Is the expansion of the universe uniform? Are there parts which are expanding faster than others?

Question

Is the expansion of the universe uniform? Are there parts which are expanding faster than others? The very fabric of the universe is expanding..but is it expanding at a constant rate? If it is not then how do we measure the distance of the stars as the light would get unevely streched in each width of space due to non-uniform expansion under Doppler 's effect?

G. Smith · Accepted Answer · 2019-07-07T16:25:33.700

1

The cosmological expansion is very highly isotropic (to about one part in a hundred thousand!), based on observations of the spectrum of cosmic microwave background (CMB) photons reaching Earth from completely different parts of the universe.

The standard cosmological model assumes that on the largest scales the universe is homogeneous as well as isotropic.

Addendum: If downvoters don’t believe this former cosmologist, perhaps they will believe Wikipedia:

Hubble demonstrated that all galaxies and distant astronomical objects were moving away from us, as predicted by a universal expansion. Using the redshift of their electromagnetic spectra to determine the distance and speed of remote objects in space, he showed that all objects are moving away from us, and that their speed is proportional to their distance, a feature of metric expansion. Further studies have since shown the expansion to be highly isotropic and homogeneous.

edited Jul 07 '19 at 16:25

answered Jul 07 '19 at 04:25

G. Smith

51,534

So do we have to correct for the ongoing expansion while the light is travelling to the Earth while applying Doppler's effect to calculate distance to the stars? – Schwarz Kugelblitz Jul 07 '19 at 04:28
Yes, that’s right. – G. Smith Jul 07 '19 at 04:28
How is that exactly done? – Schwarz Kugelblitz Jul 07 '19 at 04:28
Is is same to assume that the universe is isotopic without affecting other parameters? – Schwarz Kugelblitz Jul 07 '19 at 04:29
The mathematical details of cosmological redshift are not explainable in a comment. They would be covered in a course on General Relativity and cosmology. If you already understand GR and some cosmology, it might make a reasonable separate question. – G. Smith Jul 07 '19 at 04:31
I think you meant to ask “is it safe to assume...” isotropy. Based on the observed CMB isotropy, I would say it is. Why do you think it might not be? – G. Smith Jul 07 '19 at 04:36
Yes...'safe' got autocorrected to 'same' for some reason.I thought so because assuming that it is isotopic should end up affecting the geometry of the universe and it's consequent expansion.If we assume it to be isotopic and it is not then we would be putting unnecessary and limiting contraints on it. – Schwarz Kugelblitz Jul 07 '19 at 04:41
-1 pretty sure answer is incorrect. CMB does indicate the universe is isotropic, but not that it is expanding in an isotropic way. – Allure Jul 07 '19 at 06:23
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@Allure You’re wrong. If it were expanding anisotropically, the CMB redshift be different in different directions. – G. Smith Jul 07 '19 at 16:15
@Allure Read the Wikipedia quote that I’ve added. – G. Smith Jul 07 '19 at 16:28
@G. Smith Perhaps you have to make an additional assumption (that we are not in a special vantage point) for the uniformity of the CMB to imply isotropic expansion? – D. Halsey Jul 07 '19 at 16:57
Ah, I think we're talking past each other. The expansion might be approximately isotropic, but only because the universe is approximately isotropic. The question asks if there are parts which are expanding faster than other parts, and the answer to that is "yes". – Allure Jul 07 '19 at 21:19
I also think we are talking past each other, because you are considering small scales like humans and planets, which are not of interest to cosmologists. – G. Smith Jul 07 '19 at 21:50

score 1 · Answer 2 · answered Jul 08 '19 at 17:14

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Of course G. Smith's answer is right, but let me add a few things:

inside galaxies, space is not expanding, since gravity dominates over dark energy
now between galaxy clusters, space is expanding, dark energy is dominant, but the rate of expansion here is not as fast as in the voids of superclusters
On the large scale, the universe expands uniformly, but as you go from the space between galaxy clusters to viods between superclusters, the rate of expansion is increasing

Please see here:

https://physics.stackexchange.com/a/482814/132371

answered Jul 08 '19 at 17:14

Árpád Szendrei

28,452

Ok ..but there is no hard boundary to a galaxy right...we can only be sure of a general area in space where gravity and dark energy switch dominance? – Schwarz Kugelblitz Jul 09 '19 at 18:25
@HerambPodar yes you are correct – Árpád Szendrei Jul 09 '19 at 18:25

score 0 · Answer 3 · answered Jul 07 '19 at 06:25

The expansion of the universe is certainly not uniform. For example, you are not expanding - your local density is much higher than the critical density. Neither is the Earth expanding.

The universe is also not expanding at a constant rate. The rate of expansion is measured by the Hubble constant, which isn't really a constant - it varies with time.

score -3 · Answer 4 · answered Aug 20 '21 at 09:21

-3

Observations show that some galaxies are colliding.

This would suggest that at some point in the past they were farther away from each other.

Which means that in some regions of space, uniform expansion is a misnomer.

answered Aug 20 '21 at 09:21

Richard Shreiner

1

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Galaxies colliding is due to their gravitational attraction "overpowering" the expansion of space, so this is cannot be used as an argument. – jng224 Aug 20 '21 at 09:52

Is the expansion of the universe uniform? Are there parts which are expanding faster than others?

4 Answers4