The limit of "Ultramassive" Black Holes.

[Researcher X]

There is no limit, is there?


Both these links show not a limit, but a regulation that slows the growth of a black hole, due to radiating most of its food away:

http://www.newscientist.com/article/dn14653-how-big-can-a-black-hole-grow.html
http://www.physorg.com/news140370694.html

Expansion is also stopping really big black holes from merging, but there doesn't seem to be a fundamental physical limit that I can find reference for.

Can black holes increase indefinitely by merging? Could you have a 50 Septillion solar mass monster within known physics? (Not that the conditions to form it can be met) Are there any strange effects that would become evident as black holes become so massive? As far as I can tell, it would become harder and harder to identify it as a black hole at all.

[bombadil]

Hi Researcher X,

Thanks for the link, that was fascinating.

If some physical processes limit the growth of a black hole, then the limited growth coupled with the finite age of the universe gives you a maximum mass. So I don't think there is anything preventing a 50 septillion solar mass black hole from forming other than a lack of time.

The strange thing is that we see ~billion solar mass black holes at a redshift of 6 (1 billion years after the big bang). This means that black holes must grow really fast in the early universe, and then grow very little over the next 12 billion years.

-bombadil

[CRGreathouse]

That does not mesh well with my understanding. I though that Hawking radiation *decreased* as mass increased.

[bombadil]

That does not mesh well with my understanding. I though that Hawking radiation *decreased* as mass increased.

Hello CRGreathouse,

When we talk about radiation from a black hole, we're talking about the radiation that originates from the disc of matter accreting onto the black hole. This accreting matter radiates far more energy than Hawking radiation does.

In general, Hawking radiation is not a significant process for the growth of astrophysical black holes (i.e. black holes that more than the mass of the sun). The amount of mass that is radiated away via Hawking radiation is minuscule compared to how much matter the black holes sucks in from interstellar gas and stars.