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epenguin
2006-Feb-19, 06:45 PM
HELLO!
I'm glad to have just discovered this site: there are several things said in popular cosmology books (the only kind I can get into) that I'd like an authoritative justification for (in fact I'm beginning to find here info different from the messages of the said books, but the questions may still be worth asking.)

Firstly the Universe is expanding, or, running the film backwards, contracting. If, in reverse, distance to galaxy A is shortening at a given rate, Hubble's law makes the distance to something twice as far as A shorten at twice the rate, so everything comes together in a point at a time past estimated as 13.7 X 10^9 years ago. A child could understand that. Obvious, and simplest conclusion from the facts isn't it?

Not really. If I tell you the world population increased last year by about 0.077 X 10^9 (makes you think!) and is now about 6.4 X 10^9, you wouldn't say well if it increased by that amount every year, since 6.4 X 10^9 = 83 X 0.077 X 10^9 humanity must be 83 years old, in 1922 there were 0 people on earth. Rather you'd say that's a 1.2% annual increase, a doubling time of 57 years; if it's always been that then there were a couple of people around to start it all in about 120 AD. Which is more reasonable if still way off, the assumption that the doubling time was always what it is now has to be wrong.

If what is constant is the doubling time, then for the universe aged 13.7 X 10^9 years expanding at present rate then that is a fraction 7 X 10^-11 per year which works out as a doubling time of 9.5 X 10^9 Y. At the supposed date of origin of the Universe it would be instead of a point, a huge 0.38 of its present size. In this 'exponential model' there is much more time to fit things in. The Universe would never have been just a point, though there would be a very remote time when the density made present laws of physics break down.

It has therefore always seemed to me that there is something missing in the argument for the point (or tiny volume) and precise estimable-time origin of the universe. There must be an extra argument such as 'Einstein/Friedmann…theory predicts that the rate of expansion is constant' or 'you couldn't get the right helium abundance with the exponential model'.

Except that only while writing this I have thought of a possible answer. When looking at redshifted galaxies you are looking at them in a distant past. So a linear Hubble law means that it is linear not just at this instant but for some long time past? But I do not remember ever seeing this point, surely essential in the argument, mentioned in any of the books I've read.

This raises the question: for what fraction of that 3.7 billion years has the linear Hubble law been verified? Or had been a few years ago? (I seem to remember that some very large redshifts of more than factor 2 have been observed, but you'd need some estimate of distance independent of the redshift to be able to say the law was valid out there or back then.)

I'd be glad for an authoritative answer. This is not a question about what anyone may think but about what has actually been thought. Leave aside for the moment any evidence of acceleration of the expansion, since that has not got into the books yet, was not taken into account by their authors or sources, and may be subject to revision. Correct my figures or calculations if necessary, the exact figures shouldn't affect the arguments.

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Imprisoned in a Euclidian Newtonian well trying to leak out in every direction and all others.

epenguin
2006-Feb-19, 06:50 PM
For clarity I'd like to keep related questions distinct.

My second question is: how is the answer to my question 1 affected by evidence for acceleration of expansion? How good and how definitive is the evidence of this acceleration (considering the age of the Universe has been revised quite often). Does it affect the age conclusion? (Offhand I'd have thought that if the expansion varies smoothly and continuously with time, then, running the film backwards at first the distant points are catching up with the nearer ones even faster than before, but the closer and earlier it gets the more linear it gets so everything still finishes (i.e. started) in a point, maybe the exact age estimate changes a bit.)

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Imprisoned in a Euclidian Newtonian well trying to leak out in every direction and all others.

epenguin
2006-Feb-19, 06:55 PM
In everything I said and everything I've read it seems implicit or explicit that the early and present Universe were and are something of finite size. (Though now I think of it I don't think anyone I read ventured a size, mass or energy for the whole Universe.) Recently I read (Lineweaver and Davis Scientific American Feb 2005) that this is not true and that even many cosmologists have got it wrong. But if it's infinite now, doesn't it contract in the past to a mind-boggling infinite thing of infinite density, or to an even more mind-boggling finite thing of infinite density, which expands infinitely in a finite time?
I hope at least there is a Law of Nature that forbids it from being infinitely mind-boggling.



Imprisoned in a Euclidian Newtonian well trying to leak out in every direction and all others.

Ken G
2006-Feb-19, 07:02 PM
If I tell you the world population increased last year by about 0.077 X 10^9 (makes you think!) and is now about 6.4 X 10^9, you wouldn't say well if it increased by that amount every year, since 6.4 X 10^9 = 83 X 0.077 X 10^9 humanity must be 83 years old, in 1922 there were 0 people on earth.

Welcome to the forum epenguin, your analogy is a useful one to consider.
Here is the difference. You have only looked at current population and rate of change. It's all what you see now. But astronomers can look into the past, because light has a finite speed and the universe seems to be more or less the same everywhere at the same age (at least that is a perfectly working hypothesis). Thus to make your analogy, you would have to have the rate of growth of humanity over recorded history, at which point you would see that the exponential growth broke down within the last 83 years. Indeed, you would have a fairly complete history of just how the population has changed over time, though more than, say, a thousand years ago it would probably start to get uncertain due to limitations in the data. Astronomy is just the same, but the data doesn't start to get limited until you get back about five or ten billion years.

The second point is, in addition to the observed history of expansion, we also have the laws of physics. They are in principle a lot more specific than the laws of population growth, but we recently got two big surprises-- the expansion history only makes sense if you include dark matter and dark energy. So yes, the theory is lagging the observations, what else is new?



It has therefore always seemed to me that there is something missing in the argument for the point (or tiny volume) and precise estimable-time origin of the universe. There must be an extra argument such as 'Einstein/Friedmann…theory predicts that the rate of expansion is constant' or 'you couldn't get the right helium abundance with the exponential model'.

The way this is normally addressed is, you observe the Hubble law for the closest galaxy clusters, and using Occam's razor, you ask-- what happens if this law continues since the beginning? That's when you get more details of the initial conditions. Then you follow the standard scientific approach of looking to refine the model by including more physics. That's where your points about Einstein/Friedmann come into play. So yes, they do have to be in there, but as refinements to a simple concept. How far you want to go on those refinements is up to you, but you have to accept that at some point, the refinements are just going to be too detailed to make it into "popular parlance". This is what you object to, the fact that the basic story is pretty incomplete. But that always happens, in this case it's really not that far off from the complete models.


. So a linear Hubble law means that it is linear not just at this instant but for some long time past?
Bingo, I have should have read this part first. And yes, this should be stressed in texts, for the more clever readers like yourself.


This raises the question: for what fraction of that 3.7 billion years has the linear Hubble law been verified?
Look up a plot of the observed scale parameter as a function of time, a(t), from the observations of type Ia supernovae. That's the most complete timeline we now have. You'll see it's been pretty linear, but by no means exactly so, for about the last ten billion years out of the 13.7.

Ken G
2006-Feb-19, 07:12 PM
My second question is: how is the answer to my question 1 affected by evidence for acceleration of expansion?

A significant impact. The amount of redshift may be thought of as a record of the amount of expansion that has occured since the light was emitted.

How good and how definitive is the evidence of this acceleration (considering the age of the Universe has been revised quite often).

Depends on who you ask. But it's certainly not bad.


Does it affect the age conclusion?

Yes, you are right, it does. Ironically, the acceleration cancels some of the slowing down that happened early on, so the age comes close to what you'd get if you just naively use the current value of the Hubble parameter (and note this parameter does indeed change with the age of the universe).
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Imprisoned in a Euclidian Newtonian well trying to leak out in every direction and all others.[/QUOTE]

Ken G
2006-Feb-19, 07:17 PM
If it is infinite now, it was infinite as far back as the concept of time makes any sense. But we'll never know, it can't be observed. Note that the Big Bang is not a creation model, it is an evolution model. Either physics as we know it breaks down before you get back to a point of creation, or else something else we don't know about happened in there somewhere. Anyone who says the Big Bang starts at a point is uninformed, or is just trying to use an explanatory picture to help people who are confused. Personally, I think it creates more confusion than it resolves to start from a point. I also don't like saying "when the universe was the size of a grapefruit", etc.-- what they should say is, "when the universe we have been able to observe was the size of a grapfruit", with no implications for the "entire" universe.

Nereid
2006-Feb-20, 12:01 AM
Let me also welcome you to BAUT, epenguin!

I've merged the three threads you started, into just this one; they are all related, and I am confident that the three separate sets of questions that you asked can be adequately answered in just one thread.

FYI, if you haven't already found this, TalkOrigins' page on the Big Bang (http://www.talkorigins.org/faqs/astronomy/bigbang.html) is an excellent overview, resource, and place to go for succinct material on common misunderstandings.

epenguin
2006-Feb-20, 06:14 PM
Thank you. Yes it's best to keep them together. I just wanted to avoid a logical jumble, e.g. without acceleration, without the infinite universe I still had a problem.
Your answer and other things I'm finding have pointed me to rich information resources. (Which might inhibit me from asking other questions since the answer, so far as there can be one now, is probably there or linked.)

epenguin
2006-Feb-20, 06:44 PM
"So a linear Hubble law means that it is linear not just at this instant but for some long time past?"

yes, this should be stressed in texts, for the more clever readers like yourself.

I am most flattered. Well I'm glad I've sorted that out.
I could be wrong but I think hardly any authors mention it. As I'd always read this backwards deduction with a mental reservation I think I'd have noticed if any of quite a number of books I've read had said it. E.g. I don't think Singh, the most recent book I read does. Although it only takes a sentence or so, and they go into quite some detail aboout other confusions you might make.
Congratulations to yourself, it looks like you are principal mechanism for keeping this site on an even scientific keel.

RussT
2006-Feb-21, 10:41 PM
If it is infinite now, it was infinite as far back as the concept of time makes any sense. But we'll never know, it can't be observed. Note that the Big Bang is not a creation model, it is an evolution model. Either physics as we know it breaks down before you get back to a point of creation, or else something else we don't know about happened in there somewhere. Anyone who says the Big Bang starts at a point is uninformed, or is just trying to use an explanatory picture to help people who are confused. Personally, I think it creates more confusion than it resolves to start from a point. I also don't like saying "when the universe was the size of a grapefruit", etc.-- what they should say is, "when the universe we have been able to observe was the size of a grapfruit", with no implications for the "entire" universe.

[If it is infinite now, it was infinite as far back as the concept of time makes any sense.]

Our 'observible' universe is a sphere with a radius of approximately 13 billion light years, correct? So, if you go out in space in any direction 100 billion light years, and take that sphere with a radius of 13 billion light years, then that sphere would have been the size of a grapefruit, 13.7 billion years ago, correct?

Ken G
2006-Feb-22, 01:36 AM
[Our 'observible' universe is a sphere with a radius of approximately 13 billion light years, correct?

Actually, if you measure it in today's version of a light year, the universe we have observed is much larger than that, perhaps 30 billion LY or so (the light year has been shrinking. Or, as most would say the same thing, space has been expanding).


So, if you go out in space in any direction 100 billion light years, and take that sphere with a radius of 13 billion light years, then that sphere would have been the size of a grapefruit, 13.7 billion years ago, correct?
Yes, I think that's the idea exactly.

Ken G
2006-Feb-22, 02:10 AM
Well I'm glad I've sorted that out.
I could be wrong but I think hardly any authors mention it.

It's time for you to know the whole story, I think. The key issue is the cosmological principle, which says that the entire universe is doing the same thing everywhere at any given age. This allows you to specify the dynamics of the universe as a whole by a scale parameter as a function of age, call it a(t), which controls the distances between galaxy clusters. The scale parameter is 1 right now (by convention), and was smaller in the past. In this model, the distance to anything far away is the distance now, times the scale parameter. So how do the distances change? They change, on the largest scales, simply because of the way a(t) changes with time. But this means that a distance that is already twice as large will always be changing twice as fast-- i.e., you always get Hubble's law to be obeyed exactly. But this does not mean a(t) is linear in t, it works for any a(t). The problem is, we never measure the distance to something "now", so a difficult calculation always has to be made to translate the observations. The distinction gets more and more for very distant quasars, but is not important for relatively nearby galaxies. So what some people mean by the Hubble law is just a local measure of the fact that the cosmological principle is viable, and it is the cosmological principle itself that gives you the global meaning of the Hubble Law. It is true that books are rarely clear on this point! The observations that the whole universe goes to very high densities at a fixed age in the past is then just a statement that a(t) gets very small indeed about 13.7 billion years ago, as is consistent with general relativity.

Congratulations to yourself, it looks like you are principal mechanism for keeping this site on an even scientific keel.
I try to insert what insights I can, and try to be right as often as I can without ruining the fun of wild speculation. But there are many others on here as well who do the same, and keep everyone on their toes.

RussT
2006-Feb-22, 09:58 AM
Actually, if you measure it in today's version of a light year, the universe we have observed is much larger than that, perhaps 30 billion LY or so (the light year has been shrinking. Or, as most would say the same thing, space has been expanding).

Yes, I think that's the idea exactly.


And this should work the same all the way to infinity, in an infinite universe, correct?

Nereid
2006-Feb-22, 02:08 PM
[Our 'observible' universe is a sphere with a radius of approximately 13 billion light years, correct? Actually, if you measure it in today's version of a light year, the universe we have observed is much larger than that, perhaps 30 billion LY or so (the light year has been shrinking. Or, as most would say the same thing, space has been expanding).A gentle reminder that this thread is in the Q&A section, so we're sticking to mainstream astrophysics and cosmology. "expanding space" is the standard interpretation of the solutions to GR equations, when applied to the universe as a whole. Please, let's keep non-standard interpretations to other parts of BAUT (such as the ATM, General Science, or Astronomy sections).

So, if you go out in space in any direction 100 billion light years, and take that sphere with a radius of 13 billion light years, then that sphere would have been the size of a grapefruit, 13.7 billion years ago, correct?Yes, I think that's the idea exactly.Er, no.

For starters, anything beyond the observable horizon cannot be tested, so the sort of 'what if' thinking in RussT's question can only be answered within the framework of a particular theory. There are a great many theories that would then be available to be supplied as an answer, >10100 if you are interested in following the M-Theory/String Theory developments. Among these vast numbers of theories (more than the number of photons in the observable universe!), why choose one in particular?

RussT
2006-Feb-23, 08:55 AM
Nereid;

What is the problem here?

This is just applying the Cosmological Principle to the Big Bang Theory, isn't it?

Or that is what it should mean, right?

Hmmm, or is it?

Ken G
2006-Feb-23, 09:14 AM
Yes, RussT, actually the cosmological principle really is the mainstream model (whereas "expanding space" is not, by the way, that is actually a particular choice of how to picture the mainstream model, but I won't bother to go into that distinction here because I've done so elsewhere). So I would still answer your questions in the affirmative, within the contexts of the Q+A section, with a nod to Nereid's point that the question is hypothetical given the impossibility of knowing if the universe extends beyond what we can observe, or even if the cosmological principle is correct.

RussT
2006-Feb-23, 11:50 AM
actually the cosmological principle really is the mainstream model (whereas "expanding space" is not,

Ken, I don't understand what you could possibly mean when you say that "expanding space" is not mainstream?

Also, this is exaccerbating, because everytime I try to get a solid answer about what mainstream says is happening at the casual horizon with the galaxies just on the other side of it, I keep getting the answer that the Cosmological Principle "Must" apply. We must asume that the space there is the same as it is here, etc, etc, etc. Then when I come up with a perfecty good example I get the kind of responses like the above.

Nereid
2006-Feb-23, 02:16 PM
It can certainly get confusing, and to some extent that confusion is a reflection of different ways of thinking about the nature of the universe.

For example, the point of view that I take (well, I try to be consistent) distinguishes between what's observable1 (and what's not), between what's within the domain of mainstream theories (and what's not).

Thus, any time you ask about a domain that's beyond that of a theory, I would say you're speculating. An example: BBT in the Planck era.

Any time you ask about a part of the universe that's not observable, I would say you're speculating. An example: the universe beyond ~30 billion light years (the actual horizon is model dependent - as Lineweaver and Davis (among others) have pointed out).

So, when you write "get a solid answer about what mainstream says is happening at the casual horizon with the galaxies just on the other side of it", you are mixing up two things - what's observable (in this case, what's not), and what the best theory we have says (in this case, without the hugely important caveat about what's being assumed and what's not).

In regard of the way I think of the universe, I freely acknowledge that others may take a different view. For example, some may say we really are 'composed of electrons, quarks, etc', and that there really is 'a (mysterious) form of energy ('dark energy') which is accelerating the expansion of space'. I wish those who hold these kinds of views the very best, as they wrestle with the reality of what experiments testing the EPR paradox show (for example). I also wish them the very best as they re-adjust reality, as new observations and experiments dethrone older theories, and as new theories are shown to be consistent with those observational and experimental results.

1I've noticed that this term is sometimes confused with observed. 'Observable' means something like 'what we could see, with the most perfect telescopes etc that we can imagine building, based on the well-established physics we have today'; the -ed word is simply the sum total of all the plates, FITS files, mag tapes, etc.

Ken G
2006-Feb-23, 03:01 PM
Ken, I don't understand what you could possibly mean when you say that "expanding space" is not mainstream?

I was't saying it isn't mainstream, I was saying it is not a model. It is a commonly used way of picturing the way the mainstream model works. That's not the same thing as saying it is the only language that can be employed to properly explain the mainstream model. (And note the difference between "expanding universe" which is pretty close to an observed fact and "expanding space" which is merely a picture-- what is space anyway?).

RussT
2006-Feb-24, 01:26 AM
It can certainly get confusing, and to some extent that confusion is a reflection of different ways of thinking about the nature of the universe.

Yes it can be confusing sometimes, but I don't feel confused, as much as I feel frustrated, in trying to nail down answers. If we say that a model is correctly saying what is happening at the casual horizon (which from my view, is just how far we can actually see with our current technology), then that should mean some pretty specific things for galaxies just past that range.

It also seems like mainstream uses the Cosmological Principle whenever it suits their needs and then when we try to use it against you, you simply say that it is outside what we can know, so is therefore irrelevent, and speculative.



In regard of the way I think of the universe, I freely acknowledge that others may take a different view. For example, some may say we really are 'composed of electrons, quarks, etc', and that there really is 'a (mysterious) form of energy ('dark energy') which is accelerating the expansion of space'. I wish those who hold these kinds of views the very best, as they wrestle with the reality of what experiments testing the EPR paradox show (for example). I also wish them the very best as they re-adjust reality, as new observations and experiments dethrone older theories, and as new theories are shown to be consistent with those observational and experimental results.

Care to expound on this a little more, if not here, then in my thread?...

RussT
2006-Feb-24, 01:28 AM
I was't saying it isn't mainstream, I was saying it is not a model. It is a commonly used way of picturing the way the mainstream model works. That's not the same thing as saying it is the only language that can be employed to properly explain the mainstream model. (And note the difference between "expanding universe" which is pretty close to an observed fact and "expanding space" which is merely a picture-- what is space anyway?).


Very good point! Yes, an expanding universe is very different from "Space itself Expanding".

Nereid
2006-Feb-24, 01:55 AM
Yes it can be confusing sometimes, but I don't feel confused, as much as I feel frustrated, in trying to nail down answers. If we say that a model is correctly saying what is happening at the casual horizon (which from my view, is just how far we can actually see with our current technology), then that should mean some pretty specific things for galaxies just past that range.So, what - specifically - are you asking? Is it something about what's observable? Or is it what - given certain, clearly stated assumptions - a particular theory (or model) predicts (or doesn't)?
It also seems like mainstream uses the Cosmological Principle whenever it suits their needs and then when we try to use it against you, you simply say that it is outside what we can know, so is therefore irrelevent, and speculative.Perhaps you could be more specific? In my experience, it is very easy to draw what seems like a perfectly logical, innocent connection (or conclusion), only to find - later - that you've erred (e.g. invalid application of a principle, wrt to the theory; assuming applicability beyond the domain of the theory).
Care to expound on this a little more, if not here, then in my thread?...Sure. Could I ask you a favour, please? Start a thread, in the General Science section, with the text you've just quoted (please acknowledge me as the author). Then follow it with something like "how widespread is Nereid's view, among scientists?" (You could call the thread something like "Theory and Reality in Science - What Do Scientists Think?")

RussT
2006-Feb-24, 10:56 AM
Sure. Could I ask you a favour, please? Start a thread, in the General Science section, with the text you've just quoted (please acknowledge me as the author). Then follow it with something like "how widespread is Nereid's view, among scientists?" (You could call the thread something like "Theory and Reality in Science - What Do Scientists Think?")


No problem...it's done.