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My understanding is that the universe did not have enough time to thermalize before the epoch of recombination, so many patches of the sky were not in causal contact with each other, which means they should have different temperatures according to the original FLRW metric tensor.

How does this relate to the perfect blackbody radiation observed by Planck (2018)? Does GR predict something lumpy and inconsistent making it a less-than-perfect blackbody curve? It's my impression that you need inflation in order to make it a 'perfect' black body. Is that correct?

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The Shepard asked: "Does GR predict something lumpy and inconsistent making it a less-than-perfect blackbody curve?"

Relativity doesn't predict any of that, this problem arises from quantum fluctuations:

Wikipedia wrote: "One consequence of cosmic inflation is that the anisotropies in the Big Bang due to quantum fluctuations are reduced but not entirely eliminated."

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  • $\begingroup$ The quantum fluctuations have nothing to do with the thermalization of the universe prior to recombination. Either the photons were in causal contact with each other, or they weren't. I'm asking, in the scenario where the photons were not in causal contact, would that produce a less-than-perfect blackbody. $\endgroup$ Oct 23, 2023 at 12:28

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