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The universe expands at an accelerating rate, which makes me wonder if at a certain point the speed of expansion between two points will exceed the speed of light. In that case, it might eventually become impossible for two gravitationally bound clusters of the universe to exchange light. I recall reading that future civilizations born after a certain point would be unable to observe the distant galaxies that we can now, and would have no way of knowing galaxies besides their own exist.

My question: Is this possible, and if so could it have already happened; the entire observable universe is one of multiple clusters that have since sped away from each other? Or can this be ruled out by other known facts about the history of the universe?

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    $\begingroup$ It's an interesting question! I've made a small adjustment to the "faster than light" part so that the rest of your question doesn't depend on it completely. $\endgroup$ – uhoh Dec 31 '19 at 23:45
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    $\begingroup$ The Universe has always expanded faster than light for sufficiently large distances, but that is no hindrance for us to see them. I’m almost sure I’ve explained in another answer, but perhaps it was on physics.SE. $\endgroup$ – pela Jan 1 at 2:27
  • $\begingroup$ @pela Perhaps you're thinking of your answer in the duplicate I proposed, or the one in the question it's linked to. $\endgroup$ – PM 2Ring Jan 1 at 9:35
  • $\begingroup$ Also see the Scientific American article by Davis & Lineweaver about expansion: people.smp.uq.edu.au/TamaraDavis/papers/SciAm_BigBang.pdf $\endgroup$ – PM 2Ring Jan 1 at 9:37
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    $\begingroup$ Yes, I think the ones linked to by @PM2Ring and antispinwards should answer your question. $\endgroup$ – pela Jan 1 at 20:25
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(My answer here isn't correct, as pointed out by the first comment on this post, please refer to it.)

Yes, it certainly is possible - and it already has happened. The age of the universe is 13.8 billion years, yet it is something like 90 to 100 billion light-years in diameter. This means that we definitely can't see all of it, not because of our own observational limitations, but because the light from those parts of the universe just hasn't reached us yet - and perhaps it never will.

As for the future, yes, if cosmological expansion overpowers gravity at close range (i.e. the Milky Way and Andromeda, and the world "close" is relative here) then eventually many galaxies and clusters will disappear out of view due to light not being able to "keep up" with the expansion.

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    $\begingroup$ Sorry, this is incorrect. The "90 to 100 billion light-years in diameter" you refer to (more precisely 92.5 Glyr) is the observable Universe, i.e. the part we can see. The whole Universe is probably much larger, possibly infinite, but light farther away hasn't reached us yet. The part of the Universe receding from us faster than light is only 14.4 Glyr away. $\endgroup$ – pela Jan 1 at 20:28
  • $\begingroup$ Oh... darn, yes, you're right. Sorry about that - I'll edit my answer to refer to your comment here. Thank you for pointing that out. $\endgroup$ – Calc-You-Later Jan 1 at 20:30

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