Could entropy be reversed eventually in the far future?

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Discussion Overview

The discussion centers around the concept of entropy in the context of the universe's far future, particularly whether entropy could be reversed after reaching a state of heat death. Participants explore theoretical implications related to the Poincaré recurrence theorem, the effects of cosmic expansion, and the role of the cosmological constant.

Discussion Character

  • Debate/contested
  • Exploratory
  • Technical explanation

Main Points Raised

  • Some participants propose that the Poincaré recurrence theorem suggests a potential for entropy reversal in a closed universe, though its applicability in a flat universe with accelerated expansion is questioned.
  • Others argue that the concept of reversing entropy is nonsensical, comparing it to asking if temperature or heat capacity could be reversed.
  • There is a contention regarding whether a maximum state of entropy can exist in an ever-expanding universe, with some asserting that it cannot.
  • Participants discuss the idea that in the far future, with very few particles, the entropy of the cosmic event horizon may dominate, leading to an asymptotic approach to maximum entropy.
  • Some mention that in the presence of a cosmological constant, the universe may asymptote to de Sitter spacetime, which has a finite entropy for any given region.
  • Questions arise about the temperature in such a universe, with responses indicating it would drop to a finite horizon temperature rather than absolute zero.
  • There is speculation about mechanisms that could allow for new structures, such as black holes, to form even after the last black hole evaporates, linked to density perturbations.
  • Participants inquire about the implications of a vanishing cosmological constant on the concept of maximal entropy, suggesting that without it, maximal entropy might never be reached.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of entropy in the universe's future, particularly concerning the existence of a maximum entropy state and the implications of the cosmological constant. The discussion remains unresolved with no consensus reached.

Contextual Notes

Limitations include assumptions about the universe's expansion, the definitions of entropy and temperature, and the implications of theoretical constructs like the Poincaré recurrence theorem and de Sitter spacetime.

Suekdccia
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TL;DR
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?
 
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"Could entropy be reversed" makes no sense. Entropy is a thermodynamic quantity. Could temperature be reversed? Could heat capacity be reversed?
 
Suekdccia said:
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.
 
PeterDonis said:
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.
 
256bits said:
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.
 
PeterDonis said:
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?
 
Suekdccia said:
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.
 
PeterDonis said:
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)
 
PeterDonis said:
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?
 
  • #10
Suekdccia said:
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.
 

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