[ExI] Expansion of the Universe

[Anders Sandberg]

On 2012-12-29 21:19, Ben Zaiboc wrote:
> Maybe not necessarily.  What always occurs to me when I read about the expanding universe, is the possibility of something analogous to sound waves in air, applied at a hugely huger scale.  Maybe the universe is reverberating to the big bang, or some similar long-ago event, and we are currently in the expansion phase of a universal 'sound-wave'.  Maybe in a few billion years the galaxies will start compressing together again, then in a few billion years after that, expand again, etc.
>
> Any theoretical objections to this idea?  I've never seen it discussed.

You are essentially arguing that the large-scale metric is dominated by 
the Weyl part of the the metric tensor (gravitational waves) and that 
the universe is not isotropic. I think some people have looked at it, 
since the standard models are isotropic, but there is *no* evidence for 
very large scale anisotropy. The closest things are hints that there 
might exist some weird twists and axiality to large-scale spacetime, but 
I have not seen any follow-up from the original claims.

What you suggest is very very large scale isotropy, so that the 
wavelengths are large enough not to be visible to us right now. I don't 
have enough "feel" for the Friedman equations to tell how this would 
play out. But I suspect the isotropic and homogeneous expansion would 
swamp the waves: they become more dominant in collapsing Mixmaster 
universes, so I suspect the opposite would be true for an expanding 
universe.




-- 
Anders Sandberg
Future of Humanity Institute
Oxford University