Spacetime expansion - time dimension expansion

  • #36
exander said:
I was thinking about it a lot, and I am not sure how to explain cosmological red shift under conditions that just distance between comoving worldliness changes. How can this produce the effect of red shift? I always assume that the space in which the wave travels gets stretched.
I think the following excerpt from Manton, Nicholas; Mee, Nicholas. The Physical World: An Inspirational Tour of Fundamental Physics explains it nicely:

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  • #37
exander said:
I was thinking about it a lot, and I am not sure how to explain cosmological red shift under conditions that just distance between comoving worldliness changes.
You can explain it with mathematics. As above!
 
  • #38
Hill said:
I think the following excerpt from Manton, Nicholas; Mee, Nicholas. The Physical World: An Inspirational Tour of Fundamental Physics explains it nicely:

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This is interesting, but kind of vague. This looks to me more like the light wave hadn't changed, but the measure changed. Meaning everything shrunk. Can We interpret this as matter shrinking, and because of that the wave seems to have larger wave length? Is there a shrinking matter theory?
 
  • #39
exander said:
This is interesting, but kind of vague. This looks to me more like the light wave hadn't changed, but the measure changed.
Light doesn't inherently change. All redshift or blueshift is a result of the relationship between the source and receiver: relative motion, relative gravitational potential; or, separated by expanding space. Or, a combination of all three!
 
  • #40
exander said:
This is interesting, but kind of vague. This looks to me more like the light wave hadn't changed, but the measure changed. Meaning everything shrunk. Can We interpret this as matter shrinking, and because of that the wave seems to have larger wave length? Is there a shrinking matter theory?
This is standard GR. I can't find anything shrinking in this derivation. Which step is vague?
 
  • #42
exander said:
This looks to me more like the light wave hadn't changed, but the measure changed.
No. What changes is, heuristically, the angle in spacetime between the light ray's worldline and the comoving worldlines it passes. Each successive comoving worldline is at a slightly different angle in spacetime from the light ray's worldline. And that angle in spacetime is what determines the measured energy/frequency/wavelength of the light according to the comoving observer.
 
  • #43
PeterDonis said:
You are incorrectly describing what expands in our universe. It is not "spatial dimensions". It is the set of worldlines of comoving observers, which are observers who always see the universe as homogeneous and isotropic. The worldlines of these observers can be used to construct a very convenient coordinate chart for describing the universe, and in this chart the expansion of the set of worldlines can be, and often is, described as "space expanding". Which is unfortunate because it misleads people into thinking that the expansion is something it's not.
Sorry, the claim about comoving observers that see the universe as homogeneous and isotropic, actually means that each spacelike hypersurface of constant comoving coordinate time has a spatial positive definite metric homogeneous and isotropic.
 
  • #44
cianfa72 said:
the claim about comoving observers that see the universe as homogeneous and isotropic, actually means that each spacelike hypersurface of constant comoving coordinate time has a spatial positive definite metric homogeneous and isotropic.
Yes. But any such spacelike hypersurface is an abstraction; it's not a physical thing that we can observe to be "expanding". The actual galaxies we see are physical things that we can observe, and we can take their average motion and see that it corresponds to expansion.
 
  • #45
PeterDonis said:
No.
exander said:
This is interesting, but kind of vague. This looks to me more like the light wave hadn't changed, but the measure changed. Meaning everything shrunk. Can We interpret this as matter shrinking, and because of that the wave seems to have larger wave length? Is there a shrinking matter theory?

Einstein has used the idea of "shrinking" or "expanding" rulers in his popularization, "Relativity: The Special and General theory". See for instance https://www.bartleby.com/lit-hub/re...ry/xxiv-euclidean-and-non-euclidean-continuum. Einstein, however, uses the idea mainly as an example of one way of understanding non-Euclidean geometry, not applying it specifically to an expanding universe.

By making use of the following modification of this abstract experiment, we recognise that there must also be cases in which the experiment would be unsuccessful. We shall suppose that the rods “expand” by an amount proportional to the increase of temperature. We heat the central part of the marble slab, but not the periphery, in which case two of our little rods can still be brought into coincidence at every position on the table. But our construction of squares must necessarily come into disorder during the heating, because the little rods on the central region of the table expand, whereas those on the outer part do no

But it's really mostly a philosophical issue. It's philosophical because there isn't any obvious experiment to distinguish between a growing object and a shrinking ruler. The more common philosophy is to say that standardized rulers don't shrink - that's inherent in the idea of what a ruler does.

The idea of "shrinking rulers" occurs in a few other places, I believe MTW mentions it in "Gravitation", though I don't have a specific quote, and I recall at least one other paper that talks about this idea in the context of General Relativity, though it's name and author escape me. But while the general idea has occured to people, it hasn't gained much traction. I'm not aware of anyone publishing a peer-reviewed paper that specifically suggests applies this general idea to the expanding universe. I think the idea may run into some difficulties with instantaneous action at a distance , but that's my own thought, as I said I'm not aware of any peer-reviewed papers that discuss such an idea.

This is a bit of a longer answer than "No", but in the end the result is pretty similar.
 
  • #46
pervect said:
The idea of "shrinking rulers" occurs in a few other places
Thorne in his book "Black Holes & Time Warps: Einstein's Outrageous Legacy" describes and discusses the two "paradigms" in chapter 11. He concludes,
It is extremely useful, in relativity research, to have both paradigms at one’s fingertips. Some problems are solved most easily and quickly using the curved spacetime paradigm; others, using flat spacetime. Black-hole problems (for example, the discovery that a black hole has no hair) are most amenable to curved spacetime techniques; gravitational-wave problems (for example, computing the waves produced when two neutron stars orbit each other) are most amenable to flat spacetime techniques. Theoretical physicists, as they mature, gradually build up insight into which paradigm will be best for which situation, and they learn to flip their minds back and forth from one paradigm to the other, as needed. They may regard spacetime as curved on Sunday, when thinking about black holes, and as flat on Monday, when thinking about gravitational waves. This mind-flip is similar to that which one experiences when looking at a drawing by M. C. Escher, for example, Figure 11.2. Since the laws that underlie the two paradigms are mathematically equivalent, we can be sure that when the same physical situation is analyzed using both paradigms, the predictions for the results of experiments will be identically the same. We thus are free to use the paradigm that best suits us in any given situation. This freedom carries power. That is why physicists were not content with Einstein’s curved spacetime paradigm and have developed the flat spacetime paradigm as a supplement to it.
 
  • #47
Hill said:
Thorne in his book "Black Holes & Time Warps: Einstein's Outrageous Legacy" describes and discusses the two "paradigms" in chapter 11.
Note, though, that when Thorne says the two paradigms are "mathematically equivalent", he is glossing over an important technical issue. Locally, yes, you can use either description. But globally, the "field on flat spacetime" description might not work, because the global topology of the spacetime might not be the same as the global topology of flat spacetime, meaning that globally, it is impossible to view gravity as a field on flat spacetime. This is true of black hole spacetimes, for example.
 
  • #48
PeterDonis said:
Note, though, that when Thorne says the two paradigms are "mathematically equivalent", he is glossing over an important technical issue. Locally, yes, you can use either description. But globally, the "field on flat spacetime" description might not work, because the global topology of the spacetime might not be the same as the global topology of flat spacetime, meaning that globally, it is impossible to view gravity as a field on flat spacetime. This is true of black hole spacetimes, for example.
Where can I learn more about this limitation?
 
  • #49
Hill said:
Where can I learn more about this limitation?
I have not found any discussion of this in any detail in the literature, I expect because nobody actually uses the flat spacetime paradigm to analyze black holes. The main uses of the flat spacetime paradigm are in analyzing gravitational waves and weak fields.
 

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