Not a physics guy, but this suddenly got my curiosity.
In: Planetary Science
No, black holes can even merge with other black holes to become even larger black holes. The biggest ones are absolutely terrifying.
Only in the sense that there is a finite amount of absorbable mass in the universe.
Other than that no, Black Holes are insatiable, if they could they’d be able to absorb all mass in the universe.
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.
If you keep feeding a black hole, it will keep growing. But they’re very slow messy eaters. If you have a lot of material falling into the black hole, that material will collect into what’s called an accretion disc. In that disc, the friction from all of the matter spinning about makes it glow so much that the heat radiation pushes it away from the black hole, denying it most of the meal until what’s closer in gets gobbled up. But even that doesn’t get fully eaten. Through processes that are still not fully understood, a good part of that matter is pushed perpendicular to the axis of rotation and becomes powerful relativistic jets that shoot out at a good portion of the speed of light.
So, the answer is that, while there’s no upper limit on how much can absorb, there *is* a limit on how much it can absorb at any one time. And, the more you try to feed it at once, the bigger the mess it produces during its meal.
There is an unknown force named dark energy caused by dark matter that competes with gravity depending on distance and density of the space. So galaxies and black holes won’t compeltely merge into one massive hole.
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