Bad jokes aside, is there a limit to how much material a black hole can get before “something” happens to it? (eg explodes, dissolves, turns into a whale named Stan, whatever).

Well I don’t know and I don’t think there is a conclusive answer. Some have theorized that this universe is inside a really big black hole. When a BH gets larger, for some reason the gravity inside is suppose to go down, instead of increasing. I think that if this universe is actually inside a BH however the gravastar theory makes more sense, which is a alternate theory. Reason being in a BH everything falls forever towards the center, in a gravistar everything falls forever towards the edge. Since the universe is expanding it would seem to be the latter, but then just because we experence time in one direction doesn’t mean that it’s the way it’s actually going.

OK that’s enough from me on this one for now, so I await someone who knows what they are talking about.

On a related note, how much dark matter could a black hole suck if a black hole could suck dark matter?

IANA Physicist, but in any of the books I’ve read, there’s no known limit to how big a black hole can get. IIRC, there is an unstable equilibrium point as far as size goes. A black hole smaller than a certain size will shrink via Hawking radiation until it disappears in a violent burst of energy, and a black hole larger than that size will grow until… I dont’ know. Until it wins, I suppose.

There’s no limit. The more matter a black hole sucks in, the more “sucking power” it gets (since the gravitational force exerted by an object is proportionate to its mass.)

I know it’s a joke, but for what it’s worth a black hole *can* suck dark matter, since dark matter is subject to the force of gravity.

More precisely, the gravitational acceleration ("g"s) and tidal forces at the event horizon go down as the black hole gets more massive. This is because the event-horizon radius (the distance from the center of the black hole at which nothing including light can escape) is directly proportional to the black hole mass, but gravitational acceleration decreases as an inverse square of distance and tidal forces (the difference in acceleration between the near and far end of an object) decreases as an inverse cube of distance.

For example, if Black Hole A is twice as massive as Black Hole B, the event horizon of Black Hole A will be twice as far from its center. This means that the gravitational acceleration at the event horizon of Black Hole A will be half (doubled mass divided by four – square the doubled distance) and the tidal force will be one-fourth (doubled mass divided by eight – cube the doubled distance).

For a galactic-mass black hole, the g-forces and tidal forces aren’t too bad as you cross the event horizon. Of course, you’ll then be on a one-way trip to the singularity at the center, where these forces increase to infinity, so don’t get too cocky about it.

Thanks, **Steve**, for that fascinatin’ bit of information. Stuff like this is exactly why I first decided to subscribe to SDMB!

Exactly: there’s no limit. I think the OP is thinking of a black hole as a pocket in space that can get filled up. It’s more like a sinkhole where the fabric of spacetime itself has collapsed, rather than your backyard.

my brain hurts now…

But if there is no limit to how big a black hole gets, then eventually the whole universe can fall into a black hole… but the black hole is in the universe…which is in the black hole…ouch.

If your space ship is being pulled in by a black hole I imagine that would suck quite alot.

With a density of about 1X10^-26 kilograms per cubic meter, a radius of 1.3X10^26 meters (13.6 billion light years), and hence a mass of 9.2 X 10^52 kg, the Shwarzchild radius of the observable universe is (barring math errors and innaccurate mass estimates) is 14.4 billion light years. We are *already* inside a black hole.

I’m I the only one who now has running through her head…

How much space could a black hole suck if a black hole could suck space?

This is not correct. Every black hole decays by Hawking radiation; it is just that small ones decay much faster. I forget the equations but the result is that a black hole with the mass of a proton decays virtually instantaneously, but one the size of the sun would take longer than the age of the universe. This is complicated by the fact that any nearby matter will fall in and enlarge it. But eventually, there will be no matter nearby and the decay will go on indefinitely. I think I read somewhere that in only 10[sup]128[/sup] years all the black holes in the universe will have decayed. At that point, the entropy of the universe will be at a maximum and no further energy is available for work.

The idea isn’t that the Universe is *inside* a black hole, but that it is, itself, a black hole. This would be the case iff the Universe is closed, that is to say, will end in a Big Crunch. In that case, the Big Crunch singularity is the singularity at the center of the black hole, and sure enough, we’d all be headed unavoidably towards the singularity.

However, current data seem to suggest that this is not the fate of our Universe, and that it is therefore not a black hole. You can’t calculate just based on the density, as **Squink** did, because dark energy is very significant in our Universe. So if you’re going to ask whether the Universe is a black hole, you’d need to use the Schwartzschild-deSitter equations (which are not completely solved), not just Schwartzschild.

**iamthewalrus(:3=** is correct that there’s an unstable equilibrium size, at least in the present Universe, because there’s a lower limit on how much a black hole can eat. If nothing else, any black hole in the Universe will be eating a meager diet of cosmic microwave background photons. It’s not much, but radiation from a stellar-mass black hole (or larger) is even less, so all of those black holes are actually still growing. A hypothetical black hole a millionth the mass of the Sun (we don’t know whether any of those actually exist) would be radiating at about the same rate it was eating, and would therefore sit at the equilibrium point. A lighter one would radiate faster, and eventually evaporate away in a burst of energy.

It’s not just dark energy which messes with calculations for such huge black holes.

What with matter being distributed uniformly throughout the universe, you can arbitrarily pick the location of the center of a black hole, and calculate the position of its event horizon. Doing that more than once quickly leads to a situation where galaxies, such as the Milky way and M31 in Andromeda, are simultaneously located within a single event horizon, **and** within the event horizons of two disjoint holes.

Since we can see M31, the math must be wrong.

Not really. The center of a black hole is an event, not a worldline (that is to say, it’s a point in spacetime, with no duration), and it’s in the future. So any event in the present Universe you call the center is wrong.

I didn’t know that, but now that the equation has blown up so spectacularly, I’ll have to think about it.

Thanks, **Chronos**.