Abyssal

Three articles came in pretty much at the same time over the past couple days. MIT published a recent 85-sol black hole merging with a smaller one, and an explanation of event GW190412 last year which involved a 29.7-sol with a (much!) smaller one. Somewhere around there: Hawaii offers the Generic Object of Dark Energy, which behaves like a black hole, but isn't.

What I learnt from the former is that a "mass gap" looms in black hole theory. From the beginning: when particles are given sufficient energy (that is, they approach the speed of light), quantum effects apply which spawn off additional particles. This holds true of photons, also: when bright enough, they birth an electron and antielectron. So, in the event of an iron-fusion implosion of a star, there's a mass at which the energies are too high - they create enough particles that an event horizon is not formed. The star ceases to implode, and ex-plodes. Entirely.

I am (now) told this goes boom if the ex-star's mass was such as to run the black hole higher than 65-sol. Apparently at 200-sol the star's gravity is high enough again that the event horizon forms anyway (at 120-sol, the rest blowing off). That mass gap, between 65-120, is the "pair instability" for which we shouldn't see natural black holes.

Based on the other articles, one also from MIT, one obvious answer is "derrrp what if two 40-sol black holes merge". GW190412 was a merger and, MIT think, one of the parties of that merger, itself, came from an earlier merger.

But Hawaii asks, what if a given "black hole" is a "g.e.o.d.e.", a ball of dark-energy with an outer spinning layer. Apparently the "geode" repels, not attracts, when the spin is high enough. If they don't spin much then they just act like black holes.

SNAPSHOT 10/29: The black hole census.