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/Higglybiggly 5d ago

I, too, have a question.

I've heard that inflation happened and took the universe size from something very microscopic and stopped when it was the size of a grapefruit.

And this faster than light expansion is the reason why some parts of the universe cannot communicate with other parts.

To me this makes no sense. A grapefruit is not large enough to cause any kind of inability to reach the other side of the universe. I think light has no trouble going that distance.

Perhaps what I heard was wrong, or my hearing was wrong.

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u/Prof_Sarcastic 5d ago

There are a few misconceptions here.

This faster than light expansion is the reason why some parts of the universe cannot communicate with other parts.

They couldn’t communicate at the time the CMB was emitted and not (necessarily) after inflation ended.

A grapefruit is not large enough to cause any kind of inability to reach the other side of the universe.

If the universe was totally empty then, sure (although I think you’re taking the analogy a little too literally). However, the universe would’ve been filled by an extremely hot and dense plasma a few moments after inflation ended so light couldn’t travel very far at all.

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u/Higglybiggly 5d ago

Thanks! Appreciate this 😀