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If the universe keeps expanding at an accelerated rate (given by the cosmological constant) then the universe would approach a DeSitter spacetime where there would be a cosmological horizon that would radiate just as the event horizon pf a blacl hole radiates Hawking radiation

I thought that once this state is reached, the universe would stay like that, but I recently discovered that this horizon could evaporate just like a black hole and the cosmological constant would dilute (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6560872).

Is this true? Even if that happened and the expansion would stop being accelerated by a cosmological constanr, what would happen then after?

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The standard picture is that any static patch of de Sitter spacetime is similar to a black hole turned inside out, and it radiates by a similar mechanism, but there is no infinity for the radiation to escape to, so there is no net evaporation. Instead there's an equilibrium with a nonzero amount of Hawking/Unruh radiation in the interior.

I'm not sure what to make of the paper you linked (by T. Markkanen). On the face of it, it makes no sense that de Sitter spacetime would evolve into Minkowski spacetime. They aren't solutions of the same field theory—or if they are, one of them is a false vacuum, and you might get catastrophic vacuum decay but not a gradual relaxation.

Markkanen says

Based on thermodynamic arguments the seminal study [Gibbons and Hawking] concluded that unlike black holes de Sitter space is stable. However quite interestingly, also by invoking thermodynamic concepts in the equally impactful work [Padmanabhan] it was argued that the de Sitter horizon in fact does evaporate.

But Padmanabhan's paper is a 100+ page survey in which the argument in question is just part of one section (10.4), so the high citation count can't be taken as evidence of support for that argument. Besides, Padmanabhan doesn't argue that the horizon evaporates; he just stops short of saying that it definitely doesn't. I don't understand why he doesn't consider it obvious that the emitted radiation will fall back into the horizon at an equal rate, since there's nowhere else for it to go.

Looking at papers that cite Markkanen, I found a paper by Moreau and Serreau, published a year later, which is not very accessible but seems to claim Markkanen is wrong. But it isn't specifically about Markkanen's paper and only cites it as part of a long list of previous work.

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  • $\begingroup$ thank you. Only one thing. You say that all radiation emitted by the DeSitter horizon would be reabsorbed at the same rate. However, if I understand this correctly, if the universe ends up in a DeSitter space then there would be a fixed temperature above absolute zero, so wouldn't there be radiation in the universe that would come from the horizon? Could there be also massive particles (like protons): sciencedirect.com/science/article/abs/pii/S0550321305005985 ? @benrg $\endgroup$
    – vengaq
    Commented Aug 4, 2023 at 22:55
  • $\begingroup$ Also, another reference that Markkanen usually cites from Padmanabhan is this one (arxiv.org/abs/gr-qc/0311036). Does this one change or add anythin in your answer? @benrg $\endgroup$
    – vengaq
    Commented Aug 4, 2023 at 23:10
  • $\begingroup$ @vengaq Yes, there would be Hawking/Unruh radiation in the static patch. It doesn't fall back instantly, just eventually. But the peak wavelength of the blackbody spectrum is on the order of the Hubble distance, so this is all purely theoretical. A proton is pretty unlikely. The other paper of Padmanabhan's is another 100-pager, but I guess Markkanen would be citing the end of §6.2, which is almost identical to the end of §10.4 in the other paper, and my reaction is the same (I don't understand why he acts like the radiation won't be reabsorbed). $\endgroup$
    – benrg
    Commented Aug 5, 2023 at 1:20
  • $\begingroup$ re-reading this answer I have come up with another question: Imagine some kind of "special" material that somehow absorbed the particles coming from the cosmological horizon so that they would not return to the horizon. Would the horizon still evaporte just as a black hole would? If it would, would it be gradual? And if it would be a gradual evaporation, receding the horizon up to infinity as a result, wouldn't the horizon essentially be radiating forever as it does? @benrg $\endgroup$
    – vengaq
    Commented Nov 22, 2023 at 21:07
  • $\begingroup$ @vengaq The radiation is thermal, so any absorber would eventually heat up to the same temperature and then emit at the same rate. Also, the de Sitter horizon doesn't grow when it radiates, it shrinks. If by a very unlikely thermal fluctuation the radiation all stayed inside, I'm not sure what would happen, but maybe you'd end up with a Schwarzschild-de Sitter black hole. $\endgroup$
    – benrg
    Commented Nov 22, 2023 at 23:22

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