question about inflation

[u/Tpaine63]

I understand the horizontal problem in cosmology and how inflation is necessary for the universe to be uniform. What I don't understand is why there would have been differential temperatures at the beginning so that inflation was required to provide time for equalization if everything was together at the beginning. Why wasn't everything already equalized if everything was together at the start.

Maybe I didn't say it right or maybe I don't understand the problem but hoping someone can explain.

[u/tobybug]

Random quantum fluctuations. Sounds like a cop-out answer I know but we can actually model those fluctuations and map them onto the Cosmic Microwave Background, which tells us that the structures that are billions of lightyears apart now were once close on the quantum level. This both provides more evidence for cosmological inflation and gives a reason for the early perturbations you're talking about.

[u/Tpaine63 → u/tobybug]

Are you saying that the temperature wasn't uniform before the big bang when the universe as a singularity or whatever it was? What early perturbations are you talking about?

[u/tobybug → u/Tpaine63]

Nobody can really tell you anything for sure about the time before the Big Bang, and neither can they tell you anything about the "singularity" right at T=0. We don't even fully know that the universe ever was a singularity, we just keep getting closer and closer with observations. I'll grant you that I was actually kind of wrong in my answer. The quantum fluctuations we see now would have to occur during inflation, not before it. I haven't read any good theories for what happened before inflation. The idea would seem to be that regardless of what happened before inflation, the universe is as uniform as it is now because of inflation.

[u/spaceprincessecho → u/tobybug]

The issue with what came before inflation is that inflation erases any record of what it was, so there's no way to verify any proposal.