# IF the universe is infinite

1. Sep 23, 2010

### Oldfart

Was it always infinite, starting at the moment of the big bang, or did it somehow evolve to an infinite entity?

A newbee needs to know...

OF

2. Sep 23, 2010

### NobodySpecial

Infinite is a bit tricky when talking about space. It depends on what shape (in 4Dimensions) the universe and space-time is. It's possible to have a universe that has infinite volume but only a finite diameter for instance!

The shape of the universe also determines it's ultimate fate, if it will expand forever, or stop or collapse. At the moment the data looks like it will just about expand forever.

3. Sep 23, 2010

### George Jones

Staff Emeritus
This all changed in 1998. Prior to 1998 and the discovery of dark energy (non-zero cosmological constant), it was thought that: if universe is closed (positive spatial curvature), it will stop expanding and collapse; if the universe is open (flat or negative spatial curvature), it will expand forever. Data after 1998 indicate that the universe is close to flat, but that universe will expand forever even if it closed.

4. Sep 23, 2010

### NobodySpecial

That's a relief !
Didn't know they had fixed on the amount of dark energy needed to keep open a lambda>1 universe, always seemed a bit naval gazing since lambda apparently = 1.

5. Sep 23, 2010

### Kevin_Axion

Is it possible that it's just locally flat? Because I don't think this conjecture has been disproven.

6. Sep 24, 2010

### Chalnoth

Well, yes, the dark energy density now is quite dominant. It is conceivable that it will still recollapse, but this requires some rather exotic physics that allow the dark energy to decay away in addition to having a closed universe. But at the current time the dark energy density is very roughly 3/4ths the energy density of the universe, with curvature being constrained to contribute less than one percent to the current expansion rate. If the dark energy is truly a cosmological constant, as the universe expands the contribution to the expansion from dark energy will increase (because everything else dilutes while the cosmological constant stays the same), while the contribution from curvature will decrease along with everything else.

P.S. I'm pretty sure you meant omega, not lambda.

7. Sep 24, 2010

### NobodySpecial

Yes, I even put in the tex \lambda and thought I had done something wrong when it didn't draw an $$\Omega$$ - just one of those brain failure moments !

8. Sep 26, 2010

### Oldfart

Thanks for your replies! Though from your replies, I'm not sure that I asked my original question correctly. So I'll try once more with some more detail...

As I understand it, there is at least a possibility that the universe is infinite in extent (or infinite in volume). If you agree, let's assume just for purposes of this thread that it is, in fact, infinite.

If the universe is infinite now, was it always infinite? I have difficulty picturing a universe that "became" infinite at some point in time. And on the other hand, I have difficulty picturing a universe that was always infinite, including back in the days when it was born from a point or a pea-sized thingy. So I need your help to get my brain wrapped around an answer to this conundrum. Or, perhaps I just need some help with the definition of infinity. Thanks for listening...

OF

9. Sep 26, 2010

### Sakha

I'm pretty ignorant in this topic, but as I understand, the Big Bang theory explains that the universe was 'a point' and it expanded, hence the Bang. It was proven (can't remember by who but my heads points to Hubble) that the universe is still accelerating and expanding.

10. Sep 26, 2010

### Fredrik

Staff Emeritus
As I was reading the thread, I also felt that your question had not been answered. I don't think there was anything wrong with how you asked it. There are three main classes of solutions that describe homogeneous and isotropic universes. In two of them (negative curvature and zero curvature), the universe is infinite at all times, and in the third (positive curvature), it's finite at all times.

To understand how "always infinite" is consistent with the big bang, just imagine an infinite line with distance markings on it. Imagine that the markings are f(t) light-seconds (or whatever unit you prefer) apart at time t, for some smooth strictly increasing function f defined for all t>0 (but not for any t≤0). If you consider any two markings in the limit t→0, the distance between them (defined by the function f) goes to 0. The "big bang" is just a colorful name for the funny stuff that happens in the limit t→0.

Note that there's no t=0 in the theory, so the phrase "at the moment of the big bang" doesn't make sense (if we're talking about the original big bang theory). This doesn't just mean that we "don't know what happened before". This is the theory that tells us what time is, and it doesn't even mention a t≤0, so we can't either. Right now, your intuition is telling you that there must have been a time t=0 and times t<0, but experiments have proved that theories that describe time in a way that's consistent with our intuition are a lot less consistent with reality than GR, so this is not a good time to trust anyone's intuition.

Last edited: Sep 26, 2010
11. Sep 27, 2010