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This question is an upshot of a previous question asked by me.

The FRW metric of the Universe is based on homogeneity and isotropy of the universe on a length scale of 100 Mpc or larger. If we go back in time, do we expect the assumption of homogeneity and isotropy to be valid on even smaller and smaller length scales (i.e., the assumption expected to become better) or even larger and larger scales (i.e., the assumption expected to become poorer)?

I guess the assumption must become better because it is assumed to be valid close to inflation when the universe was tiny. But I'm not sure and cannot convince myself.

Qmechanic
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SRS
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2 Answers2

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We know that the cosmic microwave background is isotropic to within less than one part in a thousand (once the observed CMB has been adjusted to account for the Earth's motion). So it seems that the large-scale history of the universe is one of general isotropy and homogeneity with small scale perturbations (which may have originated very close to the Big Bang) being magnified by gravity into galaxy clusters, galaxies, solar systems, stars, planets etc.

gandalf61
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You assume the universe is isotropic and homogeneous through time as a whole, this is unchanging. The assumption is equally valid through time also, as all that is changing is the spacing between all matter, at an equal rate. You have to bear in mind this is an assumption made to help with ease of calculations, and isn't strictly speaking the case.

That said, there is some interesting reading about how the universe had to be slightly non-uniform at its beginning for planets, stars, solar systems etc all to form.

G.Bruce
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