Even if we (or aliens somewhere in the universe living at that time) somehow invent something that can go just short of the speed of light, will intergalactic travel still be impossible in roughly 15 billion years? Another way to put it, will even the nearest galaxy be receding from us faster than the speed of light in roughly 15 billion years? To me, that would seem to require dark energy to increase dramatically, but seems to be the conclusion after thinking on the following points (although the expansion of space itself probably alters this estimate significantly):

In some further thought on the premises I posed in this linked question, I came to the realization that the universe may be "rapidly" changing: Is there a physical limit to how far we can go?

Edit: This is different than that question in that I am focused now on the future state of our universe (when - the focus of this question) vs. the location (where - which was the focus of the prior question).

Known facts:

  • There is a part of the universe, about 15 billion years away, that is receding beyond the speed of light, and hence we can never reach.
  • If we were to pick a direction, there are a finite number of galaxies between here and that 15 billion light year position, (and I therefore deduce at some point, we would not be able to reach the next galaxy.)
  • Space is fairly uniform, so similar conditions should be present on a large scale
  • It would take at least 15 billion years to reach that edge, not accounting for space expansion.

My thought / question with the above in mind, does this mean that if we were to keep traveling, it is inevitable that we will eventually pass the last galaxy we can reach, and then all galaxies would be out of reach (except possibly the last one)? Does this mean that in 15 billion years (then adjust for the expansion of space) that we won't even be able to reach the nearest galaxies, even if we find a way to travel at the speed of light?

  • $\begingroup$ Possible duplicate of Is there a physical limit to how far we can go? $\endgroup$
    – ProfRob
    Aug 9 '19 at 13:39
  • $\begingroup$ I don't see how this differs from your previous question. $\endgroup$
    – ProfRob
    Aug 9 '19 at 13:41
  • $\begingroup$ @Rob Jeffries It is related, based on it, but I am asking here about the "when" vs. the distance. $\endgroup$
    – Jonathan
    Aug 9 '19 at 13:48
  • 1
    $\begingroup$ Premise reminds me of Zeno's paradoxes. $\endgroup$
    – Mike G
    Aug 9 '19 at 19:10
  • $\begingroup$ It’s not impossible to reach a part of the Universe the recedes faster than the speed of light (theoretically, at least; there are practical hindrances. $\endgroup$
    – pela
    Aug 10 '19 at 22:24

Yes, over time the accelerating expansion of the universe (assuming the $\Lambda$CDM model is right) will separate gravitationally bound clusters of galaxies from each other with exponentially growing distances. However, this will not happen in 15 billion years.

This excellent paper analyses the expansion in detail. What takes about 17 billion years is for the distances to increase by 2.71 times. They predict that typical clusters will grow isolated in about 120 billion years, with the Local Group becoming isolated in 175 billion years from the other nearby groups.

In this "era of isolation" the only reachable galaxies are the ones in one's local cluster. However, they will tend to merge over time into one big elliptical galaxy (like the Milky Way and Andromeda are likely to do in a few billion years). Extragalactic astronomy also becomes really boring since the remote clusters are redshifted so much they cannot be observed.

If one just travels outwards from Earth in some direction with a high velocity one will eventually find oneself in a last cluster beyond which there is nothing reachable.


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