# Half closed universe

1. Mar 23, 2012

### sea100

if the universe is not infinite in size is that mean a closed universe ?

2. Mar 23, 2012

### balaonspace

infinity is a nice number for mathematicians but in physics you can put a finite number on everything, big or small, if not then there is a singularity.
From current theories we can explain that the universe has a finite size, which can be calculated approximately from speed of light(c) and age of the universe(t). approximately the radius of a spherical universe is 'ct' and hence the volume.
You can do the same rigorously using FLRW metric and GR.
cosmologists use the term closed universe to explain on of the possible end of the universe. a closed universe is one which will contract and end in a big crunch.

3. Mar 23, 2012

### nitsuj

what is a closed universe?

4. Mar 23, 2012

### phinds

nonsense. we can do no such thing. you are talking about the OBSERVABLE UNIVERSE while using the term "universe" (incorrectly) which is not what the question was about, it was about the universe.

5. Mar 23, 2012

### phinds

Yep, has to be. The actual question of WHETHER the universe is infinite or not is an open one.

6. Mar 23, 2012

### phinds

Think of the surface of a baloon. NOT the ballon, JUST the surface. If you are limited to traveling only on the surface, then traveling in any straight line brings you back to where you started and that's a closed surface. "Closed" for the universe can be more complicated, but one POSSIBLE closed universe is such that if you travel in a straight line, you get back to where you started (this is a bit more complicated than I'm making it sound, but that's the fundamental idea).

7. Mar 24, 2012

### clamtrox

This is not what is usually meant by closed. In usual terminology, closed universe is one which has enough energy in it to stop expanding and collapse back to itself.

8. Mar 24, 2012

### Chronos

In cosmology, the term 'closed' refers to the topology of the universe. Phinds characterization is essentially correct.

9. Mar 24, 2012

### clamtrox

10. Mar 24, 2012

### balaonspace

You are relating the concept of infinite universe with constant Ωk or k in RW metric. Whatever the value of the constant is it will only tell you the topology of the universe and not the size of the universe. The size will always be finite

11. Mar 24, 2012

### phinds

Well, you clearly know something that is not known by most physicists, since most consider it an open issue as to whether the universe is infinite or finite. You should spread the word that it is DEFINITELY finite. Some will be astounded. I WOULD suggest that before you do that you find some physics to back it up.

12. Mar 24, 2012

### balaonspace

Are you suggesting that the size of the universe is infinite since dawn.
because according to my understanding if big bang created the time and space then at this epoch an accelerate expanding space should be a finite value and cannot be infinite. I might be wrong but i want to be convinced that i am indeed.

13. Mar 24, 2012

### phinds

It is not known whether the universe started as finite or infinte. Certainly if it started finite, it is still finite and if it started infinite, it is still infinite. I make no statement that it is/was one or the other, but I challenge your statement that you know it is/was finite. What is your source for such a belief?

14. Mar 24, 2012

### Mark M

Not necessarily. If the universe is flat, it can still be finite by being in the shape of the surface of a 3-torus.

15. Mar 25, 2012

### balaonspace

Thank you. I get it. Its just my idea that infinity makes no sense in physics. But I dont know how one can check the hypothesis that the universe started infinite.

16. Mar 26, 2012

### Cosmo Novice

We cant check it. We can make certain assumptions though, we can use the cosmological principle of homogeneity and we can make a pretty good assumption that past the OU is more of the same - whether that is applicable to infinity or just a much larger U than the OU (their are studies which place a lower bound on the size of the Universe if finite.)

Curvature has a big role to play in the topological "flavour" of the universe, whether open/closed or finite/infinite.

If the Universe is finite now, then it had a finite beggining, if the Universe is infinite now, then it has always been infinite. Remember as well than when talking about the "size" and "shape" of the Universe that if you treat any part of it differently (imbue some area of space with a conecpt of a center or an edge) then you invalidate the cosmological prinicpal.

Now I do not often give a personal opinion, but my gut feeling is that beyond our OU is just more of the same for infinity. I think there are probably hubble volumes totally devoid of mass (which does not invalidate homogeneity when assuming infinity.)

Anyways thats just my two cents!

17. Apr 2, 2012

### Khashishi

Mathematically, you can have an open space that's not infinite, for example, the inside of a ball, not including the shell. But, there are some restrictions on what shapes the universe can be. People usually assume homogeneity and isotropy--rather strong assumptions. These assumptions basically constrain an open space to an infinite space. But, if you don't assume homogeneity then you can have open finite spaces.