Is there a limit on the mass of a blackhole? How big can it get?
Theoretically? Nope. They can get as big as they want to. In practice one has to be careful. You can't have a black hole which has twice the mass of the universe right?
Why twice the mass? Why not exactly equal to the mass of the universe? Where would it get anymore mass?
Forget twice - it was a mere example where I chose a number at random. All I was saying is that you can't make a black hole as big as you want if you're lacking the material to make it. However nobody knows how much mass there is in the universe.
No but the lower limit is more interesting. Nothing in theory as far as I know imposes a lower limit on black holes but that doesn't mean that any low mass black holes exist. It is a question of how could low mass black holes come into existence. Apparently many physicist think that a lot of black holes were formed early in the history of the universe, perhaps at the same time when the rest of matter came into existence. Hawking radiation means that smaller black holes would have a shorter life span but only in isolation. Black holes are likely to become the centers of gathering masses like galaxies and star clusters, where they tend to gain more mass. So are there small black holes in isolation out there somewhere, left over from the beginning? Is there any physical event that can create small black holes?
Speculations about quantum gravity suggest the lower limit would probably be on the order of the Planck mass, about 10^-8 kilograms (close to the mass of a flea), in which case the black hole's radius would be around the Planck length.
I question how large they can be because everything will eventually be consumed by a black hole. In the center of all galaxies is a large black hole. Some spinning, some not. These black holes are eventually going to meet. As they grow, the gravitational feilds will get stronger and attract other, stronger black holes. Until only two are left. These two will eventually collide and combine. I need some equation or constant that would explain this universe size black hole, with a singularity whose mass is equal to the entire universe, to create the big bang. Or would this black hole evaporate into a universe? I don't think so.
The universe is a black hole. That's why light can't escape from it. The mass of the black hole is 1 in universal units.
The universe is not believed to be a black hole, because black holes have a singularity at the end of time (not the beginning of time). The universe is *probably* not a time-reversed black hole (a white hole) either.
See for instance the sci.physics.faq Is the big bang a black hole
for much more detail.
Well, not by some maybe, but by others? Thanks for the link. I always knew that the "universe is a black hole" thing was strained. I just didn't realize how taut it was.
The Black Hole is derived from the singular situation 2GM>rc2to the spherically symmetric Schwarzschild solution. The cosmological solution is a different case and it would be inappropriate to confuse the two.
There is no scientific basis for such an assumption. Why would you think this would be true? For example; suppose there is a star which collpases into a black hole. At a distance from the star there will be no observed changes in the gravitational field. None! The only thing that changes is the star itself in the the size decreases so much that you can get closer and closer to the original center. Eventually you'd get so close to the center that you'd cross the event horizon and be swallowed up. But there are no changes in the gravitational field at distances which are greater than the original stars radius.
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