Space finite/space infinite/we don't know---which do you believe?

[Martin]

Do you believe space is finite, or do you think it's infinite? Or do you believe we don't have enough evidence to tell yet?

 

It's a fascinating question. Next year two new satellite observatories, Planck and Herschel, are scheduled for launch---their data may help resolve this!

 

I'm posting this on behalf of Agent. He had the excellent idea of a poll on this issue. But he neglected to include any way for science-minded people (who require scientific evidence for their belief) to take part in the poll!

 

So to correct that, I've reformulated the poll.

Edited October 22, 2008 by Martin

[npts2020]

Part of the question comes down to the definition of infinity. Is it merely any number so big as to be incomprehensible or is it an impossible concept (what is infinity plus one)? By the first definition I would certainly say the universe is currently infinite, the second is far less certain and borders on impossible IMHO.

[Sisyphus]

I haven't really been following the discussion or particularly understand the methods at work, so this might be a dumb question, but is it possible we can definitively conclude that there's no way to tell? Kind of a "cosmic uncertainty principle" or some such? It's something I was thinking about reading a thread a little while ago about the observable universe vs. "real" universe, and trying to draw (maybe flimsy) analogies with quantum uncertainty. If nothing beyond the "information horizon," for all practical purposes, even "exists," maybe a third possible answer to the question is, "it doesn't matter?"

 

Part of the question comes down to the definition of infinity. Is it merely any number so big as to be incomprehensible or is it an impossible concept (what is infinity plus one)? By the first definition I would certainly say the universe is currently infinite, the second is far less certain and borders on impossible IMHO.

 

Infinite means not finite. So it's specifically not your first definition. As for the second, I don't think that demonstrates that infinity is self-contradictory, just that its fundamentally different from finite quantities, and not merely "the biggest." If the universe were infinite, it would have no finite volume, no limit, no "folding back on itself," etc.

[Martin]

Hey, we have five responses already! Thanks to everyone who has registered their choice.

 

Part of the question comes down to the definition of infinity...

 

Right! Or rather, the definition of the adjective infinite. It means not finite. In this case we're talking about the volume of space. It is finite if you can say it does not exceed, say, a trillion cubic lightyears. If the volume exceeds any number like that which you can name, then it is not finite.

 

It may be easier to think about in lower dimension, like the length of a line.

A line can have finite length or infinite length. Finite if you can put a limit on and say the length does not exceed, say, a trillion miles. Otherwise the line goes on indefinitely, with no limit to its length, and we say its length is infinite.

 

I wouldn't use the abstract idea of infinity in this case. What's at issue is whether or not space has a finite volume, kind of a practical matter. Notions of finitude and non-finitude seem like more abstract than we really need here.

 

... is it possible we can definitively conclude that there's no way to tell?...

 

Sure that's possible! It might turn out that way or it might not. We will be learning a lot more about this soon, assuming Planck is launched as planned.

 

And it will always depend at some point on how you weigh the evidence---and the consensus of the scientific community. They tend to eventually settle down into some kind of agreement about issues like this.

 

If you think that, then you should come down in favor of the third option, Sisyphus: We don't have enough data to decide.

(Indeed according to your suspicion, we never will have sufficient data!)

Edited October 22, 2008 by Martin
multiple post merged

[YT2095]

I reckon it will turn out to be finite, and the fact that we don`t know for sure yet is what makes the idea of exploration exciting!

[Mr Skeptic]

I believe that it is infinite, but also that it will be impossible to prove that. We can only see a finite portion of our universe, and while that local configuration may be indicative of an infinite universe, there would be no way to prove that the open configuration extends beyond the borders of the observable portion. If we can figure out how our universe could have been made, we might know if it is infinite or not, but again that would not be proof unless we can show that it is more than just theory.

 

In practice, the observable universe is finite and unless we can observe or go beyond the cosmic horizon, it does not really matter if the universe is infinite or just big.

[Martin]

Eight responses on the poll, so far.

It's interesting the different takes on it that people have.

Personally I've always liked the idea of space being finite, going back to before WMAP when we just had COBE data and a confidence interval of [1.00, 1.02]. That was back in 2003. The professional community is still leaning in favor of space being infinite, but over the years as I have been watching, cosmologists have become increasingly open to considering the possibility that space is finite and increasingly often they analyze data based on that assumption (instead of the usual flat infinite space assumption.)

 

It's pretty clear both are possible and we just don't know at this point. I've had some lengthy discussions about this, including one with a Princeton cosmology PhD who insisted that the prevailing view was infinite and that the community was NOT becoming more open to the other possibility. But I think the recent literature 2006-2008 shows a trend that bears me out.

 

Mr Skeptic, you favor space infinite which puts you in good orthodox scientific company. And you also say the issue remains undecided, which is what the experts say too! So you are unquestionably on the mainstream majority side. But I disagree with you on one point!

 

You say "it does not really matter if the universe is infinite or just big." I think it matters enormously! It affects our understanding of physical law. Because the answer affects what can have preceded the beginning of expansion. A lot of research is going into this now and a book is scheduled to come out in April 2009. Physical law is the basis of technology, so it is not merely academic. If space were to turn out finite, then the big bang would have been a process in a finite region, which says a lot about what processes could have given rise to it.

 

Basically I turn around what you said, Skeptic. You said:

"If we can figure out how our universe could have been made, we might know [if it's finite] ..."

What I'm saying is, if we can figure out whether it is finite (which Planck data will help decide starting 2009) then we will have a start on understanding how it could have been made.

 

Really interesting seeing people's different takes on this. Thanks all!

[Realitycheck]

At any given second in time, regardless of relativistic issues, the universe must be finite. However, the issue could be the potential that it could grow in time. Is there a limit or does it keep on expanding forever or does it finally slow down to a stop?

[Mr Skeptic]

You say "it does not really matter if the universe is infinite or just big." I think it matters enormously! It affects our understanding of physical law. Because the answer affects what can have preceded the beginning of expansion. A lot of research is going into this now and a book is scheduled to come out in April 2009. Physical law is the basis of technology, so it is not merely academic. If space were to turn out finite, then the big bang would have been a process in a finite region, which says a lot about what processes could have given rise to it.

 

Basically I turn around what you said, Skeptic. You said:

"If we can figure out how our universe could have been made, we might know [if it's finite] ..."

What I'm saying is, if we can figure out whether it is finite (which Planck data will help decide starting 2009) then we will have a start on understanding how it could have been made.

 

Both those points I suggested are in fact related. I do not think that it is possible to show that the universe is infinite by observing it, and therefore the only way to conclude that it is infinite would be by figuring out how it was made. However, if as you say we can find out by observing whether it is finite or not, then it would help with understanding of the laws of physics (or metaphysics ), which I definitely understand is valuable. However, I don't know how you would demonstrate that if our observable universe is shaped so that it would be infinite, that it must necessarily be that way outside of the observable horizon.

[jjuris]

everything has boundaries at their specific point in space/time. At any given point in time, that's as far it can expand. What's next, who knows... but at that point the universe has a finite nature.

[Dr. Jekyll]

At present I would say that we do not have enough data or/and lacking some theoretics to determine the case. But, if I would place a bet I would go for a finite universe; that I "believe." (If we discard scenarios/assumptions as, e.g., other, additional universes that could sum up to "infinitely many finite universes.")

[npts2020]

Good discussion. One thing I would definitely call finite is our ability to percieve and understand the universe.

[Dr. Jekyll]

Just a mind twisting remark. Mathematically, universe could be finite in volume but spatially infinite. (Disregarding established models)

[YT2095]

OR the universe still being Finite, but an Infinite number of them! each one just a little bit different to our own

[dichotomy]

I’m going with infinite space and a finite universe (yes, I’m suggesting that many universes sit within a space medium). Everything is a pattern of sorts, so why shouldn't that apply to a universe too?

 

But obviously, we don’t have enough evidence to conclude this, or the other speculations yet.