# Is the Universe Infinite or Finite?

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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### Replies to This Discussion

In that case, you're wrong about the infinitude of the universe in both directions, because infinity is a purely mathematical notion.

On a macro scale, the universe has an end in both space and time (there are actually maps of the universe if you want to google, and you'll see that it has a definite structure with edges to it). On the micro scale, it's now understood that space itself is granular, and like zooming in on a digitized photo, you reach a unit smaller than which you can't go called the Planck Length. Thus, some physicists describe space as granular or foamy.

BTW, it used to be thought that empty space was just nothing, but that is not strictly speaking true. Because space is granular or foamy, that helps explain how it can be stretched and/or compressed by gravitational forces. Thus, space was thought in the olden days to be filled with ether, then that was rejected and it was thought to be just emptiness. However, emptiness doesn't explain the effects of gravity very well, so now we are back to thinking of space itself as a weird kind of substance.

So, you can stop all that wondering and think about something else real.

You act as if these things you talk about are proved when the fact is they are just hypothesis with no real evidence behind them. Tjose maps mean nothing as they are only of the observable universe and even if space is finite there is more unobservable universe that  there is no way for us to know anything about. anyway when you research what atrophysicists have to say about the universe being infinite or not you will find the majority say they have no real answer.

The planck length is mainly theoretical as it is impossible for us to probe the universe at this scale, atleast so far.

so in the end the truth is we dont actualy know any of this yet

Which is exactly what Sean Carroll said in the first place.

Yes, and maybe I'm not 5'10" tall because it can't be disproved that there isn't more of me than meets the eye.

The fact is, when talking about the universe one is stuck with what is logical and consistent with what is known, not observations or (even less likely) experiments.There is no control group for the universe, but we make sense of it as best we can using what we know.

I notice you aren't saying that the majority of physicists and cosmologists disagree with what I wrote.

Thats an unfair comparison because they are rather sure there is alot more of the universe outside of what we can observe due to the speed of light. that is why from earth it seems we are in the center of the universe.

The majority of them dont say you are wrong but neither do they say you are right, they simply say they dont know. and when you consider they are very smart people who understand more about this stuff than i ever could i tend to take their word on it. also i am not going to  argue that i do know because i would likely just be making a fool of myself

Well, one thing is for sure: there isn't an infinity of stuff in the universe. That's because "infinite" only has metaphorical or poetic uses outside of mathematics.

By your standards of skepticism, what do we actually know absolutely 100% about anything? Science is always ready to revise itself, so one might say that science doesn't really KNOW anything but only has speculation they are only 99 and 44 percent sure of but not 100%

it is not about being 100% sure the problem is we dont really have a clue one way or the other.Here is Neil Degrass Tyson when asked the same question

Well done Rocky...so now anyone that argues that we can now deduce the answer to this must contend with both Sean Carroll and Neil DeGrasse Tyson. I think I would tap out at that point.

My understanding of the physics leads me to believe Sean was right when he said "Nobody knows." Because if it is too big to measure, we cannot know. We don't even know for certain how to define what the universe is, or if there is more than one.

Astronomy and astrophysics are not laboratory sciences, and thus one doesn't have standards such as reproducibility of results, control groups, or any of the other double-checks used in other sciences as diverse as chemistry or sociology. Thus, if one wants to play skeptic, one can doubt everything.

One thing is pretty certain, though, and that is that the universe isn't infinite because infinity is a mathematical concept having no real world application. The paradoxes related to infinity which are possible in math but impossible in reality prove this.

the problem is either which way you cut it you have to deal with infinity or 0(nothing) which are both mathematical concepts with no real world aplication. ie if the universe is infinite it must have an edge . but then what is beyond the edge as it is expanding into it.

and even empty space is not nothing

If the universe is infinite, it can't possibly have an edge because then it should also be beyond the edge and fill up whatever additional space is available. But this is just further proof that the concept doesn't apply to reality,.

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