speed of light separation of two objects that measure each other to be moving at (up to) the speed of light away from a common point.

malaidas
2015-Jun-19, 12:25 AM
From our perspective though. If we apply that x is moving away from y at 20mph and y is moving away from z at 20mph which is situation described by universal expansion, you get that z moving away from x at 40mph. What you end up with is that to every frame of reference you are stationary but that everything else acceratates away as a factor of distance but nothing is actually moving at that speed rather the playing field is getting bigger.

pzkpfw
2015-Jun-19, 01:06 AM
From our perspective though. If we apply that x is moving away from y at 20mph and y is moving away from z at 20mph which is situation described by universal expansion, you get that z moving away from x at 40mph. What you end up with is that to every frame of reference you are stationary but that everything else acceratates away as a factor of distance but nothing is actually moving at that speed rather the playing field is getting bigger.

It's true that neither z nor x is necessarily moving through space at 40 MPH, and likewise a very very distant Galaxy isn't moving through it's local space at a speed greater than light.

But, the very very distant Galaxies will eventually be moving faster than light, relative to us (something a local object can't do). I didn't think post #19 was clear on that, in saying "at no point expansion violating the speed limit".

(If anything, it's possibly more correct to say it is the expansion that 'violates the speed limit', as opposed to the objects in space at those far locations.)

Noclevername
2015-Jun-19, 11:24 AM
Why is everyone saying all happened in less then a second? Why was this first second so important that people say so many happened in such a short time? I keep hearing everywhere alot of stugg hapened in the first second. As if someone blinked and everything appeared

Something big happened during the first instant. We aren't sure what, but it happened in the context of all existence being filled with the maximum possible capacity of dense energy, that at that instant began expanding. As it did, it got cooler and less dense as all that energy suddenly found itself spread out over more space, thinning it out.

What came before that, at t=0, (the beginning of time as we know it) is impossible to know.

EDIT: I should add, when mass/energy is packed that densely, it does wonky things to the laws of physics. Like the singularity at the center of a black hole. Now imagine all the universe is like that...

https://en.wikipedia.org/wiki/Chronology_of_the_universe

Jeff Root
2015-Jun-19, 11:42 AM
Why is everyone saying all happened in less then a second?
It is an assumption. It tends to be treated by posters here
as if it were fact.

The expansion of the Universe was predicted by and is
described by general relativity, which is our current best
theory of how space, time, and energy relate to each other.
Observations beginning at about the same time as that
prediction was first made showed that widely-separated
galaxies are moving away from one another. The farther
apart two galaxies are, the faster they are moving apart.
Those observations tend to confirm the prediction of the
expansion made by general relativity. To see what might
have happened in the past, general relativity is used to
calculate the changes in space, time, and energy backward
through time, starting at the current time, from what is
currently observed. It was found that 13.7 billion years
ago, all the galaxies, all the matter in the Universe must
have been squished together in the same place. Because
the space itself has been expanding since then (according
to our best theory of how space works, general relativity),
all that squished-together matter filled all the space there
was at that early time. Using GR to extrapolate as far
back in time as possible, we reach a time we can refer to
as t=0, 13.7 billion years ago, when everything was in
exactly the same place. The entire Universe was squished
to zero volume and infinite density at that instant.

From that extrapolation comes the significance of the first
tiny fraction of a second of the Universe.

However, that is undoubtedly NOT what actually happened.

General relativity doesn't know anything about particles.
Things like photons, electrons, quarks, and so forth. General
relativity doesn't take their existence into account at all.
That doesn't matter very much for most of the extrapolation
back in time, but when we get close to t=0, it starts to be
critically important. What those particles do when they are
packed very tightly together could affect the extrapolation
profoundly. Our theory of particles (quantum theory) is not
yet able to say how particles behave under those conditions
of extreme density and temperature. So we don't know how
the extrapolation done using general relativity might be
affected.

Even worse, we don't know how the Universe came about,
or how the expansion started. New physics is required to
describe that. It might be that the matter was created over
a period of time, so that the density and temperature in the
first fraction of a second were never extremely high. It might
be that the creation of matter and expansion of space have
been going on forever, so there was no t=0 at all.

We do know that around one minute after the time general
relativity predicts for t=0, the density and temperature were
high enough for fusion of protons into deuterium, tritium,
helium, and lithium to begin. We also know that the fusion
only lasted for a couple of minutes before the density and
temperature fell so much that the fusion could not continue.
We know that from the relative quantities of hydrogen,
deuterium, helium, and lithium that exist throughout the
Universe in places where they haven't been changed much
by thermonuclear processes in stars. But before the time
that fusion began, all we can do is extrapolate backwards,
and the extrapolation is very likely to be wrong at some
point.

-- Jeff, in Minneapolis

ToBOrNotToB
2015-Jun-19, 07:56 PM
Thanks for all the answers. Now I know why everyone says all happened in less then a second. About all the universe being like the center of a black hole I don't even want to imagine. Infinite this, infinite that, no thanks, it's beter not to imagine ... uh :)

Last night I was watching on internet a physician called Lawrence Krauss presenting a drawing in which he explained how the galaxies are moving away from eachother, and why wherever you are in the universe it appears like you are in the center of it. But there is this thing which I don't get. Would be nice to know more opinions about it.

It made a very clear picture of everything, with one exception. You (the center of the universe) appear static comparing to all other galaxies which are moving, as if you would stay still. I wanted to ask this before but I didn't know how, now that I see the image I know how to. If all galaxies are moving away from us, where are we moving to? I don't belive we are staying still. Bellow the image of the drawing atached.


20666

Jeff Root
2015-Jun-20, 12:36 AM
where are we moving to?
It has been pretty well established that there is no such
thing as "absolute position" or "absolute motion". That
is, there is no way to say where a thing is or in what
direction a thing is moving except relative to other things.
Because the Universe (or the part of it we can see) is
expanding quite uniformly, it is possible and sometimes
convenient to describe motion of galaxies relative to the
overall "flow" of galaxies around them. Any galaxy might
have motion that is slightly different from the overall flow
of galaxies around it. This is called "proper motion" or
"peculiar motion". Our galaxy appears to currently be
moving at about 600 kilometres per second in the direction
of the constellation Leo, relative to galaxies around us.

The overall "flow" of galaxies has no direction. There is
nothing to measure it relative to. All galaxies are simply
moving away from each other, at least those that are
already widely-separated. The farther apart two galaxies
are, the faster they are moving apart. Galaxies which are
close together may be orbiting each other or moving closer
together.

So aside from our peculiar motions (which could include
the motion of the Solar System around the center of the
Galaxy, and the motion of the Earth around the Sun), there
is no way to say that we are moving in a particular direction,
or even that we are moving at all.

-- Jeff, in Minneapolis

.

Cougar
2015-Jun-20, 11:33 AM
Using GR to extrapolate as far
back in time as possible, we reach a time we can refer to
as t=0, 13.7 billion years ago, when everything was in
exactly the same place. The entire Universe was squished
to zero volume and infinite density at that instant.

As you say, using General Relativity alone, this kind of conclusion results. But such a state is impossible and meaningless. It is telling us that GR "breaks down" at such an early time, since it yields this nonsense answer. All theories have a range of applicability, and the first seconds of the universe are outside of GR's range.


What those particles do when they are
packed very tightly together could affect the extrapolation
profoundly. Our theory of particles (quantum theory) is not
yet able to say how particles behave under those conditions
of extreme density and temperature. So we don't know how
the extrapolation done using general relativity might be
affected.

I think this is incorrect. Large particle accelerators can indeed simulate "those conditions of extreme density and temperature" well within the first second after the beginning of the expansion, and these experiments are what inform our theory of quantum physics. Of course, such experiments are also limited in how far back they can simulate, but from what I've read, such experiments have reproduced states as far back as 10-17 seconds after the beginning.


It might be that the matter was created over
a period of time, so that the density and temperature in the
first fraction of a second were never extremely high. It might
be that the creation of matter and expansion of space have
been going on forever, so there was no t=0 at all.

I believe such conjectures have been ruled out since their implications are inconsistent with observations.


But before the time
that fusion began, all we can do is extrapolate backwards,
and the extrapolation is very likely to be wrong at some
point.

Well, we do have more than blind extrapolation. We observe that the CMB photons coming from opposite directions in the sky are almost exactly the same temperature. But these oppositely incoming photons are coming from regions that are so far apart, there's not enough time in the history of the universe that such regions could have ever been in contact with each other, i.e., they could not have been causally connected. How, then, could their temperatures be almost exactly the same? This implies, pretty compellingly, that there must have been a brief period of superluminal inflation early within the first second. We don't say this as a "fact," but rather as an explanatory model that is consistent with what we observe.

Jeff Root
2015-Jun-20, 10:27 PM
Using GR to extrapolate as far back in time as possible,
we reach a time we can refer to as t=0, 13.7 billion years
ago, when everything was in exactly the same place.
The entire Universe was squished to zero volume and
infinite density at that instant.
As you say, using General Relativity alone, this kind of
conclusion results. But such a state is impossible and
meaningless. It is telling us that GR "breaks down"
at such an early time, since it yields this nonsense
answer. All theories have a range of applicability,
and the first seconds of the universe are outside of
GR's range.
I disagree that GR "breaks down", that the result is
"nonsense", and that the first seconds are outside of
GR's range.

General relativity works just fine all the way back to a
time of t=0, the instant at which it predicts everything
to have been at exactly the same place. The math does
not fall apart. GR gives a clear prediction of a singularity.

That result is not "nonsense". It simply conflicts with
what we believe we know about the behavior of matter.
It seems unlikely that such a singularity could occur if
matter is present. At the least, it seems weird. Infinite
density seems like an impossibility.

So one question that should naturally arise is, "Was any
matter present at t=0?" If there was no matter, then not
only was the density not infinite, it was zero!

The thing is, there is no explanation of how all the matter
that exists now could have come about if it didn't exist at
t=0, so you are assuming that it DID exist at t=0, so you
have this problem with infinite density.

We KNOW that new physics is required to describe events
close to t=0. I don't know what that new physics might
be, but it COULD explain how matter was created where
there was none previously. I personally expect that that
is the case, and I personally expect that this creation took
some period of time -- that it wasn't instantaneous. Even
if it only took a few picoseconds. But it is also possible
that it took much longer, or has been going on forever.




What those particles do when they are
packed very tightly together could affect the extrapolation
profoundly. Our theory of particles (quantum theory) is not
yet able to say how particles behave under those conditions
of extreme density and temperature. So we don't know how
the extrapolation done using general relativity might be
affected.
I think this is incorrect. Large particle accelerators can
indeed simulate "those conditions of extreme density and
temperature" well within the first second after the beginning
of the expansion, and these experiments are what inform
our theory of quantum physics. Of course, such experiments
are also limited in how far back they can simulate, but from
what I've read, such experiments have reproduced states as
far back as 10-17 seconds after the beginning.
That's what I said.

Assuming your figure of 10-17 second is correct, we
know a little bit about what conditions COULD have been like
at that time. It ignores the new physics which we KNOW is
required at or very shortly before that time. Maybe we really
have "reproduced" conditions as far back as 10-17 second,
but going back before that is entirely extrapolation.




It might be that the matter was created over
a period of time, so that the density and temperature in the
first fraction of a second were never extremely high. It might
be that the creation of matter and expansion of space have
been going on forever, so there was no t=0 at all.
I believe such conjectures have been ruled out since their
implications are inconsistent with observations.
No. Some possibilities have been ruled out. Mostly, certain
combinations of possibilities have been ruled out. But the
range of remaining possibilities is still vast. It still includes,
for example, the possibility of "eternal inflation", in which
"our" Universe is not the only universe spawned by this
creation process. In that case there was no t=0, or t=0
represents just a point in the expansion when something
changed locally, and "our" Universe resulted. It is at least
conceiveable that this "eternal inflation" has been going on
forever, without any beginning.

I would like to go very much farther into the details of what
constraints are put on the possibilities by observations of
the CMBR, but that is way beyond my capability.




But before the time that fusion began, all we can do is
extrapolate backwards, and the extrapolation is very likely
to be wrong at some point.
Well, we do have more than blind extrapolation. We
observe that the CMB photons coming from opposite
directions in the sky are almost exactly the same
temperature.
That is an extremely important observation. But note that
those photons were emitted hundreds of thousands of years
after the Big Bang occurred.



But these oppositely incoming photons are coming from
regions that are so far apart, there's not enough time in
the history of the universe that such regions could have
ever been in contact with each other, i.e., they could not
have been causally connected.
This notion seems to be almost universally accepted.
I have never accepted it. One reason I haven't is that
it depends on the assumption that the GR description of
the expansion holds all the way back to t=0. Which you
rejected at the beginning of your post!

If expansion and creation of matter occurred together in
the Big Bang, then it does not follow that regions which
are now widely separated were not initially in causal
contact. It isn't a question of there being time in the
history of the Universe for them to communicate with
each other. They could have initially been in causal
contact, and would have almost exactly the same
temperature because they have a common origin.



How, then, could their temperatures be almost exactly
the same? This implies, pretty compellingly, that there
must have been a brief period of superluminal inflation
early within the first second. We don't say this as a
"fact," but rather as an explanatory model that is
consistent with what we observe.
That is certainly a possibility consistent with what we
observe. It has the distinct advantage of explaining
several different observed phenomenae.

But it is also possible that the expansion did not begin
from a state of infinite density and infinite temperature,
but rather that matter and spacetime were created
together, from a single origin, and as a result, the
temperature of all the matter created could naturally
be almost exactly the same.

I think the evidence suggests that such a creation event
probably lasted less than a second-- perhaps much less
than a second. But it isn't obvious to me that it conflicts
with inflation theory, or even with eternal inflation. So I
can't rule out any of those possibilities.

-- Jeff, in Minneapolis

Cougar
2015-Jun-21, 01:28 AM
GR gives a clear prediction of a singularity.

Which means exactly what?


The thing is, there is no explanation of how all the matter
that exists now could have come about if it didn't exist at
t=0...

Actually, I don't know of any explanations that have the matter there at the beginning. At that point, it's just energy content of the vacuum, with negative pressure.



Maybe we really
have "reproduced" conditions as far back as 10-17 second,
but going back before that is entirely extrapolation.

Well, yeah, the Grand Unified Theories are fairly conjectural, but there's also some convergence in their unification energies that is supportive of the idea. What do you call an extrapolation that is thereafter supported in some detail by observation/measurement?



But the
range of remaining possibilities is still vast. It still includes,
for example, the possibility of "eternal inflation"....

This certainly seems to be the popular one, lol. Journalist Amanda Gefter seemed to take this as the current prevailing theory, but I don't think she's the only one, since she was informed by her interviews with leading contemporary scientists.

I don't deny such a possibility (I much enjoyed Vilenkin's 2006 book), but mere possibilities don't really cut it. [What kind of idiom1 is that?] There are perhaps quadzillion possibilities, but if none are tested, well....


That is an extremely important observation. But note that
those photons were emitted hundreds of thousands of years
after the Big Bang occurred.

Yes, but if there are regions outside causal contact now, there must have already been regions outside causal contact at 370,000 years. Or are you saying there is no Horizon Problem (https://en.wikipedia.org/wiki/Horizon_problem) and there never was a Horizon Problem?

_______________
1 It is estimated that there are at least twenty-five thousand idiomatic expressions in the English language (https://en.wikipedia.org/wiki/English_language).

Jeff Root
2015-Jun-21, 03:48 AM
GR gives a clear prediction of a singularity.
Which means exactly what?
I don't know what is unclear to you about that statement.
Given the most basic assumptions about the Universe,
general relativity predicts that the expansion began at
a singularity, in which time and space began. Since I
said in an earlier post that general relativity describes
the relations between space, time, and energy, I'll say
right now that I think GR also predicts that energy began
to exist at the singularity, along with spacetime, though
I haven't followed the equations closely enough to be
able to confirm that.




The thing is, there is no explanation of how all the matter
that exists now could have come about if it didn't exist at
t=0...
Actually, I don't know of any explanations that have the
matter there at the beginning. At that point, it's just
energy content of the vacuum, with negative pressure.
The prediction of the "energy density of the vacuum"
(as made by quantum mechanics) is said to be about
120 orders of magnitude.larger than the measured
vacuum energy density for the observable universe.

Ouch.

I consider the idea of "vacuum energy" to be poorly
defined.

I consider matter and energy interchangeable, at least
to the extent that neither exists without the other. So
when I said "matter", I could just as well have said
"energy". I considered doing so, but I liked "matter".

I consider all particles to be "matter", including photons.
And I consider all electromagnetic radiation to consist of
photons, even if they can only be detected as waves.
If it has energy, I say, it is matter. Gravitational fields
arise from matter.

(Note that mass is not the same as matter.)




That is an extremely important observation. But note that
those photons were emitted hundreds of thousands of years
after the Big Bang occurred.
Yes, but if there are regions outside causal contact now,
there must have already been regions outside causal contact
at 370,000 years. Or are you saying there is no
Horizon Problem (https://en.wikipedia.org/wiki/Horizon_problem)
and there never was a Horizon Problem?
Depending on what you mean by "never was", I may
be saying that I think there never was a horizon problem.

The "horizon problem" is the idea that parts of the Universe
which are now widely separated could never have been in
causal contact, according to the basic prediction of general
relativity which describes the cosmic expansion, yet those
parts had almost exactly the same temperature when the
CBR was emitted.

I say that the solution to the horizon problem may be that
all the matter-energy of the Universe resulted from a single
"event" (which may have been protracted in time), in which
all that matter-energy was originally in causal contact, so
naturally it would all have the same temperature. The
expansion also resulted from that same event, so it was not
possible for different parts of the Universe to not have been
in causal contact at the beginning. They could only move
out of causal contact due to the expansion. The density of
the matter-energy would never have been extremely high,
as it is presumed to have been in the basic GR prediction
and in the Inflation scenario prior to inflation. But new
physics would be required to describe how that occurred.

-- Jeff, in Minneapolis

Cougar
2015-Jun-21, 11:48 AM
Since I
said in an earlier post that general relativity describes
the relations between space, time, and energy, I'll say
right now that I think GR also predicts that energy began
to exist at the singularity, along with spacetime, though
I haven't followed the equations closely enough to be
able to confirm that.

Well, Tony Rothman, GR prof at Harvard, has followed the equations closely. He describes it like this:



"We mentioned that the FLRW cosmology begins with a singularity. This is a much more serious breakdown than a flat tire or a cracked engine block. It is, in fact, a physical impossibility -- a region where the laws of physics break down altogether and even spacetime comes to an end."


A singularity is something like division by zero. No can do.


The prediction of the "energy density of the vacuum"
(as made by quantum mechanics) is said to be about
120 orders of magnitude.larger than the measured
vacuum energy density for the observable universe.

That's for the current vacuum that we live in. We were talking about the state of the universe around the time t=0. Regardless of what theory you like, I think we can say that conditions were considerably different during that early epoch.


I consider the idea of "vacuum energy" to be poorly
defined.

Well yeah, that's why QM comes up with such a monstrously bad estimate for it. I think that's because it's an attempted measure of the combined energy of virtual particles that obey the Uncertainty Principle, and which appear for only a brief instant and then disappear again. This measurement cannot be made with, like, a thermometer.


I say that the solution to the horizon problem may be that
all the matter-energy of the Universe resulted from a single
"event" (which may have been protracted in time), in which
all that matter-energy was originally in causal contact, so
naturally it would all have the same temperature.

Of course. That's the only reasonable conclusion. But that's not a solution to the problem.


They could only move
out of causal contact due to the expansion.

But during the first 7 or 8 billion years, the expansion was slowing. AIUI, that means regions that were outside our light horizon would be entering our visible universe (if we'd been around then). In other words, distant regions would be coming into causal contact, not moving out of it. The slow change from deceleration to acceleration of the expansion has not had enough time or effect to explain the distance between the CMB coming from opposite sides of the sky.

Jeff Root
2015-Jun-21, 10:20 PM
Well, Tony Rothman, GR prof at Harvard, has followed
the equations closely. He describes it like this:


"We mentioned that the FLRW cosmology begins with
a singularity. This is a much more serious breakdown than a
flat tire or a cracked engine block. It is, in fact, a physical
impossibility -- a region where the laws of physics break
down altogether and even spacetime comes to an end."


A singularity is something like division by zero. No can do.

Is your point there a repetition of the assertion that the
singularity predicted by general relativity cannot have
happened? If so, you are wrong.

First, a singularity is NOT a region. A singularity is a point.
That doesn't necessarily mean a point in space. In this case
it is a point in time. The singularity predicted by GR is not a
stretch of time, it is a point in time.

Second, a singularity is not a place where physical laws
"break down". It is a place where some value goes to zero,
or to infinity, or both. Generally one value goes to zero
while its reciprocal goes to infinity.

The Earth's poles are classic examples of singularities that
actually exist and don't cause any real problem. When you
reach the north pole, north suddenly no longer exists, and
every direction becomes south. Of course, those are just
co-ordinate singularities, not physical, and are no different
from a zillion other singularities that could be pointed out
all around us. Basically, whenever anything reaches a value
of zero, you have a singularity. General relativity simply
predicts, via the FLRW metric, that the Universe began from
zero volume at a particular time. The chances are pretty
good that that prediction is correct. Your objection, and
Tony Rothman's objection, is that the GR/FLRW prediction
also predicts all the mass-energy that exists now to exist at
the beginning. That's why he refers to a "region", because
the prediction of an absurdly high density isn't just at the
singularity, but for a time after the singularity as well.

I thought I made it perfectly clear that the scenario I am
describing avoids that absurd density by postulating that
the mass-energy came into existence as the spacetime
came into existence, and that it would do so over some
(maybe extremely short) period of time. That eliminates
the problem with the "singularity". There could still be a
singularity -- a point in time at which the Universe began --
but it would not have had the absurd density which is the
cause of your objection.

If GR's prediction of a singularity isn't correct, chances are
pretty good that the reason it isn't correct is that the time is
off, because the expansion began more gradually than the
basic GR / FLRW calculation predicts.



That's for the current vacuum that we live in. We were
talking about the state of the universe around the time t=0.
Regardless of what theory you like, I think we can say that
conditions were considerably different during that early epoch.
Okay. I won't argue with that!




I say that the solution to the horizon problem may be that
all the matter-energy of the Universe resulted from a single
"event" (which may have been protracted in time), in which
all that matter-energy was originally in causal contact, so
naturally it would all have the same temperature.
Of course. That's the only reasonable conclusion. But
that's not a solution to the problem.
What??? Why do you say that? It should be obvious that
It can be a solution. I think it probably is the solution.




They could only move out of causal contact due to the
expansion.
But during the first 7 or 8 billion years, the expansion was
slowing. AIUI, that means regions that were outside
our light horizon would be entering our visible universe
(if we'd been around then). In other words, distant regions
would be coming into causal contact, not moving out of it.
The slow change from deceleration to acceleration of the
expansion has not had enough time or effect to explain
the distance between the CMB coming from opposite sides
of the sky.
I said clearly, and you quoted it, and you even agreed,
IMMEDIATELY above, that everything in this scenario
would have started out in causal contact. There was
NOTHING that did not start out in causal contact. So the
amount of time that went by and events which occurred
after that are irrelevant. You could disagree with the
premise, but you just agreed with it. So why do you
immediately ignore the premise and disagree with the
conclusion?

-- Jeff, in Minneapolis

Cougar
2015-Jun-21, 11:30 PM
I said clearly, and you quoted it, and you even agreed,
IMMEDIATELY above, that everything in this scenario
would have started out in causal contact. There was
NOTHING that did not start out in causal contact. So the
amount of time that went by and events which occurred
after that are irrelevant. You could disagree with the
premise, but you just agreed with it. So why do you
immediately ignore the premise and disagree with the
conclusion?

I'm saying that it appears that there must have been superluminal inflation after the primordial causal contact. I believe that the mainstream view has it that the virtual particles that appeared during inflation got inflated and provided the initial density fluctuations that later manifested as the temperature fluctuations in the CMB.

Else, how do the sources of the cosmic microwave background photons on opposite sides of the sky get to impossibly far apart?

As wiki puts it:


Basic concept

When one looks out into the night sky, distances also correspond to time into the past. A galaxy measured at ten billion light years (https://en.wikipedia.org/wiki/Light_years) in distance appears to us as it was ten billion years ago, because the light has taken that long to travel to the viewer. If one were to look at a galaxy ten billion light years away in one direction, say "west", and another in the opposite direction, "east", the total distance between them is twenty billion light years. This means that the light from the first has not yet reached the second, because the 13.8 billion years that the universe has existed is not a long enough time to allow it to occur. In a more general sense, there are portions of the universe that are visible to us, but invisible to each other, outside each other's respective particle horizons (https://en.wikipedia.org/wiki/Particle_horizon).

WayneFrancis
2015-Jun-22, 02:25 AM
Thanks for all the answers. Now I know why everyone says all happened in less then a second. About all the universe being like the center of a black hole I don't even want to imagine. Infinite this, infinite that, no thanks, it's beter not to imagine ... uh :)

Last night I was watching on internet a physician called Lawrence Krauss presenting a drawing in which he explained how the galaxies are moving away from eachother, and why wherever you are in the universe it appears like you are in the center of it. But there is this thing which I don't get. Would be nice to know more opinions about it.

It made a very clear picture of everything, with one exception. You (the center of the universe) appear static comparing to all other galaxies which are moving, as if you would stay still. I wanted to ask this before but I didn't know how, now that I see the image I know how to. If all galaxies are moving away from us, where are we moving to? I don't belive we are staying still. Bellow the image of the drawing atached.


20666

One thing to note is all of those dots, to them selves, are not moving. In actuality none of them are. The space between them is changing. This is why you can consider any place the centre of the universe or more importantly that there is no centre with regard to space. We can consider there to be a centre with regard to time tho.

So the various galaxies are not moving relative to each other when you look at cosmic expansion. It is only this distance between them that is increasing. There is proper motion but these proper motions are small in scale to the recession velocities we are dealing with at these scales.

Krauss is a very good communicator as long as you don't get offended by his philosophical positions on religion but he doesn't get into that on his purely science talks so much. Oh and he's a physicists, a physician is a medical doctor :) I would not trust Krauss with a scalpel to remove something from my body :)

ToBOrNotToB
2015-Jun-22, 11:54 AM
One thing to note is all of those dots, to them selves, are not moving. In actuality none of them are. The space between them is changing. This is why you can consider any place the centre of the universe or more importantly that there is no centre with regard to space. We can consider there to be a centre with regard to time tho.

So the various galaxies are not moving relative to each other when you look at cosmic expansion. It is only this distance between them that is increasing. There is proper motion but these proper motions are small in scale to the recession velocities we are dealing with at these scales.

Krauss is a very good communicator as long as you don't get offended by his philosophical positions on religion but he doesn't get into that on his purely science talks so much. Oh and he's a physicists, a physician is a medical doctor :) I would not trust Krauss with a scalpel to remove something from my body :)

Hehe, sorry, my bad, of course I ment physicist :D

Oki, doki, so...
Whats the diference between space expanding and galaxies moving away from eachother. Isn't the same thing?

I mean, let's reduce it to a more simple approach, one that my dumb head can understand :D ... Let's just assume for a second we are in the of the universe and there are 4 galaxies around us, moving away from us, as folowing: 1 North of us, 1 South of us, 1 East of us and 1 West of us.

Let's also assume space can only increase in the 4 cardinal directions (North, South, East, West). Now, if all are moving away from us in one of those 4 directions, or let me put it your way, the space between us and the other 4 is increasing in one of the 4 coresponding directions, doesn't that make us a static point? Otherwise how is us getting further and further away from the other 4 of them?

But it might all be a missunderstanding. After I readed jeff's post I think there is a small confusion. I keep hearing the next quote "all the universe is expanding and accelerating". That's a bit missleading because as jeff stated earlier, space betwen some galaxies is actually decreasing, not increasing. So not all the univers is expanding and accelerating, but only some parts of it.

What I wonder is what could cause some galaxies to move away from one another while others get closer? ... Assuming of course all the universe started from a singularity. Doesn't make too much sense.:)

pzkpfw
2015-Jun-22, 07:47 PM
...
Let's also assume space can only increase in the 4 cardinal directions (North, South, East, West). Now, if all are moving away from us in one of those 4 directions, or let me put it your way, the space between us and the other 4 is increasing in one of the 4 coresponding directions, doesn't that make us a static point? Otherwise how is us getting further and further away from the other 4 of them?
...


That Galaxy to our North also has a Galaxy to its North, South (us), East and West. They are all moving away from it. The same goes for the Galaxies to our West, East and South.

Those Galaxies can consider themselves a static point, just like we could, but there's no objective "truth" to that. We can't all be static.


...
What I wonder is what could cause some galaxies to move away from one another while others get closer? ... Assuming of course all the universe started from a singularity. Doesn't make too much sense.:)

Locally, there can be effects from gravity. The expansion of the Universe isn't (yet) "pulling" Earth apart, or "pulling" Earth away from our Sun, or "pulling" all Galaxies away from their neighbours. Gravity is keeping them together.

If all matter were expanding away from all other matter completely evenly, we wouldn't see the Universe that we do.

ToBOrNotToB
2015-Jun-22, 10:49 PM
...
Locally, there can be effects from gravity. The expansion of the Universe isn't (yet) "pulling" Earth apart, or "pulling" Earth away from our Sun, or "pulling" all Galaxies away from their neighbours. Gravity is keeping them together.

If all matter were expanding away from all other matter completely evenly, we wouldn't see the Universe that we do.

The expansion of the Universe pulling galaxies appart is an effect. I was actually thinking what could cause that effect. Since once all the universe was a singularity once, then there was the big bang, then the inflation, then gravity, shouldn't actually all the universe act the same way?

This is yet again only an asumption but is the misterious dark energy 100% responsible for the expansion? Is it posibble that the expansion could be caused by something else, something more understandable, like, i don't know, some sort of colissions? I know for example the black hole at the center of a galaxy can throw out a star from the galaxy in certain conditions, I was wondering if something similar might be hapening on a larger scale.

Otherwise, without, such events, why isn't the universe acting the same way?

Cougar
2015-Jun-23, 02:18 AM
The expansion of the Universe isn't (yet) "pulling" Earth apart, or "pulling" Earth away from our Sun, or "pulling" all Galaxies away from their neighbours. Gravity is keeping them together.

Correct. Or drawing them together. Expansion, however, is only detectable when there is a lot of space between us and a distant object, like a billion lightyears worth of space. Expansion is really weak, but it's cumulative - it adds up.

Jeff Root
2015-Jun-23, 03:31 AM
Whats the diference between space expanding and
galaxies moving away from eachother. Isn't the same thing?

I mean, let's reduce it to a more simple approach, one that
my dumb head can understand :D ... Let's just assume for a
second we are in the of the universe and there are 4 galaxies
around us, moving away from us, as folowing: 1 North of us,
1 South of us, 1 East of us and 1 West of us.
I'm guessing that you meant to say "Let's just assume for a
second we are in the center of the universe". If you meant
something else, please say. Assuming that we are in the
center of the Universe doesn't change anything, so it isn't
an assumption that needs to be made for your question.



Let's also assume space can only increase in the 4 cardinal
directions (North, South, East, West). Now, if all are moving
away from us in one of those 4 directions, or let me put it
your way, the space between us and the other 4 is increasing
in one of the 4 coresponding directions, doesn't that make
us a static point? Otherwise how is us getting further
and further away from the other 4 of them?
Allow me to play grammarian for a second and point out that
you should have said "how are we" rather than "how is us".
I don't know why you made that mistake so I don't know
whether it is helpful for you to have it pointed out.

The galaxy in the center can certainly be considered to be
static in that scenario. But you could look at any of the five
galaxies and consider that galaxy to be static, while the other
four are moving. There is no way to distinguish between a
galaxy which is moving and one which is static, because there
is no difference between them.

That's what we mean when we say that all motion is relative.
The only way to tell whether something is moving is to see
whether it is moving relative to some other thing that you are
considering to be static. As long as it isn't accelerating, you
can consider anything to be static.

This gives a clue why some things are referred to as moving
but widely-separated galaxies are referred to as getting farther
apart from one another "without moving". All the galaxies in
a small group have about the same speed relative to galaxies
in another small group far away from the first one. The speed
difference between the two groups is considered to be due to
the expansion of space, because it applies to all groups of
galaxies throughout the visible Universe.

We would still say that the galaxies are moving, rather than
that space is expanding, were it not for the fact that there is
no visible edge or center to the Universe. If we could see an
edge, we'd be able to say where the center is, and we might
then suppose the center to be static while all the galaxies
around it are moving away. But with no visible edge, every
place in the Universe looks like the center, and every place
can equally claim to be static. And this kind of geometry was
found by Albert Einstein to correctly describe the behavior of
spacetime *before* the cosmic expansion was discovered!
The observed expansion fits the description provided by
general relativity, which tends to confirm the validity of GR.
So general relativity's geometric description of spacetime is
the description used to describe the observed expansion, and
that description is of expanding space, rather than galaxies
moving through space.

Observationally, there isn't much difference, but the GR
description works very well and seems to be correct. If you
want to see a clearer distinction between relative motion of
objects through space versus the expansion of space, look
at places that have very strong gravity. Look close to the
event horizon of a black hole. There, the difference becomes
much more apparent.



But it might all be a missunderstanding. After I readed ...
"Read" is a funny word. The present tense and past tense
are spelled identically, but pronounced differently. That
must be terribly hard to learn just through ... reading!



... jeff's post I think there is a small confusion. I keep
hearing the next quote "all the universe is expanding and
accelerating". That's a bit misleading because as jeff stated
earlier, space betwen some galaxies is actually decreasing,
not increasing. So not all the univers is expanding and
accelerating, but only some parts of it.

What I wonder is what could cause some galaxies to move
away from one another while others get closer? ... Assuming
of course all the universe started from a singularity. Doesn't
make too much sense.:)
A lot of people ask why not all galaxies are moving apart.
It seems quite simple to me, and if you look at it as the
expansion of space versus the proper motions of galaxies,
I think it is even simpler.

Space throughout the entire Universe is expanding. It might
be expanding the same amount everywhere-- in every volume.
Or it might only be expanding between clusters of galaxies.
The expansion is so weak that it can't be detected at small
scales. We can't tell whether the space inside clusters of
galaxies is expanding or not, and we sure can't tell whether
it happens inside individual galaxies. The galaxies and
clusters of galaxies are held together by gravity. Gravity
opposes expansion. Inside individual galaxies and clusters,
gravity is strong enough that if the expansion of space is
happening there-- and again, maybe it is, or maybe it isn't--
the expansion is too weak to have any observeable effect on
the visible matter in that space. Gravity prevents the matter
from moving when the space it is in is expanding. It is only
over very great distances that the gravitational attraction
between galaxies becomes weaker than the expansion of
space, so widely-separated galaxies are able to go with the
expansion.

If you consider the Hubble flow to be due to the expansion
of space, and the motions of galaxies within clusters to be
proper motions, it may be easier to comprehend. Galaxies
which are moving closer together have proper motions due
to their mutual gravitational attraction, as a result of being
close to each other.

-- Jeff, in Minneapolis

.

WayneFrancis
2015-Jun-23, 04:21 AM
Hehe, sorry, my bad, of course I ment physicist :D

Oki, doki, so...
Whats the diference between space expanding and galaxies moving away from eachother. Isn't the same thing?


Then easiest answer here is that there is no actual acceleration which means there is no SR affects going on.
Take 3 points in space and synchronize clocks at those points. Let space expand and each will say the other 2 are moving away from them. But there will actually be no SR component to that recession. None of the points feels any acceleration at any time regardless of what the other 2 points say about their recession velocity.
Take 3 ships in non expanding space and try to reproduce the same separation and at least 2 of ships will have to undergo acceleration causing a SR affect.
We know distant objects can recede at faster then the speed of light but we also know that no body with mass can be accelerated to the speed of light.




I mean, let's reduce it to a more simple approach, one that my dumb head can understand :D ... Let's just assume for a second we are in the of the universe and there are 4 galaxies around us, moving away from us, as folowing: 1 North of us, 1 South of us, 1 East of us and 1 West of us.

Let's also assume space can only increase in the 4 cardinal directions (North, South, East, West). Now, if all are moving away from us in one of those 4 directions, or let me put it your way, the space between us and the other 4 is increasing in one of the 4 coresponding directions, doesn't that make us a static point? Otherwise how is us getting further and further away from the other 4 of them?


The space is expanding between them. They are not moving through an existing space.




But it might all be a missunderstanding. After I readed jeff's post I think there is a small confusion. I keep hearing the next quote "all the universe is expanding and accelerating". That's a bit missleading because as jeff stated earlier, space betwen some galaxies is actually decreasing, not increasing. So not all the univers is expanding and accelerating, but only some parts of it.



No space between galaxies is not every decreasing. Distance is but this is due to actual motion through space. In this case if you picked random spots around that actual motion the vector you would need to take to get to said moving object would change with time unless you where at one of two special points where the object was going directly towards or away from those 2 points. This is very different from space expanding.

So all parts of space are expanding but some objects are close enough that their gravitational attraction overwhelms the cosmic expansion.



What I wonder is what could cause some galaxies to move away from one another while others get closer? ... Assuming of course all the universe started from a singularity. Doesn't make too much sense.:)

Gravity. The only time you get galaxies moving closer together is because they are gravitationally bound and that attraction is greater than the rate at which the space between them is expanding.

ToBOrNotToB
2015-Jun-23, 08:34 AM
Allow me to play grammarian for a second and point out that
you should have said "how are we" rather than "how is us".
I don't know why you made that mistake so I don't know
whether it is helpful for you to have it pointed out.

...

"Read" is a funny word. The present tense and past tense
are spelled identically, but pronounced differently. That
must be terribly hard to learn just through ... reading!


-- Jeff, in Minneapolis

It helps me improve my english, thank your for pointing me that. English it's not my first language and I haven't had the chance to practice it lately too much, so it might be due to lack of practice. Please excuse my grammar mistakes.

Also please do excuse me for my posts appearing very late. My posts do not appear live as I press on the post reply button, they appear after they are moderated by a moderator, sometimes it takes a few minutes, sometimes it takes hours.

I do not mean we are the middle of the universe Jeff. We could consider as being because of everything we have here on Earth, but if we realy are, I belive noone can tell, at least not yet. That's not what I was pointing at, but rather looking to understand some things in the universe, about how we fit in the big picture. Also to make a sense out of the way it all happens, by using simple analogies.

I'm too dumb in science and cosmology to understand some things the way you guys do, that is definately true, but I'm trying to find out, if maybe, there could be some other more palpable explanations then dark matter and dark energy, these famous dark forces which we might never know how to handle or use. I think one of the scientists, can't remember his name now, but I'll look for it over the internet, stated "Dark energy is just us being ignorant and lazy".

And thank you all again for taking the time to answer. :)

ToBOrNotToB
2015-Jun-23, 11:51 AM
Gravity. The only time you get galaxies moving closer together is because they are gravitationally bound and that attraction is greater than the rate at which the space between them is expanding.

Ok, then again let me ask as a more simple analogy, applied to our own solar system. If we launch a space shuttle from Earth in space, it reaches outside Eath's gravitational pull, and then we stop it completly, as in 0 speed, complete stand still. Does the space shuttle get atracted by the Sun and it will eventually fall into the Sun?

Cougar
2015-Jun-23, 12:06 PM
"Read" is a funny word. The present tense and past tense
are spelled identically, but pronounced differently.

This bothers me, particularly since the past tense of lead ("Who will lead the parade?") is led.

Of course, there is no end to the oddities in the English language.

Cougar
2015-Jun-23, 12:28 PM
Since once all the universe was a singularity once, then there was the big bang, then the inflation, then gravity, shouldn't actually all the universe act the same way?

Yes. Except it changes with time. It evolves. As others have said, the gravity of neighboring bodies can overwhelm the weak expansion of space and bring them together. The nearby Andromeda galaxy and the Milky Way are moving toward each other, for example.


....is the misterious dark energy 100% responsible for the expansion?

Not as we currently envision it. When the superfast inflationary expansion stopped, the universe's expansion is thought to have kept going, like coasting, at a much more leisurely pace. The dark energy is invoked to be responsible for the recently (1998) discovered acceleration of the expansion. (https://en.wikipedia.org/wiki/Accelerating_universe)

WayneFrancis
2015-Jun-23, 04:13 PM
Ok, then again let me ask as a more simple analogy, applied to our own solar system. If we launch a space shuttle from Earth in space, it reaches outside Eath's gravitational pull, and then we stop it completly, as in 0 speed, complete stand still. Does the space shuttle get atracted by the Sun and it will eventually fall into the Sun?

Well technically the gravitational attraction goes on for ever it just reduces in a inverse square law. If you get a space shuttle out of Earth's orbit it is still in orbit around the sun. You'd have to get it so that it looses the velocity it got from coming from the Earth that is also in orbit around the sun. So basically fly in a direction away from the orbit that the Earth is orbiting. But yes if you got the space shuttle to go away from the Earth at 30km/s in the right direction then it would have no orbital speed relative to The Sun and fall into The Sun.

The trick is "stopping" is always relative to something else. Something "stopped" relative to the Earth is still orbiting the sun. Most things in space have relative speeds to other things thus don't tend to "fall into" them. They tend to orbit them.

Jeff Root
2015-Jun-24, 02:37 AM
It is common in English-language descriptions of orbits
to say that an object orbiting a gravitating body is "falling"
around that body. Thus the International Space Station is
falling around Earth, the Moon is falling around Earth, and
Earth, along with everything on it, and everything orbiting
it, is falling around the Sun. The Sun, along with everything
orbiting it, is falling around the center of the Galaxy.

I actually don't like this use of the term "falling", because
to me "falling" means "going lower"-- the opposite of rising.
So when I say that something in orbit is "falling", I generally
mean that it is in part of its orbit where it is getting closer
to the body it is orbiting. I agree completely with the way
you used the word "fall" here. It is the way I use it. I just
wanted to point out that some people use "falling" to mean
the same as "orbiting". At least sometimes.

-- Jeff, in Minneapolis

Jeff Root
2015-Jun-24, 02:54 AM
Any object brought to a standstill relative to the Sun, at
Earth's distance from the Sun, would take about 64 days
to fall into the Sun.

-- Jeff, in Minneapolis

ToBOrNotToB
2015-Jun-24, 11:51 AM
Yes. Except it changes with time. It evolves. As others have said, the gravity of neighboring bodies can overwhelm the weak expansion of space and bring them together. The nearby Andromeda galaxy and the Milky Way are moving toward each other, for example.


Yup, I know that. I also red somewhere the fact that Andromeda and MilkyWay are moving toward eachother at an angle of aprox. 90 degree, kinda perpendicular one to the other (no ideea how to explain this in english, sorry). It is presummed they will meet head to head in aprox 4 billion years.

Would be interesting tho, to find out for how long they are moving toward eachother. Is it since their begining or is just recently? How far they were at the moment they were created comparing to now?

Jeff Root
2015-Jun-24, 08:00 PM
I saw something hinting that the two galaxies may have
sideswiped each other long ago, moved apart, reached their
maximum separation, and are now coming back together.
That seems likely if they are on a collision course now.

I question whether the lateral relative motion between the
galaxies is known well enough to say that they will actually
collide, or will miss and zoom past each other.

-- Jeff, in Minneapolis

ToBOrNotToB
2015-Jun-24, 08:27 PM
I saw something hinting that the two galaxies may have sideswiped each other long ago, moved apart, reached their maximum separation, and are now coming back together. That seems likely if they are on a collision course now.

I question whether the lateral relative motion between the galaxies is known well enough to say that they will actually collide, or will miss and zoom past each other.

It's actually intriguing how little it is known about this issue, but if we follow the current standard model of the obs. universe, eventually they will collide, this time, or some next time they pass near by.

Don't get me wrong (and it's only my personal opinion) when I say we should focus more on the Andromeda matter before we draw conclusions about how the universe realy works, the observable one of course. Andromeda matter is one of the reasons I'm not 100% convinced we do understand the universe (referring here generaly to the the standard model, and particulary to the dark energy matter). I think we could learn alot from observing Andromeda more. I don't know for what reason we skip it so easily.

But is just a personal opinion.

Hornblower
2015-Jun-25, 01:25 AM
It's actually intriguing how little it is known about this issue, but if we follow the current standard model of the obs. universe, eventually they will collide, this time, or some next time they pass near by.

Don't get me wrong (and it's only my personal opinion) when I say we should focus more on the Andromeda matter before we draw conclusions about how the universe realy works, the observable one of course. Andromeda matter is one of the reasons I'm not 100% convinced we do understand the universe (referring here generaly to the the standard model, and particulary to the dark energy matter). I think we could learn alot from observing Andromeda more. I don't know for what reason we skip it so easily.

But is just a personal opinion.

My bold. Because on the cosmic scale the Milky Way and Andromeda are just two tiny specks out of billions of galaxies in the observable universe. We develop understanding of the overall universe by analyzing the general large scale patterns of motion, not by dwelling on the small scale local irregularities caused by the mutual gravitation of two galaxies that are close together.

WayneFrancis
2015-Jun-25, 02:39 AM
It's actually intriguing how little it is known about this issue, but if we follow the current standard model of the obs. universe, eventually they will collide, this time, or some next time they pass near by.

Don't get me wrong (and it's only my personal opinion) when I say we should focus more on the Andromeda matter before we draw conclusions about how the universe realy works, the observable one of course. Andromeda matter is one of the reasons I'm not 100% convinced we do understand the universe (referring here generaly to the the standard model, and particulary to the dark energy matter). I think we could learn alot from observing Andromeda more. I don't know for what reason we skip it so easily.

But is just a personal opinion.

What don't you understand with regard to this? The distance from the Andromeda galaxy and us is about 2.5 million light years
Space is expanding on the large scale at a rate of 71km/s/Mpc.
A megaparsec (Mpc) is 3,261,633.44 so the galaxies are 0.7664Mpc apart.
Cosmic expansion would be 54.42km/s
Yet our 2 galaxies are speeding towards each other at a speed of 111.66km/s over 2x that speed. This is due to the gravity.
So because the 2 galaxies are close enough gravity > cosmic expansion and gravity wins the tug of war

It isn't like there is a galaxy 5 billion light years from us that is seemingly racing towards us against this cosmic expansion.
We literally have redshift data for millions of galaxies and they all point to the same thing. I think that alone is a pretty good set of data to start to draw conclusions about how the universe is working, in this case with regard to expansion.

Cougar
2015-Jun-25, 12:28 PM
I think we could learn alot from observing Andromeda more. I don't know for what reason we skip it so easily.

Oh, it has gotten lots of attention. The expected collision even has its own wiki entry. (https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision) :)



In 2012, researchers came to the conclusion that the collision is definite after using the Hubble Space Telescope between 2002 and 2010 to track the motion of Andromeda. Andromeda's tangential or side-ways velocity with respect to the Milky Way was found to be relatively much smaller than the approaching velocity and therefore we expect it to directly collide with the Milky Way in about 4 billion years.



"Collision" is a misnomer, though. In galaxies, stars are tiny compared to the space between them. The galaxies are expected to move right through each other, with next to no actual stellar collisions.


It isn't like there is a galaxy 5 billion light years from us that is seemingly racing towards us against this cosmic expansion.

Right. Once we look at galaxies that are outside our local group and outside our local cluster, beyond a certain distance, all the galaxies appear to be moving away from us. Not only that. The farther away they are, the faster they appear to be moving, and they're doing so with a very definite relation between distance and recession velocity, which has come to be known as Hubble's Law. (https://en.wikipedia.org/?title=Hubble%27s_law)

ToBOrNotToB
2015-Jun-25, 07:31 PM
What don't you understand with regard to this? The distance from the Andromeda galaxy and us is about 2.5 million light years
Space is expanding on the large scale at a rate of 71km/s/Mpc.
A megaparsec (Mpc) is 3,261,633.44 so the galaxies are 0.7664Mpc apart.
Cosmic expansion would be 54.42km/s
Yet our 2 galaxies are speeding towards each other at a speed of 111.66km/s over 2x that speed. This is due to the gravity.
So because the 2 galaxies are close enough gravity > cosmic expansion and gravity wins the tug of war

It isn't like there is a galaxy 5 billion light years from us that is seemingly racing towards us against this cosmic expansion.
We literally have redshift data for millions of galaxies and they all point to the same thing. I think that alone is a pretty good set of data to start to draw conclusions about how the universe is working, in this case with regard to expansion.

I understand what you wrote completly. No doubts about. I was particulary refering to the "dark energy" (from the overall/general universal model). I'm not doubting the expansion of the universe observed through the redshift. I'm sorry if my post was somehow confusing. Anyway is just a personal opinion which I'm entitled to, not to fully be convinced of the existing of "dark energy".

Thank you both of you. Wayne and Cougar for your answers. :)

I'm reading that page now... Oh, what? Quoting from that page: "As of 2006, simulations indicated that the future Earth might be brought near the center of the combined galaxy, potentially coming near one of the black holes before being ejected entirely out of the galaxy." ... Please tell me this is in very small percentage possible. :rolleyes:

Cougar
2015-Jun-26, 12:35 AM
I'm reading that page now... Oh, what? Quoting from that page: "As of 2006, simulations indicated that the future Earth might be brought near the center of the combined galaxy, potentially coming near one of the black holes before being ejected entirely out of the galaxy." ... Please tell me this is in very small percentage possible. :rolleyes:

Heh. Relax. Have a cool refreshing beverage. Reading further down, a different group apparently did a more detailed study...


Based on current calculations they predict a 50% chance that in a merged galaxy, the Solar System will be swept out three times farther from the galactic core than its current distance.[2] (https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision#cite_note-muir-2) They also predict a 12% chance that the Solar System will be ejected from the new galaxy sometime during the collision.[9] (https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision#cite_note-Cain-9)[10] (https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision#cite_note-Cox2008-10) Such an event would have no adverse effect on the system and the chances of any sort of disturbance to the Sun or planets themselves may be remote.[9] (https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision#cite_note-Cain-9)[10]
(https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision#cite_note-Cox2008-10)


In any case, in 4 billion years, the Earth will no longer be a hospitable place due to the Sun's increasing luminosity. We're going to have to move anyway. :D

ToBOrNotToB
2015-Jun-26, 09:37 AM
In any case, in 4 billion years, the Earth will no longer be a hospitable place due to the Sun's increasing luminosity. We're going to have to move anyway. :D

Good point. Furthermore some physicists like Michio Kaku claim we are standing straight on a gun barrel of a posible gama ray burst due to a star exploding. I'll have to look for more informations to detail this, but if so, I shall not worry myself with it. Since, Lawrence Kraus sugests, it will heappen even before we can see it, so it's pointless to even worry about it :D


What about the Sun in 4 billion years? Isn't our Sun suppose to last for 6 billion years more in the actual configuration, with the same steady luminosity for example? What are you pointing at? ... Sounds like something to find more details about :)

antoniseb
2015-Jun-26, 10:54 AM
... some physicists like Michio Kaku claim we are standing straight on a gun barrel of a posible gama ray burst due to a star exploding ...
Yes, look that up. I suspect he never published such a thing, but rather on a TV show said very speculatively that "we could be ...". I've paid attention to this topic, and am unaware of any such star that actually has its rotational pole aimed at us.

Ken G
2015-Jun-26, 01:50 PM
This bothers me, particularly since the past tense of lead ("Who will lead the parade?") is led.
(Yet instead of misspelling the past tense of "read" like "red", we see people misspelling the past tense of "lead" like "lead"! The unfortunate crossover with that dense metal is causing the read/read problem to spread into a lead/lead problem too, we're going the wrong way.)

Hornblower
2015-Jun-26, 04:05 PM
Good point. Furthermore some physicists like Michio Kaku claim we are standing straight on a gun barrel of a posible gama ray burst due to a star exploding. I'll have to look for more informations to detail this, but if so, I shall not worry myself with it. Since, Lawrence Kraus sugests, it will heappen even before we can see it, so it's pointless to even worry about it :D


What about the Sun in 4 billion years? Isn't our Sun suppose to last for 6 billion years more in the actual configuration, with the same steady luminosity for example? What are you pointing at? ... Sounds like something to find more details about :)
The Sun is gradually increasing in luminosity, which it has been doing throughout its history to date. This is caused by changes in its internal dynamics as helium builds up in the core. It has brightened about 40% since settling down on the main sequence some 4.5 billion years ago. Fortunately for the biosphere our planet has been able to adjust to it so far. A further increase at this rate, though proportionately slight compared to what will happen in the red giant stage far in the future, will be enough to cook our biosphere to death in another billion years or so.

ToBOrNotToB
2015-Jun-26, 08:22 PM
Yes, look that up. I suspect he never published such a thing, but rather on a TV show said very speculatively that "we could be ...". I've paid attention to this topic, and am unaware of any such star that actually has its rotational pole aimed at us.

It's from this youtube movie. How the Universe Works s01e05 Supernovas - Full Episode (https://www.youtube.com/watch?v=Zn3xxMOkIGc)

The star is called double RR104 if i corectly undestood him. You can watch it from minute 28:00 for about 40 seconds. Is where he sais, I quote "Belive it or not, one of the stars is pointed at us...". Also he sais, I quote again "Īt's not a question of if, but a question of when"

If I correctly understood this two stars will die sending a GRB straight on earth :)

antoniseb
2015-Jun-26, 08:45 PM
Take a look at this Universe Today article (https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0CDQQFjAD&url=http%3A%2F%2Fwww.universetoday.com%2F23342%2Fw r-104-wont-kill-us-after-all%2F&ei=HbmNVZCMIcamyAS01onYBQ&usg=AFQjCNE_W4JSiCnX1APVgNkbrqVHgmPluQ&sig2=WoYfF0T4PKMAhJQUZ-hcbQ&bvm=bv.96783405,d.aWw), it shows that WR 104 is not aimed close enough to our direction. Michio Kaku's statement was already out of date when that episode first aired in 2010.

Jeff Root
2015-Jun-27, 12:27 AM
My understanding is that stars of mass similar to the Sun
go through a T-Tauri stage while they are forming, in which
they are much brighter than they will be when they enter
the main sequence. So, for a short time, the Sun was
probably a very bright T-Tauri star.

-- Jeff, in Minneapolis

Ken G
2015-Jun-27, 02:17 PM
And indeed the Sun was brighter still, in terms of total luminosity, prior to the T Tauri stage, though I don't know if it would be brighter in the visible portion of the spectrum with its red surface all that dust hanging around. We must take every opportunity to fight the misconception that the onset of nuclear fusion makes the Sun brighter, or indeed that nuclear fusion has much at all to do with the luminosity of a star like the Sun, other than changing the composition by turning hydrogen into helium. So Hornblower was only talking about the Sun's current state and its gradual brightening as its helium content grows-- its ancient history was an even brighter history, before the Sun reached its current state of radiative equilibrium over most of its interior.

Hornblower
2015-Jun-28, 01:36 PM
And indeed the Sun was brighter still, in terms of total luminosity, prior to the T Tauri stage, though I don't know if it would be brighter in the visible portion of the spectrum with its red surface all that dust hanging around. We must take every opportunity to fight the misconception that the onset of nuclear fusion makes the Sun brighter, or indeed that nuclear fusion has much at all to do with the luminosity of a star like the Sun, other than changing the composition by turning hydrogen into helium. So Hornblower was only talking about the Sun's current state and its gradual brightening as its helium content grows-- its ancient history was an even brighter history, before the Sun reached its current state of radiative equilibrium over most of its interior.

Yes, I was referring to the evolution of the Sun and Earth after the Sun had settled down on the main sequence. During the more luminous T Tauri period and earlier, the proto-Earth was not yet anywhere near ready to support a biosphere.