Expanding Universe or Expanding Space?

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mysearch
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The question in the title of this thread was triggered after reading a couple of papers that, while accepting the assumption of an expanding universe, seem to question whether space itself actually expands. These papers are:

http://arxiv.org/abs/0707.0380" [Broken]
Reference: ‘The Expansion of Space and Redshift’ in section 2.6.4

http://arxiv.org/abs/0809.4573" [Broken]
Reference: ‘The nature of the redshift’ in section 3

While the issue of expanding space has always seem a bit vague to me, mainly because I have never seen a fully rational explanation of the physics that explains how the vacuum of space expands, I have an issue with the premise of these papers based on the current cosmological model, which maybe somebody can help resolve. There are various ‘cosmic calculators’, e.g. Ned Wright, that effectively calculate the expansion of the universe as a function of time via associating redshift [z] to the scale factor [a]. The figures below are taken from the http://www.astro.ucla.edu/~wright/ACC.html" [Broken]:

As I understand it, a redshift of [z=1089] corresponds to the decoupling of radiation and matter some 0.378 Myrs after the Big Bang. Percentage-wise this is ~0.003% of the current estimated age of the universe. However, the figures of real interest are listed as:

The comoving radial distance: 45.665 Gly.
The angular size distance: 0.041891 Gly.


Again, as I understand it, the 'angular size distance' can be interpreted as the proper distance when the light left the object, while the 'comoving radial distance' in this case approximates to the particle horizon in the current era. On this basis, a redshifted photon arriving on Earth today associated with this event started off some 41 million lightyears in distance, but has been travelling for nearly 13.7 billion years, because the space through which it has been ‘self-propagating’ expanded on route to effectively 13.7 billion light years. The source that generated this photon has now receded to some 45 billion lightyears.

If all this is basically correct and aligns to the accepted model of cosmology, I am not sure how this can be explained other than through the expansion of space. However, I don’t know enough about the practical science of measuring a photon with energy [E=hf] and then associating this to a given redshift on the basis that it was originally sourced in the CMB. However, this might be a question for astro-physics. Anyway, would appreciate any insights. Thanks
 
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  • #2
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Those papers aren't arguing about the physics of the situation, they seem to be arguing about the metaphysical interpretation and linguistic description of that physics. Even that argument is incredibly unclear however. Additionally, their analysis ignores any concepts from quantum mechanics, or quantum field theory which suggest that "the expansion of spacetime" per se would be distinguishable (and seems consistent).

Those papers don't really make any sense to me, it sounds like they're just trying to be argumentative....
 
  • #3
Chronos
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Papers written by people who need a refresher course in GR, imo.
 
  • #4
Jorrie
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If all this is basically correct and aligns to the accepted model of cosmology, I am not sure how this can be explained other than through the expansion of space.
At least part of the (semantic) problem is that space is not a material or substance that we can easily think of as 'expanding'. For all we know, it is just that more space is 'being created' and that this increases the separation between unbounded particles/objects in free space. The fact that the cosmological constant (vacuum) energy density is constant and hence the total energy from it increases, may perhaps support such a view.
 
  • #5
Ich
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If all this is basically correct and aligns to the accepted model of cosmology, I am not sure how this can be explained other than through the expansion of space.
That's a very good question.
The cited paper's point is that "expanding space" is rather a coordinate choice than some new physics: space is defined as orthogonal to an expanding set of observers. So it should be possible to describe the situation if space is defined by a non-expanding set of observers ("non-expanding space").

Generally, non-expanding space is more positively curved that expanding space. Empty space, for example, when described in expanding coordinates, has a negative spatial curvature. OTOH, a flat expanding space has positive spatial curvature if described in non-expanding coordinates.

What you get when you use non-expanding coordinates (*) are these numbers: the light originated some 7.64 Gy ago and 10.56 Gly away. However, the angular size of the source is much bigger, because positively curved space acts like a giant gravitational lens for objects far enough away (See http://www.damtp.cam.ac.uk/research/gr/public/inf_lowden.html" [Broken], third picture). Not so elegant as in a flat spatial slice, but it still works.

(*) to be exact: I'm talking about radial proper distance in the space defined by a cogruence without radial expansion.



zhermes said:
Those papers don't really make any sense to me, it sounds like they're just trying to be argumentative....
Actually, these papers do make a lot of sense (the second even more than the first).
zhermes said:
"the expansion of spacetime" per se would be distinguishable
There's no "expansion of spacetime". There's an expanding universe (undisputed), and there are expanding congruences. "Expanding space" however is an "incredibly unclear" concept, that's not the fault of the papers which try to point this out.

What about this: You show me how to locally measure "expansion of space" per se, and I'll concede that the papers make no sense. You fail to do so, and you'll concede they may have a point by saying (on local physics):
peacock said:
The very idea suggests some completely new physical effect that is not covered by Newtonian concepts.
 
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  • #7
mysearch
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While experts on such matters may correct me, it seems that the papers do, at least, raise some legitimate questions as to whether an equally valid cosmological model can be constructed based on an expanding universe that does not necessarily require expanding space.

Of course, any model that is implicitly founded on the assumption that all of space-time had to exist within the boundaries of the singularity, i.e. nothing existed outside, might reasonably argue that the space within this universe must have expanded or possibly, as suggested in #4, created new space. However, it seems to be far more ‘acceptable’ these days to speculate on the possibility that our universe is only part of a much bigger universe. Some have even speculated that while our universe did expand, it did so into pre-existing space. However, I accept that such speculation may be outside the scope of this forum.

In the example given in #1, I cited the issue of a photon with a redshift of 1089, which the current energy-density model suggests started at an initial distance of 41 million lightyears, but had a light travel time of 13.7 billion lightyears and a comoving distance of ~46 billion lightyears. Initially, I thought this implicitly required space to expand, but on reflection I am not sure that this necessarily has to be the case.

For example, if the photons in question start and end with respect to physical matter, is it possible that this matter could have been pushed apart by negative pressure, such that the distance between the start and end locations became 13.7 billion years. Of course, there would still be the issue of redshift, which each paper as cited in #1 briefly discuss, although the arguments did not seem particularly strong. However, more by way of a general question, could the original source existed in a stronger gravitational field such that time was dilated more than in the present era creating a similar effect as Doppler shift?
 
  • #8
bcrowell
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While experts on such matters may correct me, it seems that the papers do, at least, raise some legitimate questions as to whether an equally valid cosmological model can be constructed based on an expanding universe that does not necessarily require expanding space.
I wouldn't even call it a model. I'd call it an equally valid verbal description of the same model. The math is the same either way, and the predictions are the same either way. Whether you choose to refer to it as expansion of space is simply a matter of what words you use to talk about the model. I don't think there is any serious controversy among experts about the actual predictions of the model, and only a minority are even interested in debating which is the better verbal description.
 
  • #9
mysearch
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I wouldn't even call it a model. I'd call it an equally valid verbal description of the same model. The math is the same either way, and the predictions are the same either way. Whether you choose to refer to it as expansion of space is simply a matter of what words you use to talk about the model. I don't think there is any serious controversy among experts about the actual predictions of the model, and only a minority are even interested in debating which is the better verbal description.
I am really surprised by such a comment. I would have thought there were significant differences in a model that describes the expansion of the universe with or without the actual expansion of space. While the authors of the 2 papers cited in post #1 may or may not be experts, they seem to believe that, at best, that the current description leads to misconceptions:

http://arxiv.org/abs/0707.0380" [Broken]
"While it remains the staple of virtually all cosmological teaching, the concept of expanding space in explaining the increasing separation of galaxies has recently come under fire as a dangerous idea whose application leads to the development of confusion and the establishment of misconceptions."

http://arxiv.org/abs/0809.4573" [Broken]
The idea of an expanding universe can easily lead to confusion, and this note tries to counter some of the more tenacious misconceptions. The worst of these is the ‘expanding space’ fallacy.

While these quotes may have only been intended to highlight the confusion of people, like myself, who are simply trying to gain a better understanding of the cosmological model, they seem to suggest a wider debate. For example, if space doesn’t expand, by what mechanism did the universe expand? What drives this expansion? What causes the observed redshift? Sorry, but at face value, this seems more than semantics. Thanks
 
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  • #10
Ich
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While these quotes may have only been intended to highlight the confusion of people, like myself, who are simply trying to gain a better understanding of the cosmological model, they seem to suggest a wider debate. For example, if space doesn’t expand, by what mechanism did the universe expand? What drives this expansion? What causes the observed redshift? Sorry, but at face value, this seems more than semantics.
But it isn't.
What all agree upon is that the standard Big Bang model is fully described by GR. You start with the field equations, add some constraints and "initial" conditions, and you get the correct value for all the observables. The rest is - or should be - semantics.
The problem is that many people, including quite a few physicists, are not aware of this. They think that, even on the smallest scales, "expansion" is fundamentally different from "ordinary motion". Just look at some of the reactions you got in this thread.
 
  • #11
apeiron
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They think that, even on the smallest scales, "expansion" is fundamentally different from "ordinary motion".
Isn't there a fundamental difference that is not captured by GR - the fact that the expansion is also a cooling?

"Ordinary motion" is inertial - energy is not being dissipated. But the expansion of space involves a matching drop in its average temperature.

Expansion and cooling are two sides of the same coin. The big bang is a phase transition and the universe is its own heat sink.

GR models the development of the universe in terms of a geometry of distance, but we seem to need a geometric model of thermalisation, of cooling, of entropy, to capture the dual nature of "expanding space".
 
  • #12
Ich
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Isn't there a fundamental difference that is not captured by GR - the fact that the expansion is also a cooling?
No, that's not a difference.
I tried to explain what happens in https://www.physicsforums.com/showthread.php?p=2740583#post2740583".
Basically, all particles get sorted by velocity, which drains the random part of their motion (aka temperature).
 
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  • #13
bcrowell
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I am really surprised by such a comment. I would have thought there were significant differences in a model that describes the expansion of the universe with or without the actual expansion of space. While the authors of the 2 papers cited in post #1 may or may not be experts, they seem to believe that, at best, that the current description leads to misconceptions:

http://arxiv.org/abs/0707.0380" [Broken]
"While it remains the staple of virtually all cosmological teaching, the concept of expanding space in explaining the increasing separation of galaxies has recently come under fire as a dangerous idea whose application leads to the development of confusion and the establishment of misconceptions."

Note the word "teaching." They're talking about whether one verbal description or another leads to more student misconceptions. It's a pedagogical debate, not a scientific one.​
 
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  • #14
mysearch
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But it isn't.
What all agree upon is that the standard Big Bang model is fully described by GR. You start with the field equations, add some constraints and "initial" conditions, and you get the correct value for all the observables. The rest is - or should be - semantics.
While I won’t try to pretend that my understanding of GR is any greater than my current understanding of cosmology, your statement seems to imply a uniformity of opinion, presumably in informed circles, that there is little controversy in modern cosmology and GR essentially explains all. Again, while I will admit that my reading is limited, it has not given me that impression.

I assume that by the standard Big Bang model, you include some form of inflation (?) for it was my understanding that without some form of inflation, the original Big Bang model could not account for current observations. But what degree of certainty exists on the issue of inflation and does GR fully explain this process? Other sources seem to cite major problems with GR as the universe is reversed back towards the Planck scale because of incompatibilities between GR and quantum theory.

While I have only quickly read the thread referenced in post #12; the question that it raised in my head was ‘what assumptions are being made about the universe from an energy conservation perspective within of each argument forwarded?’. See issue with dark energy below as an example.

Today, the basic energy-density model seems to be dependent of both dark matter and dark energy. These concepts don’t appear to be described or fully understood by the present-day particle model of accepted physics. As such, anybody trying to learn about cosmology must, at least, question whether these concepts were introduced so that GR could explain the ‘observables’. Equally, the ability of dark energy to maintain a constant energy-density during ‘expansion’ (whatever that really means) is not an easy concept to understand unless there is some net increase in the energy of the universe. Based on the standard model, it seems that the overall energy has to have increased by a factor of ~2.7 in the last 13.7 billion years. Does this affect the thermodynamic processes assumed by the current cosmological model?

I am not looking for answer to all these questions as I realise that the PF is not offering free personal tuition (although it is a valuable learning resource) because ultimately people must take responsibility for pulling themselves up the learning curving. However, cosmology seems to be based on so many assumptions that it is difficult to accept any statement of certainty without questioning, and trying to understand, the premise on which it is based. Anyway, I wasn’t looking for an argument, simply trying to highlight an issue that I believe many must face when coming to the PF looking for a helping hand. Thanks.
 
  • #15
Ich
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your statement seems to imply a uniformity of opinion, presumably in informed circles, that there is little controversy in modern cosmology and GR essentially explains all.
Not at all. I said that the standard model is fully described by GR. (Bad wording, I admit.)
But what degree of certainty exists on the issue of inflation and does GR fully explain this process?
The degree of certainty is irrelevant: if there is a standard model of the earliest epoch, it's certainly inflation.
And no, GR does not fully explain the process. Just like it doesn't fully explain atomic structures, stellar evolution, or the financial crisis. But GR is the only theory of gravity used in the standard model.
So if we're talking about different interpretations of "space expansion", GR is now the basis of these discussions. This may be different in 50 years, but now it's GR.
 
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If not for the metric expansion of space, the big bang theory would violate special relativity.

Would it not also cause problems with the primeval atom aspect of big bang?
 
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  • #17
bcrowell
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While I won’t try to pretend that my understanding of GR is any greater than my current understanding of cosmology, your statement seems to imply a uniformity of opinion, presumably in informed circles, that there is little controversy in modern cosmology and GR essentially explains all. Again, while I will admit that my reading is limited, it has not given me that impression.
Cosmology is a high-precision science these days. When you refer to our incomplete understanding of inflation and Planck-scale physics, you're talking about stuff that happened less than a microsecond after the big bang. That has nothing to do with the question of how to interpret the standard models at later times.

Based on the standard model, it seems that the overall energy has to have increased by a factor of ~2.7 in the last 13.7 billion years. Does this affect the thermodynamic processes assumed by the current cosmological model?
General relativity does not have a global law of conservation of energy. You can't even define the total energy of the universe. There is a good discussion of this on p. 457 of Misner, Thorne, and Wheeler.
 
  • #18
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I wouldn't even call it a model. I'd call it an equally valid verbal description of the same model. The math is the same either way, and the predictions are the same either way. Whether you choose to refer to it as expansion of space is simply a matter of what words you use to talk about the model. I don't think there is any serious controversy among experts about the actual predictions of the model, and only a minority are even interested in debating which is the better verbal description.
What about the issue of violating special relativity?
 
  • #19
apeiron
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No, that's not a difference.
I tried to explain what happens in https://www.physicsforums.com/showthread.php?p=2740583#post2740583".
Basically, all particles get sorted by velocity, which drains the random part of their motion (aka temperature).
I'm not seeing how this approach deals with a bath of photons in an expanding space. This would seem the deeper model of the universe - just cooling radiation, forget massive particles. Now the cooling (redshifting) and expansion are clearly two sides of the same coin. And any model has to be able to deal with a global coordinates story, not short-cut the description by measuring against a (good as) static frame.
 
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  • #20
Ich
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I'm not seeing how this approach deals with a bath of photons in an expanding space.
You're right, it doesn't.
But the thermal bath we're seeing stems from matter (surface of last scattering). You have this matter moving away from you, the farther the faster. So you have the light redshifted more and more, as it comes from increasingly farther sources. That's it.

BTW, I'm not trying to convince you to use static coordinates in this kind of problems. I just want to point out that it's possible to use different descriptions.
And any model has to be able to deal with a global coordinates story, not short-cut the description by measuring against a (good as) static frame.
I'm not proposing different models here. For modeling, you'd like to choose coordinates that reflect the symmetries of the situation, and are universally valid.
But that doesn't mean that different descriptions are impossible.

I don't mind which description you prefer to do cosmology.
But if you're thinking that local physics in an expanding universe is totally different from what we used to know before we heard of "expanding space", you're in good company - and dead wrong.
 
  • #21
apeiron
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For modeling, you'd like to choose coordinates that reflect the symmetries of the situation, and are universally valid.
Thanks for your explanation. I personally still think it is telling that cooling and expansion are an apparent symmetry of the situation. Can we tell the difference between an object (eg: event horizon) that is cooler or further away? If not, then "space" starts to look a little more than just a measure of distance.
 
  • #22
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Can't mix models. For example, the Hubble expansion is an observation, and separate form GRT model. The Hubble expansion seems to indicate a 3-surface (manifold) expansion. While might GRT suggest to some a hypercube expansion (faces or 4-cube?)? Separate models; which is more related to expansion of manifold?
 

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