Question

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?

Answer 1

The CMBR's near-perfect blackbody spectrum with minor temperature fluctuations supports the Big Bang and the Inflationary Hypothesis, which explains the uniformity of the universe despite the non-causal contact of distant regions.

Step-by-step explanation:

The cosmic microwave background radiation (CMBR) is an important piece of evidence for the Big Bang, featuring a near-perfect blackbody spectrum. However, the early universe was not perfectly uniform, and minor fluctuations in temperature are observable upon close examination. These fluctuations, or 'wrinkles', are key to explaining the formation of large-scale structures like galaxies and stars. Without these minor irregularities, the universe's matter would be too evenly distributed to coalesce into the structures we observe today.

The Inflationary Hypothesis addresses the seemingly paradoxical observation that regions of the sky that were not in causal contact (could not have exchanged information or energy) have almost identical temperatures. Inflation proposes a very rapid expansion of the universe, which would smooth out any initial irregularities, leaving behind an extremely uniform CMBR with only tiny temperature variations. These variations are consistent with the predictions and observations by the Planck satellite in 2018.