I Could entropy be reversed eventually in the far future?

[Suekdccia]

TL;DR Summary

Could entropy be reversed eventually with infinite time in the far future?

In the far future there will be most likely a point where a maximal state of entropy will be reached in the universe and after the last black hole evaporates there could be no more structures and no more work could be done.

According to the Poincaré recurrence theorem for a closed universe there could be a reversal of entropy in long-term periods. However in a flat universe with an accelerated expansion it is not so clear that this theorem holds.

However, are there any other mechanisms through which entropy could be ever reversed eventually after reaching heat death? Perhaps something related with quantum fluctuations?

 

[Vanadium 50]

"Could entropy be reversed" makes no sense. Entropy is a thermodynamic quantity. Could temperature be reversed? Could heat capacity be reversed?

 

[PeterDonis]

In the far future there will be most likely a point where a maximal state of entropy will be reached in the universe

Not if the universe keeps expanding. There is no maximum entropy in a universe that expands forever.

 

[256bits]

Not if the universe keeps expanding. There is no maximum entropy in a universe that expands forever.

Isn't it assymtotic to a maximum.
In the far distant future with very few particles around the entropy of the cosmic event horizon predominates.
Presently the universe has a physical deficit from the maximum.

 

[PeterDonis]

In the far distant future with very few particles around the entropy of the cosmic event horizon predominates.

Ah, that's right, in the presence of a cosmological constant things will asymptote to de Sitter spacetime in the far future, which has a constant finite entropy for any given region.

 

[Suekdccia]

Ah, that's right, in the presence of a cosmological constant things will asymptote to de Sitter spacetime in the far future, which has a constant finite entropy for any given region.

Would temperature drop to 0K in such universe?

 

[PeterDonis]

Would temperature drop to 0K in such universe?

No, it would only drop to the horizon temperature, which is a finite value that depends on the value of the cosmological constant.

 

[Suekdccia]

No, it would only drop to the horizon temperature, which is a finite value that depends on the value of the cosmological constant.

Also, here it seems to indicate a mechanism through which new structures could form black holes even after the last one evaporates with denisty perturbations (https://arxiv.org/abs/astro-ph/9701131), see F.1. and VI. D. (Even for an open universe, the question of reaching heat death seems open)

 

[Suekdccia]

No, it would only drop to the horizon temperature, which is a finite value that depends on the value of the cosmological constant.

Also, if the cosmological constant vanished, would we be again in the case whwre maximal entropy is never reached?

 

[PeterDonis]

if the cosmological constant vanished, would we be again in the case whwre maximal entropy is never reached?

Yes, because then there would be no cosmological horizon and the "amount of universe" that was accessible would increase without bound.