Heat death of the Universe in LCDM I have often read that the heat deat of the Universe occurs in cosmologies where its age can be arbitarily large, even with a cosmological constant. However the standard LCDM cosmology's conformal age is bounded, even in the arbitarily far future. It seems to me that for the Universe to necessarily reach equilibrium then it must also conformally reach equilibrium, but I don't see how that is a given if the conformal age is bounded. My question is the LCDM model how can the Universe definitely reach heat death?
 A: The fact that the conformal time is bounded in the future means that there are regions in the universe which we will not be able to get information from, so particles here cannot equilibrate with particles there.
However, the form of equilibrium you reach in LCDM is not one with a bunch of particles colliding and reaching equilibrium. Rather, you reach an equilibrium with no particles, only the cosmological constant. Basically, all the particles will leave the horizon (and the wavelength of any photons will stretch beyond the horizon). The temperature of this universe will be given by the Hubble rate, $T \sim H$, similar to the Hawking temperature of a black hole.
A: There are TWO LCDM possible assumptions related to what the universe will be like towards the end part of time.

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*All matter gravitationally bound together in a galaxy, or a collection of galaxies, will ultimately form into a single black hole. All other matter will become so far away from this black hole so that it will have no significant influence on it. This is one possible result. It depends on the assumption that Hawking radiation is only a false conjecture and is not a real phenomenon. The universe would then have many such black holes, each not having any relationship with any other black holes. There is no currently present firm experimental or observational evidence that Hawking radiation is a real phenomenon.

See https://en.wikipedia.org/wiki/Hawking_radiation.


*If Hawking radiation is a real phenomenon, than eventually the black hole described in #1 will eventually radiate all of its contents away, and the radiation particles will not interact with anything. Thus the  universe will eventually everywhere become a vacuum.

