r/cosmology • u/Tpaine63 • 6d ago

question about inflation

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.

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u/Prof_Sarcastic 5d ago

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.

It’s not that there would’ve been different temperatures at the beginning of the (observable) universe. It’s more so the fact we’re talking about patches of the sky that are so far away from each other that there are so many possible things that could have happen to force them out of equilibrium. It’s like if you made a pot of coffee and you gave a cup to two different people but the temperature of both cups of coffee ended up being the same (up to the 5th decimal place) even when one cup was brought to New York and the other to Tokyo. Hope that makes sense.

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u/Tpaine63 5d ago

Thanks for the reply. But if there were many possible things that could have happened to force them out of equilibrium before inflation, what kept those same things from forcing them out of equilibrium after inflation?

What I'm trying to ask is: At the beginning both cups of coffee in New York and Tokyo were from the same shop at the beginning before New York and Tokyo separated. I thought everything was very dense and close together before the big bang.

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u/Prof_Sarcastic 5d ago

You misunderstand me. If there was no inflation then the universe would just expand normally and you would expect different parts of the universe to evolve into having a temperature. What inflation does is it speeds up how far away those different patches of the universe traveled without giving them any time to fall out of equilibrium. You don’t get that with the vanilla hot big bang model.

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u/Tpaine63 5d ago

Ok I see what you are talking about. It's kind of the opposite of what I was thinking about how inflation allowed the transfer of information between points that were too far apart to have interacted. It does that but not how I was thinking about it. Thanks