As defined by Wikipedia:
The heat death of the universe is a suggested ultimate fate of the
universe in which the universe has diminished to a state of no
thermodynamic free energy and therefore can no longer sustain
processes that consume energy (including computation and life). Heat
death does not imply any particular absolute temperature; it only
requires that temperature differences or other processes may no longer
be exploited to perform work.
Does it even make sense to describe temperature in the system described? If so, would it be a very cold system or a very hot system?
Best Answer
Yes, and it would be very cold. The paper "finite temperature in a deSitter universe" explains that the cosmological constant (if it is really a constant) creates a "horizon" that acts somewhat like an inside-out event horizon: objects that get too far away from you are unreachable. This Horizon will radiate Hawking radiation at an extremely low temperature of 10^(-30)K; the wavelength range of light this corresponds too is the same order of magnitude as the horizon's radius (also the case for black holes). It's conceivable that some relatively compact system could have an excited quantum state so close to it's ground state that it is thermally accessible even at this extremely cold temperature. Thus, there is still some form of heat swimming around. However, there is no reservoir colder than this temperature to dump heat into so this heat can't be converted into useful energy.