# Why is the universe considered to be infinite in size?

1. Dec 8, 2003

### deltabourne

Infinity is essentially the biggest 'number' there is.. there is nothing greater than infinity, which is why infinity+5=infinity. Now the universe is said to be expanding, meaning it grows in size - if it really was infinity it could not expand at all since it was already at an infinity. It's an inherent contradiction, so is it generally accepted that the universe is infinitely large or it is finite and expanding?

I hope i'm making some sense, I didn't get much sleep last night

2. Dec 8, 2003

### jcsd

It's not certain but an infinite universe looks to be the best fit for current data and understanding of the cosmos.

Imagine the universe as an infinitely large piece of square paper, when the paper universe expands the squares on the paper get bigger.

3. Dec 8, 2003

### deltabourne

Hmm.. seems to make sense, except that what do the squares that get bigger represent? It isn't planets or celestial bodies, since those aren't getting bigger (are they?). However we know they are moving apart from blue shift, but that doesn't necessarily mean the universe is expanding. It's already large enough to have all the planets infinite distances away from each other (since the universe is infinitely large).

meh, I may be talking nonsense/semantics here, but it's just a little quirky idea that was bugging me today

4. Dec 8, 2003

### jcsd

It means that the distance between objects in th euniverse is getting bigger.

Something is redshifted when it moves away from us (compared to blueshifted when it moves towrads us) and yes it does mena that the universe is expanding.

5. Dec 8, 2003

### Ambitwistor

Regions of space.

When we say that the universe expands, we mean that all the galaxies are getting farther away from each other. That can happen whether the universe is finite or infinite. Any two galaxies are always a finite distance from each other, though that distance may increase with time.

Last edited: Dec 8, 2003
6. Dec 8, 2003

### StephenPrivitera

Among infinities, isn't it the case that some infinities are larger than others? I remember hearing something along these lines before. Something like the set of reals is a larger infinity than the set of integers.
So we're looking at finite space and saying that finite space is expanding. Overall is the universe getting larger? Or is just the space between things in the universe getting larger?

7. Dec 8, 2003

### Ambitwistor

When we talk about an infinite universe expanding, no new points are being added: the set of points is always the same order of infinity, the cardinality of the reals.

Both. The universe gets larger because the distances between all points in space get larger.

8. Dec 8, 2003

### TillEulenspiegel

The universe is considered to be infinite because all that exists is contained within it's limits.The fact of differing infinites is a mathematical abstraction based on an understanding of things like set theory and weirder stuff all based on an alphabet of symbols. I.E. There are more numbers between 0 and 1 then between 1 and infinity.

The application when it comes to these symbols and concepts when it comes to reality becomes somewhat stickier. If the universe is infinite how can it be expanding? What is it expanding into? Thats the kicker
The universe "makes" it own space as it expands it does not expand into anything =) counter intuitive but correct. You may use a sheet of paper as an example but there is nothing outside the sheet of paper . If you finally come to the accommodation , however unreasonable it seems, that there is no outside, you have it . The question on peoples minds is what is it expanding into? The answer is nothing, not really nothing because even nothing is something.. There is no there there, you have to use a Zen mindset to come to grips with it If that happens some of the other weirder concepts of QM and even Relitivistic physics becomes less a mind stunner.

9. Dec 8, 2003

### Ambitwistor

We don't actually know whether the universe is infinite. "All that exists" could be contained within the universe, but the universe could still be finite in size.

10. Dec 8, 2003

### TillEulenspiegel

Well much is not certain, but recent evidance seems to point to omega as ~1 so if correct that means a flat, open , infinite universe dosn't it?
Of course we won't know till the SNAP and Planck probes go up will we?

11. Dec 8, 2003

### StephenPrivitera

Excuse my cosmological ignorance. Didn't the universe start out finite? How does something finite become infinite?

12. Dec 8, 2003

### Ambitwistor

Well, that's if Omega is exactly equal to 1. But we can never know if that's true... the universe could be finite, but inflated so much that it's just under 1...

No, not necessarily. If the universe is infinite, then the universe started out infinite. (It's only the finite observable universe that has to start out as a point.)

13. Dec 8, 2003

### TillEulenspiegel

I don't think anyone can answer that question even though it seems simple on it's face. It has more to do with semantics, language,human cognition,Theosophy and philosophy then science. One concept is that it was not finite as the only existence was included in it's corpus, nothing else existed so to treat it as a point in space is a fallacy, there was no space. The others require the prsupposition of god or other devices to explain them.

14. Dec 8, 2003

### StephenPrivitera

Ow... my brain hurts.
What nonsense is this? So every finite element of the universe started out as a point. And to sum all these points an infinite amount of times we get an infinite space. Sounds like 0 times infinity = infinity to me.

15. Dec 9, 2003