Rate and direction of expansion of the universe

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Discussion Overview

The discussion revolves around the rate and direction of the universe's expansion, exploring whether this expansion is uniform in all directions and the implications of various theories, including the cosmological constant and quintessence. Participants also touch on related concepts such as Lorentz invariance and ongoing experiments aimed at understanding these phenomena.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that if one considers their location as the center of the universe, objects at equal distances will recede at the same rate, suggesting a uniform expansion.
  • Others inquire about ongoing or planned experiments to differentiate between cosmological constant theories and quintessence theories, which may exhibit spatial variation in expansion.
  • One participant mentions that the WMAP observation of the CMB provides strong evidence for uniform expansion, as it appears uniform to one part in 100,000.
  • There are questions regarding the implications of non-uniform expansion on local Lorentz invariance, with some arguing that local Lorentz invariance would not be violated by non-uniform expansion alone.
  • Another participant clarifies that global Lorentz invariance is already violated by the universe's expansion due to the changing metric of space.

Areas of Agreement / Disagreement

Participants express differing views on the uniformity of expansion and its implications for Lorentz invariance. While some agree on the uniformity suggested by observations, others raise questions about the potential for spatial variation and its consequences, indicating that the discussion remains unresolved.

Contextual Notes

There are limitations regarding the measurement of expansion with nearby objects, and the discussion acknowledges the complexity of distinguishing between different theoretical models of expansion.

asimov42
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Hi all,

I was wondering: to the best of our current observational knowledge, is the rate of expansion of the universe the same in all directions in space? (I realize it varies in time)
 
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If you choose your current location to be the center of the universe (you are free to do this because there is no true center) then all objects an equal distance radially from you will be receding at the same rate. The farther away these objects are the faster they will be receding so yes.
 
Thanks WannabeNewton!

Are there any experiments underway or planned to try to distinguish between cosmological constant theories for the expansion and quintessence theories (with spatial variation)?

WannabeNewton said:
If you choose your current location to be the center of the universe (you are free to do this because there is no true center) then all objects an equal distance radially from you will be receding at the same rate. The farther away these objects are the faster they will be receding so yes.
 
asimov42 said:
Thanks WannabeNewton!

Are there any experiments underway or planned to try to distinguish between cosmological constant theories for the expansion and quintessence theories (with spatial variation)?

Yes! One of the stated goals of the BOSS project (http://www.sdss3.org/surveys/boss.php) is to measure the evolution of the expansion history of the universe to a high enough level of accuracy to see if there is any time variation of the cosmological constant. I think there are also projects to measure more distant Type 1a supernovae with the same goal in mind, but I'm not sure.
 
asimov42 said:
Hi all,

I was wondering: to the best of our current observational knowledge, is the rate of expansion of the universe the same in all directions in space? (I realize it varies in time)
Well, it's a bit difficult to measure this with nearby objects, but our best observation for nailing this down is currently the WMAP observation of the CMB, which is uniform in all directions to one part in 100,000, which is strong evidence for expansion that is the same in all directions.
 
Ok, so uniform expansion via the cosmological constant does not violate local Lorentz invariance - if it was discovered that the expansion was, in fact, non-uniform in space, would local Lorentz invariance be violated?

Also, just to clarify - global Lorentz invariance is already violated by the expansion of the universe, because the underlying metric is changing (due to the expansion of space)? Is that correct? Sorry for all the questions, still learning! ;-)

Thanks all!
 
asimov42 said:
Ok, so uniform expansion via the cosmological constant does not violate local Lorentz invariance - if it was discovered that the expansion was, in fact, non-uniform in space, would local Lorentz invariance be violated?
Not at all. Local Lorentz invariance is a fundamental property of General Relativity. You'd have to have some (significant) deviation from General Relativity for local Lorentz invariance to fail. Simply having a non-uniform expansion wouldn't do that.

asimov42 said:
Also, just to clarify - global Lorentz invariance is already violated by the expansion of the universe, because the underlying metric is changing (due to the expansion of space)? Is that correct? Sorry for all the questions, still learning! ;-)
Correct! Lorentz invariance assumes a flat space-time. Any curvature, and it no longer holds.
 

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