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Currently it is not known whether the universe is finite or infinite. Is there any test that can be performed (theoretically) to know whether the universe is infinite or not?

I'm still in high school, so it'd be great if the answer provides a more qualitative test rather than a quantitative one, as I doubt I'll be able to understand the mathematics behind it.

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Given that we can only interact or observe the universe out to a certain distance, it isn't possible to test whether the universe is infinite. Even if it is, the parts of it beyond a certain distance are inaccessible to us through any means, because all information is limited to the speed of light, including the travel of gravitational waves and massless particles; so the only way we'll ever be able to tell whether it is finite is if someday an edge shows up somewhere, in which case we'll know it's not infinite.

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  • $\begingroup$ It's not true that the only way we can know whether or not the universe is by finding an edge. There are other ways to test the idea. For example, we may one day find a final model of physics which accurately predict everything we can observe, and this model may also predict a finite/infinite universe. $\endgroup$
    – Sheepman
    Apr 27, 2015 at 14:52
  • $\begingroup$ Please edit the answer to include @SamuelMontgomery 's comment. I'll accept it then. $\endgroup$ Apr 27, 2015 at 15:31
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    $\begingroup$ @SamuelMontgomery: your assumption is not safe. We may well find a theory $A$ that matches all experiments and tells us whether the universe is finite or not. That does not mean there isn't another theory $B$ that also matches all experiments but predicts the universe size to be the opposite of what theory $A$ predicts. $\endgroup$ Apr 27, 2015 at 17:00
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    $\begingroup$ @BinaryGeek: yes. Asher's answer is correct. We may one day prove the universe is finite though we're unlikely to find an edge - more likely is that we'd find the universe was topologically closed. We are very unlikely ever to be able to prove the universe is infinite. I don't like saying we'll never prove it because never is a dangerous word! $\endgroup$ Apr 27, 2015 at 18:12
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    $\begingroup$ @BinaryGreek: Apologies. The Debye scale in a plasma is basically like the visibility in fog. What's far away in that "fog" is simply not visible. The same is true for the universe as a whole. As humans we live in this particular domain of the visible universe and nothing on the "outside" can get to us. We simply can't look beyond the big bang event horizon. What could be "behind/before it" is not a scientifically testable question. So basically the max. spatial/temporal scale is well defined and walled in with an impenetrable wall. "Infinity" is thereof not even a question that matters. $\endgroup$
    – CuriousOne
    Apr 28, 2015 at 12:59

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