Is the Universe's Expansion Uniform?

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Discussion Overview

The discussion revolves around the nature of the universe's expansion, specifically whether it is uniform across all regions or if different parts of the universe are expanding at varying rates. The conversation touches on theoretical aspects of cosmology and the implications of gravitational influences on expansion rates.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that the universe expands uniformly per volume of space outside of gravitationally bound objects, such as galactic clusters.
  • Others argue that the expansion rate can vary; for example, if two clusters are twice as far apart, they recede from each other at twice the rate of closer clusters.
  • A participant notes that while the universe is generally assumed to be uniform, this assumption may be questionable, especially beyond the observable universe.
  • It is mentioned that the acceleration of the universe has been increasing since about 6 billion years ago, indicating a change in the expansion dynamics over time.
  • Another participant expresses skepticism about the uniformity of expansion on smaller scales, suggesting that asymmetries likely exist.

Areas of Agreement / Disagreement

Participants express differing views on the uniformity of the universe's expansion, with some supporting the idea of uniform expansion under certain conditions, while others highlight potential variations and asymmetries. The discussion remains unresolved regarding the extent and implications of these differences.

Contextual Notes

There are limitations in the assumptions made about uniformity, particularly regarding gravitational influences and the behavior of the universe beyond observable limits. The discussion also reflects uncertainty about the implications of the cosmological constant and its effects on expansion over time.

nst.john
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I know the universe is expanding and the expansion is accelerating but I'd the entire universe accelerating at the same rate? Or are different parts moving at different rates?
 
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No sure what your exact question is. There are two possible answers.

(1) Yes, the rate is uniform per volume of space outside of gravitationally bound objects such as galactic clusters.

(2) No, if cluster A and B are twice as far from each other as clusters C and D are from each other, then A and B are receding from each other twice as fast as C and D are receding from each other.

EDIT: on the other hand, for (2), if galaxies E and F are the same distance apart as A and B then they are receding from each other at the same speed as A and B are from each other, so in that sense, it's a yes.
 
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OK I understand the examples and see how they fit. Thanks!
 
... entire universe accelerating at the same rate?

phinds sure covered the observable universe well...what's going on beyond that is assumed
to be the same...such scientific assumptions have shown themselves in the past to be questionable...

We assume it is uniform at all points in time because the negative pressure vacuum energy, aka cosmological constant, seems to have constant energy density in all of homogeneous and isotropic space...
which usually means at intergalactic scales and larger.

Also, the acceleration of the universe varies over time and has been increasing since about 6B years ago as we entered an energy dominated expansionary phase. THAT was a rather recent and unanticipated revelation.
 
nst.john said:
I know the universe is expanding and the expansion is accelerating but I'd the entire universe accelerating at the same rate? Or are different parts moving at different rates?

In general the universe expands homogeneously, but there could easily be asymmetries in this expansion (on a smaller scale), I'd be surprised if there weren't
 

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