[askonomm]

It was my understanding that if two black holes collide, they just form a bigger black hole, but we know there's a black hole in our universe, which then would mean that there's a black hole inside of a black hole that did not merge with the parent black hole, right? Is that something that is considered possible?

[lupusreal]

I'm under the impression that we really have no clue what's going on inside of black holes, so the most we can really say with confidence is that when two black holes collide they appear from the outside to now be a single black hole.

[colejohnson66]

It’s a reasonable assumption. If two solar masses collide, their masses tend to combine[^]. Just “look” at planets that smash into each other. Ergo, a more massive black hole.

[^]: Ignoring ejections. But black holes also don’t “eject” mass. Or maybe they do? Hawking Radiation is weird.

[soulofmischief]

Actually, they can shoot out relativistic jets at the poles. https://www.nustar.caltech.edu/page/relativistic-jets

[colejohnson66]

But there, the black hole is ejecting other mass near it, not its own.

[soulofmischief]

Mm, I didn't correctly interpret their comment. You're absolutely correct.

[colejohnson66]

Yeah. I could've worded it better. By "ejections" I meant how, when two planets/moon sized masses collide, rocks shoot out into space. But because black holes have so much gravitational pull, everything theoretically just falls in.

[blueflow]

The inner black hole did not come from the outside, it formed inside and if i had to guess, it is stuck in the inside together with all the other matter, unable to interact with the outside of the outer black hole.

[askonomm]

Just thinking about this infinite recursion gives me the mental equivalent of a stack overflow.

[blueflow]

I don't think it is infinite - each universe can only have that mass/energy that fell into the outer black hole in the parent universe. At some level you'll inevitably have black holes with universes that do not have enough mass to form another inner black hole.

[soulofmischief]

Unless, although there's no reason to currently believe this, the energy requirements for physics are relative within each black hole, sort of (but not strictly) like how the speed of light is relative for all observers. And we can get a little crazier, and imagine a meta universe that is sort of like a Klein bottle in that it doesn't just recurse all the way down but somehow folds back into itself. Again, no current reason to believe anything like this but it's a mind-boggling to visualize.

[idiotsecant]

How much mass is required to form a black hole in a new universe with perhaps different physical constants? It could be that 'ability to make black holes' is a prerequisite for successful universes in the way way that good genes are a prerequisite for successful organisms. The universes that fail to spawn black holes are 'dead ends' so any life is statistically likely to find itself in a black hole spawning universe.

Maybe there is an 'incentive' for universes to form with physical constants tuned to produce black holes with the available energy in that universe.

[IngoBlechschmid]

This circle of ideals seems to be known as: https://en.wikipedia.org/wiki/Cosmological_natural_selection

[jfengel]

The trick is that bigger black holes are less dense. Supermassive black holes can have the density of water. If the universe is gravitationally closed, it would have the density of... well, just look up at night. (Actually much less than that; you see more stars because you're inside a galaxy.)

The density makes the scale recursion less mysterious.

[drusenko]

That’s interesting! When you are referring to density, are you referring to average density within the event horizon? Isn’t most (effectively all) matter concentrated in the singularity? Would love to hear you elaborate on this thought further.

[jfengel]

We can't really talk about what's inside a black hole. From outside, it has a volume and a mass, and that's all there is to know.

We can say that any particle inside the horizon is inevitably headed to the center. (That's why we can't say any more: no other information can escape.) That does lead to a problem in that all of the mass would be concentrated at a single point at the center, whose density is division-by-zero.

But I wouldn't put too much weight on that. We already know from quantum mechanics that there isn't really any such thing as a "point". The math is still a problem, but the solution almost certainly lies in that direction.

[jug]

Maybe generating a stack overflow was the true depiction of God!