This PBS video postulates a positively curved universe which is in the shape of something the narrator calls a "hypersphere".


Such a universe would have no edge, and traveling in one direction would eventually travel back to the same point in space (possibly depending upon the expansion). What is the fate of such a shaped universe? Would such a universe recombine into a single point again as a big crunch, like a big-bang rewind?

  • $\begingroup$ Science can't say anything about these things, at least nothing particularly useful. What you want such a model to do is really a choice of model properties and parameters, so you can, most likely, achieve all kinds of different outcomes. Strictly speaking science can only say something with certainty about the past. Even long term prognosis for the one and only universe we know are highly speculative. $\endgroup$
    – CuriousOne
    Jun 26, 2016 at 7:53

1 Answer 1


Given a few plausible assumptions about the universe its spacetime geometry is described by a solution to the Einstein equations called the FLRW metric. If we know the densities of various types of matter/energy present, e.g. photons/matter/dark energy/anything else, then we can calculate how the expansion of the universe varies with time.

Generally speaking the photon/relativistic matter density is only important for times very soon after the Big Bang. So if there is nothing screwy like dark matter around then the evolution is dominated by the density of matter (that's visible and dark matter), and the key parameter is the ratio of the density to the critical density. We call this ratio $\Omega$, and the geometry is related to $\Omega$ as follows:

  • $\Omega \lt 1$ - closed universe

  • $\Omega = 1$ - flat universe

  • $\Omega \gt 1$ - open universe

Our universe appears to have $\Omega$ very close to $1$ so it appears to be flat. The universe discussed in the video you link has $\Omega \lt 1$ so it is a closed universe, and a closed universe has the spatial topology of a 3-sphere. This does indeed mean that if you draw a straight line in space and continue it for long enough the line will eventually meet itself again.

So far so good, but the main point of your question is asking what is the fate of a closed universe, but that turns out not to have a simple answer. If all the closed universe contains is matter and photons then the closed universe must recollapse. That is, it will expand from the Big Bang, reach a maximum scale factor then recollapse again into a Big Crunch. But if the universe contains dark energy this can change its fate. If the ratio of dark energy to matter is high enough then even a closed universe can expand forever.

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    $\begingroup$ Or none of these scenarios is implemented by nature because general relativity is not the correct theory. $\endgroup$
    – CuriousOne
    Jun 26, 2016 at 8:57
  • $\begingroup$ Thanks for taking the time to answer. The big thing I got out of this answer is a Big Crunch is a possible fate for a closed universe even with a high ratio of dark energy. $\endgroup$ Jun 26, 2016 at 16:47

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