Is the Universe Infinite or Finite? I was having that debate with a friend of mine today. My position was that it is finite because it is expanding. My logic was that the fact that is expanding indicates that it must be finite, because how could something be infinite and getting bigger?

His position was that it was infinite though he couldn't really explain why he thought that. (to give you some context, he also believes his daily horoscope offers him meaningful guidance and that aliens built the pyramids).

You may have guessed by now that neither one of us are physicists, so to settle the argument, I proposed that we contact an expert.

Confident of my correctness, I suggested to my buddy that we should have friend of this here site @seanmcarroll (the brilliant physicist and author of "From Eternity to Here") give us an answer.

I tweeted at Sean the subject line of this thread and he was kind enough to tweet me back in a matter of minutes. He said:

"Nobody knows. Sometimes that's the answer."

He then tweeted

"There may not even be an answer. de Sitter space (e.g.) can be finite or infinite depending on how you slice it."

So not wanting to badger Sean with follow up questions, I ask you: If we know the Universe is continually expanding (which to me suggests that it has a finite size that is increasing), how can it be labeled as anything other than finite?

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When I say "the universe", I am referring to our ball of expanding spacetime that grew out of the big bang. It was infinitely small,dense and hot and now it is enormous, expanding and cooling. There's still a part of me that says that thing is finite.

Interestingly enough, there is a hypothesis (covered in Lauwrence Krauss' latest book A Universe From Nothing) that presents the idea that our universe is just a local difference inflationary rate and that other universes are created similarly. That there is a vast, infinite expanse of expanding spaccetime and that our universe came about due to one small segment of spacetime undergoing a period of extraordinarily rapid inflation. It's a very interesting read, I recommend it.

Hello Dave: I second your opinion of Krauss' book. Its worth buying and studying. Although its not a text book and he could have elaborated further on Cosmic Inflation. Chapter six was a bit skimpy.  

If it IS finite, all we can give is a minimum size, since we don't know what's beyond the observable edge caused by the Hubble expansion.  The universe could be 10, 100, a billion, or a googol (10**100) times bigger than the observable universe, for all we know or can ever know.

You should give this a watch. it is put in laymans terms and will help inform you while leaving you alot more confused

I think of the universe as a really big balloon.Though the skin of the balloon (edge of the universe) represents a finite border. The fact it's always expanding shows that's it infinite. Now, will we ever get to the 'breaking point'? And if we do, will that lead to another 'Big Bang"? (no pun intended)  No one knows.

The Universe has been observed to be finite. And their may be evidence of a Multi-verse

Basically, I agree with Carroll.  However, I would go a bit further and suggest that it is an inappropriate question in the context of our times, our present state of knowledge, the current degree of neuronal complexity of our brains, and especially the conduct of our day-to-day lives.

That is not to say that we shouldn’t be trying to find answers to the question; we wouldn’t be human, otherwise.  But, in all likelihood, Homo sapiens will become extinct LONG before coming anywhere near understanding the true, deep nature of what we term, “the universe.”  

The human brain, as it is currently structured, is profoundly underequipped to grasp the concept of either eternity OR non-eternity.  We cannot envision an end to the universe because, in our experience, there always has to be “something on the other side.”  On the other hand, we can’t imagine anything going on indefinitely, since we have not experienced such a phenomenon (except maybe the Moebius Band) in our own lives.

The apparent dilemma of “infinite” vs “finite” only exposes the severe limits of the human mind at this stage of our evolution, and that the only reason to think about it is to exercise that mind.

And, in order to exercise MY mind on this matter, I’m going to give that “Moebius Band” metaphor some serious thought. 

Perhaps the universe is both infinitely vast and infinitely small, and life is merely the balance of these two extremes.

I don't even know how to parse that. Could you rephrase it?

Well I guess what I meant is that maybe the universe in infinite, in regards to the grand scale of things, as well as extending infinitely small, meaning no end to how small it can get. We can't see past a certain limit in both grandness and smallness, with current technology anyway. I know its strange to try to imagine, but I just think that if the universe is indeed infinite in size, why shouldn't that infiniteness extend in all directions, both small and large. I probably only made my thought more confusing with my so called explanation, ha

I think Justin means.. inifinite scale. inifinite magnitude. Am I correct? Not just infinite matter, but a scale so large that it makes a galaxy the size of an atom, but is still relevant to matter somehow?

I am in university for the very small things, and it is completely unknown if there is a definitively small thing. We know there are quarks, gluons, etc... inside of atomic particles, but we cannot directly observe them, and there is currently no way to tell if there is something inside of the quarks. It would be incredible to find something smaller (like the string of string theory!). I would be very excited.



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