There is a limit of mass that a black hole can absorb?

Question

Not a physics guy, but this suddenly got my curiosity.

In: Planetary Science

Answer 1

Theoretically, no. Practically, yes. Black holes inherit the angular momentum of stuff falling in. In other words, spinning stuff makes the black hole spin. A black hole doesn’t *have* to be spinning, theoretically, but that’s not realistic because the real world has a lot of angular momentum. As it spins, the black hole drags spacetime around with it, which then drags stuff *in* that spacetime. Now, stuff that has already fallen past the event horizon can never come back out, but anything still above that horizon can stay out if it orbits fast enough.

The increased momentum from the black hole dragging spacetime around with it will keep stuff moving fast enough that it orbits rather than falling in. The more stuff a black hole swallows, the more spin it will have and less stuff can fall in.

Additionally, as stuff falls in and breaks apart due to spaghettification, it forms the accretion disk which gets *super* hot from the friction of all that matter grinding down. Accretion disks are some of the brightest objects in the universe – the disk of a galactic supermassive black hole can outshine the entire rest of the galaxy *combined*. Just like how the light pressure from a star holds it up against gravity, the light pressure from the accretion disk stops additional material from falling in.

Together, these put a practical upper limit on black holes to ~50 billion solar masses. Above that, they can’t seem to swallow material faster than they evaporate from Hawking radiation. There’s also just the practical problem that there’s only so much *stuff* for the black hole to swallow. I mean, there’s infinite stuff but not enough time.

*Theoretically*, AFAIK, you could construct a universe where all of the stuff around a black hole falls directly in and all the angular momentum cancels each other out so the black hole isn’t rotating and there’s no accretion disk. In this idealized universe, there’s nothing stopping the black hole from continuing to grow for as long as you keep giving it stuff. In reality, that’s not going to happen.