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The mass of a star is limited to about a couple of hundred solar masses. The fusion rate scales strongly with density (which is why the most massive stars have extremely short lifetimes) so if a star was massive enough (above the Eddington limit) then the radiation pressure would blow it apart.

Now a black hole also has radiation, but by definition it is not capable of escaping due to gravity. But is there in some way a limit of the size. Imagine that all the matter in the universe formed a black hole. Should that be possible or is there a law which forbids creating it?

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  • $\begingroup$ "Imagine that all the matter in the universe formed a black hole..." Isn't that a description of the universe pre-big bang? (At least by popular models) $\endgroup$ – BuvinJ Mar 9 '16 at 23:25
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General Relativity does not limit the size of a black hole, and we have good (indirect) observations of black holes with a mass of more than a billion suns.

Black holes are not object that are held together by gravity, and which could be ripped apart if forces opposing that gravity were increased. They are (in GR) singularities: a complete collapse of the matter to a point. There is no "object" inside a black hole that could be emitting radiation.

Now if all the matter in the universe were to be a black hole, then that singularity would be in all our futures. We wouldn't "see" the singularity because it is in the future, never in the past. In fact a universe in which is one black hole could look rather like the one in which we are living.

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  • $\begingroup$ Why can't there be any radiation within a BH? Isn't there enough space just within the Schwarzschild radius? $\endgroup$ – Marijn Mar 9 '16 at 20:40
  • $\begingroup$ There can be radiation inside the black hole, but it isn't going anywhere but to the singularity. Once something crosses the event horizon, the singularity is in its future, and it is certain to reach it in a finite amount of time. There can't be any radiation coming from the singularity, because the singularity is never in the past. (Time is weird inside a black hole, but look up "penrose diagram" ) $\endgroup$ – James K Mar 9 '16 at 21:54
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There's no theoretical limit. If you had enough energy to move stars or galaxies, you could in theory keep feeding a black hole until it became enormously large, larger even than the Milky way for example. But there are practical limits past which black holes are unlikely to grow.

The two reasons for this are that 1), black holes aren't efficient at taking in matter. They can spit out as much as 90% of the energy from the matter that falls into them, and 2) once they reach a certain size, black holes are too large to form accretion disks, so matter tends to orbit around them rather than funnel into them.

Source and Source.

As to your 2nd question

Imagine that all the matter in the universe formed a black hole. Should that be possible or is there a law which forbids creating it?

I've pondered this one myself and I have no idea the answer. Is there a size past which Dark Energy would overcome gravitation? Dark energy operating inside the black hole might overcome the gravitation past a certain size, but that's just my novice speculation and I think the black hole would need to be billions of light years across for that to happen.

I don't know the answer to that one. I'd be curious if anyone does though.

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