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If black holes swallow stuff, will a big black hole eventually swallow the universe? The universe will eventually go dark, because there will not be enough matter to make stars, and all that will be left is a bunch of black holes and they can merge. So will the universe get swallowed by a big black hole someday?

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    $\begingroup$ Yes, black holes can merge, but on a very long time scale they will evaporate due to Hawking radiation. I think you'll enjoy looking at this timeline: en.wikipedia.org/wiki/… $\endgroup$ – PM 2Ring Jun 19 '18 at 4:07
  • $\begingroup$ The mass is already there, so there's no reason why it should happen in the future if it wasn't happening already. The short answer is that there's too little mass for everything to fall into a black hole. It won't happen outside of a big crunch scenario and the big crunch isn't expected to happen. There will be some coallesing of local galaxies but everything falling into a black hole is unlikely. Black holes are too small and the matter in galaxies, too spread out. $\endgroup$ – userLTK Jun 19 '18 at 4:42
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If black holes swallow stuff, will a big black hole eventually swallow the universe?

As far as I know, no. Because the universe is expanding, and we have good evidence that the expansion is accelerating. Yes, some people talk about the Big Crunch, but there's no evidence for it. I don't think there ever will be. I say that because it's space that's expanding, and a gravitational field isn't collapsing space. The universe didn't collapse when it was smaller and denser, so I see no way it can collapse in future.

The universe will eventually go dark, because there will not be enough matter to make stars, and all that will be left is a bunch of black holes and they can merge. So will the universe get swallowed by a big black hole someday?

As above. Even if star formation ceased and a whole lot of matter ended up in black holes, the universe would still be expanding. As far as we know. The expansion would have to reverse for the black holes to coalesce. Mind you, it might be a good thing if it did. As Stephen Hawking said, the universe is like a black hole in reverse. If the universe ended up as one big black hole, it would be ending up the way (we think) it started out. Then you'd have a cyclical universe.

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  • $\begingroup$ There is actually a wikipedia article dedicated to the ever expanding universe (en.wikipedia.org/wiki/Future_of_an_expanding_universe) going into some of the rather weird things that might happen in the very distant future. $\endgroup$ – Adwaenyth Jun 21 '18 at 8:48
  • $\begingroup$ @Adwaeny : I've seen that. In fact I've written an article on the fate of the universe myself. Talking of weird things, I think the Big Rip isn't quite as weird as people think. $\endgroup$ – John Duffield Jun 21 '18 at 15:57
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all that will be left is a bunch of black holes and they can merge

That is not clear actually. Keep in mind, black holes do evaporate via Hawking radiation. It just takes a very long time.

So the ultimate fate of the universe might be a uniform soup of cold photons instead.

But this is all highly speculative.

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