Black holes are maximally entropic but universe predicted to end in heat death instead?

Question

As I understand it black holes have the highest density of entropy per area (and thus per volume for spheres at least) of any form of matter and spacetime.

Also we expect the far future of our accelerating universe to asymptotically approach heat death with every particle infinitely spaced.

If at every timeslice entropy of the universe increases, why does it "miss" this boundary condition of a final single massive black hole?

Is a single supermassive blackhole at the center of the observable universe less entropy than a thermal bath of the same matter+energy in the same volume (obs uni)? Or is it more like phase space will never pass through the point of a single suppermassive blackhole because we started with inflation and have a positive CC?

Answer 1

I'm not an expert and I may deprecate or delete this answer if someone writes a better one.

De Sitter space in de Sitter's original coordinates resembles a Schwarzschild black hole turned inside out. It looks a lot like a maximum entropy solution, and indeed it seems to be commonly believed that if the cosmological constant is positive then the number of degrees of freedom in the universe is finite and equal to the entropy of its de Sitter horizon. (Though since the horizon has a nonzero temperature, I guess the real maximum entropy state would be an equilibrium with some Hawking radiation in the interior.)

These coordinates don't cover the whole space; they only cover the causal diamond of an arbitrary pair of points at future and past infinity. But this fits in nicely with the idea of event horizon complementarity, which suggests that that portion of the space is really the whole universe, and contains all of the physics of every other causal diamond.

In short, the universe may be "collapsing" outward to an inside-out black hole.

Here's one paper that discusses this (not necessarily the best one, just the first one I found): "Disturbing Implications of a Cosmological Constant" by Dyson, Kleban, and Susskind, hep-th/0208013, doi:10.1088/1126-6708/2002/10/011.