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I was wondering, given how in any other area of life making an explosion spherically symmetric is more or less impossible is there any reason to expect that the universe is? I appreciate that the FLRW has the advantage of being an exact solution, but is there any reason to suppose its an accurate one? Does WMAP allow a measurement of any inhomogeneity in the rate of expansion, or is it simply able to probe lumpiness.

I should probably have learned this in lectures but if I'm honest at the time I found cosmology a homogeneous, spherical cow in a vacuum too far.

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The reason cosmology is taken to be spherically symmetric is because the observed macroscopic universe actually is spherically symmetric on large scales. The evidence for this is the blackbody radiation spectrum, which is the same in all direction to 1 part in 100,000.

The spherical symmetry cannot be an accident, it requires an explanation, and this explanation is provided by inflation. If you assume that the universe had an approximately deSitter phase leading into the FRW phase, during the deSitter phase, it is driven to spherical symmetry, because the only stable maximum entropy configuration of a positive cosmological constant universe is the deSitter vaccuum, which is spherically symmetric.

A smooth end to inflation preserves the spherical symmetry, and sets the stage for the symmetric expansion that follows. The spherical symmetry is an extremely good approximation, in agreement with experimental data, not an ad-hoc simplifying assumption that makes the solution of the equations nice (although it does that too). The universe, it turns out, is a spherical cow!

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The metric actually follows from two very general properties of (or really assumptions about) the universe (on the large scale associated with cosmology): uniformity and isotropy. That is all that is needed. It is not just that it is spherically symmetric - it is that it is spherically symmetric from ANY point.

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