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If all matter was created in the Big Bang (not counting spontaneous generation of particle-antiparticle pairs), wouldn't that imply that a finite (if very large) amount of matter exists in the Universe?

By the same token, wouldn't that imply that the Universe is spatially "finite", i.e. a compact (if boundaryless) 3D manifold?

If not, why not?

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    $\begingroup$ Related: physics.stackexchange.com/q/9419/2451 , physics.stackexchange.com/q/1915/2451 and links therein. $\endgroup$
    – Qmechanic ♦
    Oct 2, 2020 at 19:41
  • $\begingroup$ Have you read about Friedmann universes with $k=0$ and $k=-1$? $\endgroup$
    – G. Smith
    Oct 2, 2020 at 20:51
  • $\begingroup$ @G.Smith I mean, writing down equations is one thing, but being physically possible is another, and I have a sneaking suspicion that solutions of those equations with $k = 0, -1$ might be unphysical to begin with. $\endgroup$
    – Rivers McForge
    Oct 3, 2020 at 0:49
  • $\begingroup$ Mainstream physics does not consider these models to be unphysical. $\endgroup$
    – G. Smith
    Oct 3, 2020 at 3:48

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All we really know is that all of the matter in the observable universe was a very uniform, very hot, expanding plasma about 13.7 billion years ago. We don't know where that came from. "Big bang" is just a name for our ignorance.

Whatever process produced that state could plausibly have produced a region of finite or infinite size. Whether it's finite or infinite, it could be just one part of a larger universe. At this point we just don't know.

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  • $\begingroup$ I very much like this answer. @benrg $\endgroup$
    – Buzz
    Nov 14, 2020 at 19:10

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