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I've heard from a number of people that the Universe is infinite, and from my perspective, I can't see how this can actually be known, especially given that the Universe started out a finite size and grows at a finite rate.

Regarding the possible similarity of my question: I want to know how it could be and how we would know, rather than if it is. This is significantly different.

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  • $\begingroup$ No, I want to know how it could be, rather than if it is. Granted, there are some similarities, but I think my question is distinct enough that it's aimed at significantly different information. I went ahead and checked that question too, and I didn't get all the information I wanted from it. $\endgroup$ – Pulchritude Mar 4 '16 at 14:54
  • $\begingroup$ Oh I'm sorry, how we know it is, not how it could be $\endgroup$ – Pulchritude Mar 4 '16 at 15:11
  • $\begingroup$ I don't think we "know" the universe is infinite, just like we don't "know" the big bang happened. Its just a best guess based on our current understanding and theories, as described by Sir Cumference in his answer. $\endgroup$ – Dean Mar 4 '16 at 15:14
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We don't know for sure, but it certainly fits into our theories. There is, of course, no way to actually test if the Universe is infinite, but right now we think it is likely.

Also, if you read my updated answer on your other post, the Universe has always been infinite in size. I explain over there how it actually works: space is created in between everything, and thus one could say the Universe is expanding.

These objects can actually drift away from each other faster than the speed of light. That is, light from them eventually won't make it to us, since they'll be drifting away too quickly.

Now, this doesn't actually go against Einstein's theory that the speed of light is the fastest thing in the Universe. Einstein said that nothing can travel through space faster than light — but here, space itself is actually being created between the objects. Distances are increasing because space itself is dilating, and thus we can drift apart from other objects faster than light.

Really, there is no limit (as far as I know) to how fast we can drift away. Farther objects will keep drifting faster and faster away, since our gravity has a much weaker effect on them.

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  • $\begingroup$ Really wish people would explain downvotes. $\endgroup$ – Sir Cumference Mar 4 '16 at 21:14
  • $\begingroup$ But the theory that the universe is expanding is just an untested extrapolation of the Hubble constant. Hubble found that, when plotting the distance of a galaxy vs its redshift, that there is a linear fit. Which does not necesserely mean that they are flying away faster. It could for example also mean that the light "fatigues" during its travel and lowers its frequency. One explanation for this would be, that according to special relativity gravity slows down time. What if the light somehow gets "slowed" down during its longer travel by more gravity sources? Fact is we dont know for sure $\endgroup$ – RononDex May 2 '16 at 14:46

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