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I am only an interested layman in cosmology and GR. In recent years, we have learned that black holes indeed exist in the universe and that almost all galaxies contain massive black holes in their centers. Black holes, in essence only devour other (stellar) matter and they also swallow each other as has been demonstrated by the gravitational wave discoveries. Is it possible that black holes eventually will "eat" most or even all normal and dark matter of the universe? And could this, and a merging of black holes, lead to a situation where all the universe ends up being a few or even only one black hole?

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  • $\begingroup$ -1 No research effort? Googling your title is a start. $\endgroup$ – sammy gerbil Jan 17 '18 at 21:52
  • $\begingroup$ @sammy gerbil - With your recommendation it would obviously suffice to google any question you can ask here on this site or engage in a study in the scientific literature. I thought that an advantage of SE would be that you could get a qualified answer in a reasonable time, which would also be of interest to other SE participants. $\endgroup$ – freecharly Jan 17 '18 at 22:11
  • $\begingroup$ Prior research is expected (as the voting button indicates), and an internet search is quicker than asking on SE. It does not prevent you from asking here, you can still ask for clarification or further information. Users here do not usually display their academic qualifications, whereas on Quora (the top google hit) they do. $\endgroup$ – sammy gerbil Jan 17 '18 at 23:52
  • $\begingroup$ @sammy gerbil - That's right. I have already found this Quora place following your suggestion of a Google search.They had a couple of knowledgable people there (with credentials) answering a similar question.I did some prior "research" on Wikipedia but there was nothing relevant. On the other hand I have bad experiences with Google searches because so much unreliable stuff appears whre one doesn't know if it is trustworthy. $\endgroup$ – freecharly Jan 18 '18 at 1:06
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I think black holes have been misrepresented in popular culture. They aren't giant cosmic vacuum cleaners devouring everything in their path, they are gravitational objects which exist and have their own stable orbits. Things do fall into them, but they aren't sucking everything towards them at some extreme rate.

Ultimately most cosmologists believe the universe will keep expanding until everything tends toward absolute 0 temperature and there is no energy in a sense, the black holes that are there will exist until pretty much the end of time, but even those will probably slowly radiate away due to Hawking radiation until there is literally nothing. Black holes generally are too far away from each other to feel the gravitational influence of even their closest black hole neighbor so the likelihood of them all consuming each other and merging is extremely narrow, especially as with cosmic expansion they are getting further apart at all times.

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  • $\begingroup$ I understand your argumentation. But considering cosmic time scales, the chances of other stellar object and other black holes falling into an existing one (as we have recently witnessed) are probably not so small thus accumulating more and more mass in an existing black hole. Also, how did the extremely massive black holes form at the center of most galaxies? And I would not consider the orbits of black holes "stable". Not even the orbits of our planets are stable in the long run. $\endgroup$ – freecharly Jan 17 '18 at 19:49
  • $\begingroup$ By having stable orbits what I mean is that things can form stable orbits around them. The supermassive black hole in the centre of our galaxy is massive but its not nearly on the order of magnitude of the mass of the whole galaxy and doesn't account for much at present. Without doing calculations it is hard to know whether the big freeze would happen before or after all matter became a black hole. Maybe there would be a point reached where the black holes radiate out more than they acrete and they would shrink. I don't really know the answer to this, but if someone did that would be nice. $\endgroup$ – LUPHYS Jan 18 '18 at 1:17
  • $\begingroup$ So you think that it might be a justified question whether the massive black hole in the center of the galaxy could in the very long run absorb the mass of our galaxy before it evporates? And that scientific calculation might give an answer to it? $\endgroup$ – freecharly Jan 18 '18 at 2:24
  • $\begingroup$ @freecharlie. I have found some good treatments of a very old universe, Googling does help. Forget the details but something close to just too few things near each other to interact, very diffuse and low density, sure some particles with infinite lifetimes, and so on. The BH don't eat each other up, too low a density because of the accelerated expansion, so they decay $\endgroup$ – Bob Bee Jan 18 '18 at 2:24
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No. There is a misconception that a lot of laypeople seem to be picking up recently from popularizations, which is that all the matter in the universe will first be sucked into black holes, and then recycled into photons in the distant future through Hawking radiation, so that the only thing left will be photons. This is wrong.

Black holes are not super-powerful cosmic vacuum cleaners that can suck in everything there is. When a black hole forms through gravitational collapse, such as the collapse of a dying star, its mass stays the same, and that means that at some fixed distance such as a billion kilometers, its gravity is no stronger than it used to be. A black hole is in fact a very small object, typically about the size of a city. This makes it a small target, and the space between the stars is vast, so a huge amount of luck is required if it is to swallow another astronomical object through a chance collision.

So if we look at the cosmic cereal box a gajillion years from now, what will be on the list of ingredients? It will contain stable, massive subatomic particles such as electrons.[Baez 2004] (The exact roster of particles depends on unknown particle physics such as dark matter and proton decay.) Any photons with ordinary energies, such as those originating from the present-day cosmic microwave background, will play a negligible role as their energy density becomes diluted faster than the mass-energy density of material particles. (This process has already been going on for billions of years. It's is the reason why the cosmic microwave background, which was dominant in the early universe, is now a negligible part of the universe's mass-energy.) However, the universe will contain photons with extremely low energies, which an observer (if one could be present and could detect them) would describe as Hawking radiation from the cosmological event horizon.[Hu 2011] These photons will have a temperature on the order of 10^-30 K, meaning a typical energy of 10^-34 eV and a wavelength of 10^28 m.

Adams and Laughlin, "A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects", Rev. Mod. Phys. 69 (1997) 337, http://arxiv.org/abs/astro-ph/9701131

Baez, "The End of the Universe," 2004, http://math.ucr.edu/home/baez/end.html

Penrose, Causality, quantum theory and cosmology. In On Space and Time, ed. Shahn Majid, Cambridge University Press, Cambridge, 2008, pp. 141-195. (ISBN 978-0-521-88926-1)

Hu, "Hawking radiation from the cosmological horizon in a FRW universe," Phys.Lett. B701 (2011) 269-274, http://arxiv.org/abs/1007.4044

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  • $\begingroup$ Thank you for your interesting description of the end of the universe and the references. As you know, there are supermassive black holes at the centers of most galaxies. How did they become so large as compared to the common variety of those produced by stellar gravitational collapse? I suspect by accumulating mass from around them. Has anybody calculated the growth rate of these black holes? $\endgroup$ – freecharly Jan 17 '18 at 20:07
  • $\begingroup$ On the other hand, what would happen if you consider a universe only filled with black holes like the ones at the centers of the galaxies. This would be something like a "black hole gas". What is the probability in such a black hole gas of black holes encountering each other and merging? Do black holes have a scattering cross section for merging? $\endgroup$ – freecharly Jan 17 '18 at 20:09
  • $\begingroup$ Thank you again for the very good references! I just read the article by John Baez. Delightful! $\endgroup$ – freecharly Jan 17 '18 at 21:12

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