Maxwell's Demon and cosmology

Maxwell’s Demon is a concept originally introduced in the study of thermodynamics, to determine if the 2nd law of thermodynamics could in principle be broken. It posited an intelligent agency that could interact with matter at the atomic scale and thus skew statistical laws governing thermodynamic processes. It occurred to me that Maxwell’s Demon could also be invoked in cosmological questions, since particle interactions at the beginning of the universe shaped the nature of the present universe.

In particular, I was wondering how to explain or debunk the following scenerio: at the start of the Big Bang, matter and antimatter are created in (nearly) equal amounts. Left to their own devices the two will annihilate each other, leaving a universe mostly of radiation. But let’s throw in Maxwell’s Demon. In this version, his job is to round up all the antimatter he can and compress it into black holes. Black holes classically have only mass, spin and charge, so other than those the qualities of anything dumped into a black hole vanish from the universe. So after a short time thanks to Maxwell’s Demon the young universe now consists of radiation, matter and primordial black holes.

This however raises some questions: Antimatter has positive mass and between anti-protons and positrons is overall electrically neutral like ordinary matter. So in principle there’s no way to distinguish between a black hole built of matter and one built of antimatter. And yet somehow the Demon has introduced asymmetry on a literally cosmic scale. If black holes “have no hair”, then where did this asymmetry come from? Is it balanced somehow by a counterbalancing asymmetry in the Demon itself, in that it somehow had to be able to recognize the difference between matter and antimatter? Is there in fact some way to distinguish a black hole built of antimatter? Or is gravity the great equalizer in that at some super- (or hyper-?) symmetric level, matter and antimatter are interchangable?

This is pretty much the answer, here. Gravity makes no distinction between matter and antimatter, and a black hole made out of either will be identical. Both, when they decay, will produce particles of both matter and antimatter, with no preference one way or the other. You don’t need any sort of supersymmetry to get this result; it’s inherent to all models of gravity.

Your demon, like any Maxwell’s Demon, will accumulate impressive amounts of entropy in its head in the process of doing the sorting, but while that does balance out any decrease in entropy you might find in the rest of the universe, it does not balance out the matter-antimatter imbalance you’re talking about.