[ExI] Expansion of the Universe

[Ben Zaiboc]

Anders Sandberg <anders at aleph.se> wrote:

> 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.  
...
> > 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.

Thanks, Anders, that's the kind of answer I was looking for (even though I didn't really understand it!).

So there are sound theoretical reasons (pun intended) why this idea is a non-starter.  OK, that's good enough for me.

Ben Zaiboc