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- TL;DR Summary
- Do Black Holes form in Curved Spacetime? What is and is not known about this?
We've been talking in another thread about supermassive black holes. That has me thinking about really, really big BH's - so large that the spacetime curvature and evolution of the universe matters.
Let's start by defining the density of a black hole as its mass divided by the volume enclosed by its event horizon. Thus is not technically correct, but it's good enough to support the next idea: as black holes get large, their required density drops. This follows from R ~ M.
Galactic SMBH's are not formed by a star's collapse. They are formed when enough (too many) stars get very close (too close) together, such that M > R.
One can ask how big do structures have to be before this happens. Galaxies aren't big enough. Clusters aren't big enough. Superclusters aren't big enough. However, aggregates of galaxies that are a substantial fraction of the size of the visible universe are big enough.
However - there's a big "but" here - the GR solutions that lead to BH's assume that the surrounding spacetime is static and asymptotically flat. If you have a region that is a good fraction of the visible universe, spacetime around it is anything but flat. It's not static either - the universe is expanding and its density is dropping.
This is not the same thing as "maybe the universe is a giant black hole". This is about the formation of very lartge black holes in the universe.
Let's start by defining the density of a black hole as its mass divided by the volume enclosed by its event horizon. Thus is not technically correct, but it's good enough to support the next idea: as black holes get large, their required density drops. This follows from R ~ M.
Galactic SMBH's are not formed by a star's collapse. They are formed when enough (too many) stars get very close (too close) together, such that M > R.
One can ask how big do structures have to be before this happens. Galaxies aren't big enough. Clusters aren't big enough. Superclusters aren't big enough. However, aggregates of galaxies that are a substantial fraction of the size of the visible universe are big enough.
However - there's a big "but" here - the GR solutions that lead to BH's assume that the surrounding spacetime is static and asymptotically flat. If you have a region that is a good fraction of the visible universe, spacetime around it is anything but flat. It's not static either - the universe is expanding and its density is dropping.
This is not the same thing as "maybe the universe is a giant black hole". This is about the formation of very lartge black holes in the universe.