Is General Relativity Wrong on Large Scales?

[Dotini]

GR "slightly" Wrong?

http://www.wired.com/wiredscience/2011/06/clumpy-universe/

The universe appears to be clumpier than astronomers expected, according to the largest galaxy survey to date. The extra clumps could call for a redesign of the standard model of cosmology, and maybe a new understanding of how gravity works.

“Maybe on very large scales, Einstein’s general relativity is slightly wrong,” said cosmologist Shaun Thomas of University College London, lead author of a new paper in Physical Review Letters. “This potentially could be one of the first signs that something peculiar is going on.”

The universe appears to be clumpier than astronomers expected, according to the largest galaxy survey to date. The extra clumps could call for a redesign of the standard model of cosmology, and maybe a new understanding of how gravity works.

The result could mean cosmologists need to reassess their understanding of dark energy, the mysterious force that drives the universe outward at an ever-increasing rate. Dark energy itself is supposed to be almost perfectly smooth, but clumps of dark energy could draw clumps of visible matter around them.

The extra lumps could also mean dark energy doesn’t exist at all. Instead, gravity could behave differently on very large scales than it does on smaller scales, meaning Einstein’s theory of general relativity needs an overhaul.


Respectfully submitted,
Steve

 

[bcrowell]

Here is the paper:

http://arxiv.org/abs/1012.2272

"Excess Clustering on Large Scales in the MegaZ DR7 Photometric Redshift Survey

We observe a large excess of power in the statistical clustering of Luminous Red Galaxies in the photometric SDSS galaxy sample called MegaZ DR7. This is seen over the lowest multipoles in the angular power spectra C_{\ell} in four equally spaced redshift bins between 0.45 < z < 0.65. However, it is most prominent in the highest redshift band at ~ 4 sigma and it emerges at an effective scale k ~ 0.01 h Mpc^{-1}. Given that MegaZ DR7 is the largest cosmic volume galaxy survey to date (3.3 (Gpc h^{-1})^3) this implies an anomaly on the largest physical scales probed by galaxies. Alternatively, this signature could be a consequence of it appearing at the most systematically susceptible redshift. There are several explanations for this excess power that range from systematics to new physics. This could have important consequences for the next generation of galaxy surveys or the LCDM model. We test the survey, data and excess power, as well as possible origins. "

 

[narrator]

Is this just clumping suggested of DM or does it involve other things?
Does this new data affect curvature models?
What other aspects would this affect?

 

[Chalnoth]

This is the most likely explanation, quoted from their abstract:

Alternatively, this signature could be a consequence of it appearing at the most systematically susceptible redshift.

One of the main issues here is that gravity on the largest scales is already tightly constrained by CMB observations, so it is highly unlikely that, in the end, this will end up being a real discrepancy.

 

[bcrowell]

Yeah, it does seem like kind of a weak argument: a simulation using GR predicts 1% clumping, but we observe 2% clumping. It's a very indirect way to test GR.

The Wired article says:

Instead, gravity could behave differently on very large scales than it does on smaller scales, meaning Einstein’s theory of general relativity needs an overhaul.

Well, structure formation happened when the universe's scale factor was a lot smaller than it is today, so it's not clear to me what scale is actually being probed. Also, the Einstein field equations are invariant with respect to changes of coordinates, including rescaling, so changing the behavior of gravity at large distance scales would require a pretty major change to the basic structure of GR.