As far as I know, the critical density of our observable universe coincides with the density of a huge black hole of the same radius. It could be a BH expanding at the speed of light and growing in mass/energy accordingly (maintaining critical density through time)… Any evidence in support or against this hypothetical BHU model?
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2$\begingroup$ Does this answer your question? Observable universe equals its Schwarzschild radius (event horizon)? $\endgroup$– pelaCommented Jan 19 at 10:39
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3$\begingroup$ It's really not helpful to downvote a question just for being a duplicate, and that doesn't match the stated use of the downvote. Downs are supposed to mean "lack of research effort, unclear, or not useful". Dupes can be marked as dupes and help others find the answer (stating a question many ways and linking to one answer is great!) but "somebody didn't search the right words to find the existing answer" shouldn't be a reason to downvote. (It just makes new users feel like they're unwelcome and that's not a good look.) $\endgroup$– Darth PseudonymCommented Jan 19 at 21:14
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1$\begingroup$ The black hole cosmological model is just a theory, while also being built on other hypotheticals such as the white hole and multiverse theory in some claims. The most widely accepted models suggest our universe is everything - period. So in a more conventional sense, comparing the universe to a black hole would be somewhat of a metaphor more than anything because it might share characteristics with one. Point is, as far as we know, the universe isn't actually a black hole. But in the end, we simply don't know, and since we can't see outside of the universe, it's an educated guessing game. $\endgroup$– 4NT4R3SCommented Jan 20 at 1:20
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1$\begingroup$ The answer is related to my question but does not really answer it, @pela $\endgroup$– Juan CasadoCommented Jan 20 at 11:40
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4$\begingroup$ @WhitePrime, the question does seem quite out there, I'll give you that - but it's actually a real theory that the universe is a black hole in some multiverse. It is not a widely accepted or supported one, but it's still out there. $\endgroup$– 4NT4R3SCommented Jan 20 at 19:15
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3 Answers
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The Hubble length $c/H$ is indeed the same as the Schwarzschild radius of the energy enclosed within a radius $c/H$. This is because the same physics set both. It does not imply that the Universe is a black hole.
Incidentally, it should be noted that the radius of the observable Universe $\simeq 46$ billion light years is larger than $c/H\simeq 14$ billion light years by a factor of a few.
People have legitimately thought about whether the Universe could be inside a black hole. I have no comment to make about those ideas specifically. I will only note that the cosmological solution to the equations of general relativity, the FLRW metric, is valid in any spherical region irrespective of what lies outside of it (under some simplifying but reasonable assumptions). So while we understand the expansion history of the observable Universe well, that implies little about what lies beyond it.
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It might point to a related "black hole" effect; that light from our galaxy will not "escape" the visible cosmos because of the total mass. Similarly the opposite way; the galaxies outside of the visible cosmos have their own visible cosmoses, which we are outside of - and thus cannot see. That could explain why the radii are the same, without it being a "black hole" in terms of sucking everything into the center.
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$\begingroup$ Thank you, but I don't see how this reasoning "could explain" the radii coincidence... $\endgroup$ Commented Feb 26 at 14:20
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$\begingroup$ Think of it like this: every galaxy is in the center of a giant ball of mass. With critical density of 9.47×10−27 kg/m³, that means the event horizon is 13.7bn ly. We can't see a galaxy 14bn ly away, because its light "stops" at 13.7bn. It's trapped in its "visible cosmos". Meaning the boundaries of our visible cosmos is simply where the light sources outside of it, can't reach us. If we move f.ex. 5 bn light years in any direction, the boundaries of the visible cosmos will move accordingly. $\endgroup$ Commented Feb 27 at 8:41
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$\begingroup$ I agree but, again, I don’t see the relationship with the commented answer. $\endgroup$ Commented Feb 28 at 17:49
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Well if there was a black hole, for it to be growing at all, it would have to be consuming matter outside of the event horizon, however if this event horizon is expanding at the speed of light, it would be impossible for any matter to exist outside of it, because the matter would have had to be outside the event horizon before it expanded there (the only way to accomplish this is assuming that matter can move faster than the speed of light for no apparent reason)
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$\begingroup$ Thank you. You are assuming that the observable universe IS the whole universe and it is finite... We can´t be certain about that. $\endgroup$ Commented Feb 26 at 14:22
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$\begingroup$ well if it is infinite then that's implying that there is something such as faster than light speed or something like infinite speed, and that just isn't something that is possible (to our current knowledge). Additionally, because (as far as we know, and correct me if i'm wrong) all black holes have some spin in them, the centrifugal force would force everything to the edges of the black hole (in this case the edge of the universe). for this to work, that would mean everything is moving away from a certain center point, which we know isn't happening. $\endgroup$ Commented Feb 27 at 1:16
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$\begingroup$ No, as the whole universe was already infinite at the Big Bang $\endgroup$ Commented Feb 28 at 17:53