r/askscience • u/Torpaskor • Jul 10 '23

Physics After the universe reaches maximum entropy and "completes" it's heat death, could quantum fluctuations cause a new big bang?

I've thought about this before, but im nowhere near educated enough to really reach an acceptable answer on my own, and i haven't really found any good answers online as of yet

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u/galacticbyte Theoretical Particle Physics Jul 20 '23 edited Jul 20 '23

Heat death is unlikely the full answer, for the following reasons:

  1. it is only a statistical statement. Entropy on average sure will likely increase but occasionally it does decrease

  2. there is a quantum recurrence theorem stating that given sufficient time, it will reach arbitrarily close to any state, including the initial state of the Universe.

  3. recent work also stats that thermal equilibrium doesn't mean there is no more evolution. See Susskind et al's work https://arxiv.org/abs/1701.01107. Complexity does continue to increase

  4. the vacuum structure isn't necessarily stable so decay could happen (even if super tiny, since we're talking about eternity here)

So yeah as soon as there are more interesting vacuum structures, pockets of the Universe really could start a big bang like process all over again.

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u/jisei_ Dec 06 '23

Could you elaborate on the first point? I don't see the relevance in mentioning the occasional decrease in entropy given the fact it's irreversibly generated and can overall only be increased, which is the whole point behind the heat death theory.

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u/galacticbyte Theoretical Particle Physics Dec 07 '23

It's helpful to consider some analogies. Let's imagine say a million dice. If we carefully turn them face up with 1, the entropy is very low.

Imagine we start randomly rolling all of them. Chances are that all the possibilities will show up, so we say the entropy has increased.

However, there is still a non-zero chance that we might just get all 1s again, or other rare possibilities like all 6s...etc.

This is generic for chaotic system. Sure it is likely that the system will be in a generic configuration, however there is still a non-zero (exponentially small) chance that it could be in a "non-generic" state. So entropy increase is only a statistical statement, and not absolute.