## View Poll Results: Is the Universe infinite or finite?

Voters
222. You may not vote on this poll
• It's Infinite

70 31.53%
• It's Finite

73 32.88%
• I'm not sure

79 35.59%

# Thread: Is the Universe infinite or finite?

1. Mathematician here, hopefully I can clear up some confusion in this thread:

It's finite, but its potential size is infinitely large.
What I think this means to say is that it is finite but unbounded. We can find an easy example in terms of the natural (counting) numbers 1, 2, 3... and so on. Each number is finite, but they become arbitrarily large. However, under our current understanding, it is likely that a finite (i.e., closed - see the sphere analogy below) Universe would eventually stop expanding and collapse.

An infinite thing can't expand.
Sure it can, and we can construct simple examples depending on your precise definition of expansion. To use, say, two-dimensional area in the same way you would in a geometry class, consider a bar in the plane which extends infinitely far to the left and right, but is only one unit tall. It is easy to imagine such a bar expanding so that it remains infinite in extent to the left and right but is now two units tall. In both cases, the area is infinite, but in some sense the second object is "bigger" in the intuitive sense you're referring to.

If it's finite, it has an edge
Not necessarily. Our current understanding of the Universe models it as an object called a manifold. Roughly speaking, a manifold is an object that looks like normal Euclidean space as you did it in high school geometry, but only if you "zoom in" far enough. A simple example would be the surface of a sphere - if you stand at a point on the Earth's surface, the area around you looks like a plane. The surface of a sphere is finite in some sense (mathematically, we might say it has finite measure or is compact, both of which have suitably rigorous definitions), but it has no boundary. It simply loops back around on itself. Similarly, our universe looks on a small scale like the usual three dimensions of space and one of time world that we're used to, but under extreme conditions or on large scales behaves differently. If our Universe is closed, it would be structured similarly to the sphere, except that the surface of a sphere is two-dimensional and our Universe to the best of our understanding is four-dimensional (although some speculative theories use many more).

Infinity is infinity! There's no bigger or smaller infinities!
False in almost any suitably rigorous sense. I direct you to Wikipedia's article on cardinality.

There's just no evidence of an infinite amount of energy in the universe. Sure, there is a lot of it, but no matter how far it expands or how long it goes on it's still a finite number.
The total energy can be infinite even if the density of that energy is not. Consider the plane filled with a uniform density of energy, so that the amount of energy contained in a region is proportional to the area of that region. Since we, and any other point in the universe, can only observe a finite volume at any finite time due to speed-of-light constraints, the total energy of the universe may well be infinite even if the energy we observe is always finite.

Now how is that supposed to work? If we follow the Big Bang, this universe started condensed in one spot. So you're telling me that, in 13.7 billion years, it managed to expand ~6.78 x the speed of light?

Matter cannot move faster than light under current physical understanding. But this expansion is not motion in the normal sense - roughly speaking, the way the Universe measures distances is, over time, measuring larger and larger ones even between objects that are not (with respect to their own space) moving. The effect is tiny enough to be unobservable at small scales (within, say, a galaxy), but in the voids between galaxies it dominates, and it is not limited by speed-of-light concerns because proper motion is not involved. In addition, objects that we see today emitted the light we see when they were considerably closer to us than they are now.

For instance, suppose we all lived on a straight line where distances double every year (and for the sake of keeping calculus out of things, let's assume that doubling happens instantaneously at 12:01 January 1), and where the speed of light is two units/year. An object that emitted light one year ago on December 31, 2000, when the distance between us and the object was 1 unit, would be seen by us on December 31, 2001 as it was on December 30, 2000 - but as we see it, that object is now two units away from us even if it didn't "move" with respect to its own space. Such an object would cease to be visible eventually. This analogy is very simplified and ignores the fact that there is no absolute notion of "distance in space" in the Universe, but it serves to illustrate the point.

2. Originally Posted by Pugbot
I believe Carl Sagan says "Finite but boundless" and that is truly the quickest way to sum it up.
A nice tl;dr that I'd also tend to agree with.

3. Originally Posted by Didactic
Because if it expands into infinity, then even a finite object then becomes infinite. Infinity + 1 is still infinity.
The infinity that it's expanding "into" isn't part of the universe, it's something else or nothing at all. If space itself isn't infinite then neither is the universe because space contains energy and it's space and energy which are governed over by the laws of physics, and that's the definition of the universe; spacetime, energy and the laws of physics.

Seriously, I'm puzzled as to how you think that just because space can expand indefinitely, it must mean space is infinite. And as I've said, it only expands into infinity at t=infinite. Which is where we aren't, and where we'll never be, so it's silly to say that the universe is now infinite just because it's expanding into infinity (although there technically doesn't have to be anything with volume "outside" space).

It might be infinite for other reasons but not because of that. The only clue so far that physicists have that points towards an infinite universe is the topology of space. If physicists thought that the fact that space is expanding is enough to declare that space is therefor infinite, then they would've done so, and there would be no open question as to the finiteness of space/universe.

---------- Post added 2012-12-24 at 05:44 PM ----------

Originally Posted by Dezerte
A nice tl;dr that I'd also tend to agree with.
Well, with all due respect, Carl Sagan died 20 years ago. Physics is an ever-changing science, and those that don't adapt to the changes and accept them as they are aren't proper physicists. Back then, there was no reason to think that space was infinite. Today, we pretty much know that it is. And that pretty much means the universe is unbounded since space is its defining part.

4. When we get a big enough telescope, the universe will figure out if it is finite or infinite.
What?

If there is no end we don't call it "infinite", otherwise we would be calling it now and making these statements.

If we get a big enough telescope and there is still no end then obviously we need a even bigger telescope.

Also telescopes in this thread don't make any sense.

5. It's finite, but practically infinite since it expands faster than anything could reach its border.

6. Originally Posted by Cranica
False in almost any suitably rigorous sense. I direct you to Wikipedia's article on cardinality.
That's maths. Cantor's set theory don't really have anything to do with physics.

Or are you referring to something else?

7. ## You are absolutley right, and I can give you the proof.

Originally Posted by Activi-T
I believe it to be both. An infinitely expanding finite closed system. What I mean by that is you can't go past the the 'bubble wall', or the outer edge if you were to suddenly be transported there, into the medium the universe is expanding into but the expansion of the closed system is infinite (no 'big crunch' point). That is my layman's opinion at least.[COLOR="red"]

---------- Post added 2012-12-21 at 10:02 PM ----------

I.e the stuff inside the universe is finite (energy and the likes) but the expansion (empty space/distance between things) will tend towards infinity.
I wanted just to say that you are "right". But you thinked a little reversed. Its not really finite or normal infinite. But its special infinity. Special infinity got no limitation in existence but it has limitation in size. The big bang is wrong because Universe got no start. If you want to help me spread this theory which I try hard then would I tank you. Look at my picture of Universe with black background at;

www dot invinsibleforce dot com (describe special infinity) you must understand black stars, black hole is wrong, its stars thats imploding.

Well if you can't understand I thank you for been reading this at least.

Check my equation of an infinity core. The entire Universe is the special infinity reactionchain who is "finite" in size.

talk with me on skype; karate-martin at hot mali dot com

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