I gather the precise validity of the Friedman equations depends on the universe being "homogeneous and isotropic", on a large-enough scale, yet not so large a scale as to render the terms meaningless.
But we keep finding the universe not to be uniform, even on a 100M parsec scale. A much larger scale would be uncomfortably close to the perceived size of the universe itself, flirting with that meaninglessness. So, it seems hard to know how the results are affected by such non-uniformity, or how much correction is needed.
But we keep finding the universe not to be uniform, even on a 100M parsec scale. A much larger scale would be uncomfortably close to the perceived size of the universe itself, flirting with that meaninglessness. So, it seems hard to know how the results are affected by such non-uniformity, or how much correction is needed.