# How do we know the universe is infinite?

Tags:
1. Jul 26, 2015

### Rupert Young

In this documentary they discussed some research experiments which concluded that the universe is infinite. I didn't really understand it. Can someone explain how we know that the universe is infinite?

Wouldn't this also mean that the universe was infinite at the big bang?

2. Jul 26, 2015

### FactChecker

3. Jul 26, 2015

### Bandersnatch

Since most of us can't watch the doc, can you confirm a guess about its contents?
Was it about measuring flatness of the universe? Perhaps something about geometry, angles in a triangle adding to more/less than 180 degrees? Baryon acoustic oscillations?

If not that, can you quote or paraphrase the part that was unclear?

4. Jul 26, 2015

### Rupert Young

Yes, I remember angles adding to more/less than 180? Was that referring to flatness?

5. Jul 26, 2015

### rootone

As far as I know the case for the (whole Universe, as opposed to observable), being spatially infinite (or not), is still very much debatable.
mathematically it can be argued both ways, observationally we don't have enough real data to draw conclusions.

The observable universe is definitely finite, almost as a matter of definition.

Last edited: Jul 26, 2015
6. Jul 26, 2015

### Chronos

Flatness is only suggestive of an infinite universe. There are finite topologies [e.g., torus] that would also exhibit flatness. There no known way to definitively show the universe is infinite, while there are a number of ways to prove it is not. None of these finite universe models has been validated, so, the general consensus is the universe is probably infinite.

7. Jul 26, 2015

### Chalnoth

I don't think this last bit is accurate. Because the evidence is lacking, which option is preferable is entirely up to personal preferences.

8. Jul 26, 2015

### phinds

Yes, and as Chronos said that does not imply an infinite universe, just that the universe MIGHT be infinite.

You really have to be very careful in getting your science from TV. They have really neat graphics and pics and so forth, and they do get a lot of stuff right, and the programs can be very entertaining but they get a lot of stuff wrong, some of it egregiously so and the problem is that if you don't already know the actual science you can't tell which parts are right and which are wrong.

9. Jul 26, 2015

### Orodruin

Staff Emeritus
It should however be noted that a torus breaks another of the usual assumptions in cosmology, isotropy. With isotropy and homogeneity, there are only three options.

10. Jul 26, 2015

### stedwards

Do you mean an embedded torus, a torus generated by identifying opposite edges of a square, or a hyper torus by identifying opposite faces or a cube? As far as I know, the hyper torus is isotropic and homogeneous, and a candidate for flat space.

11. Jul 26, 2015

### William White

surely the only answer to the question is

"We don't"

(If you are outside of the UK, its quite easy to watch the BBC using a British Proxy)

12. Jul 26, 2015

### Staff: Mentor

13. Jul 26, 2015

### rootone

It works for me, although I did see this doc before.
A second take is well worth it for me, and as far as popsci goes it's good enough.
I'm in Ireland, and availability of BBC stuff is a bit strange, though not usually a problem.

Last edited: Jul 26, 2015
14. Jul 27, 2015

### Orodruin

Staff Emeritus
It is only locally isotropic. A geodesic on the flat torus ends up returning to the original point (with a winding number of one) only in some special directions, making the space non-isotropic.

15. Jul 27, 2015

### Rupert Young

Could someone run through the reasoning for the flat infinite universe, I'd like to understand theory itself?

16. Jul 27, 2015

### phinds

Flatness is not a theory, it's a measurement. To within the accuracy of the measurements, we measure the universe as flat, but that is not a guarantee that it IS flat (be a hell of a coincidence it is isn't, but that's an opinion, not evidence). If it IS flat, that is not a guarantee that it is infinite. See post #6

17. Jul 27, 2015

### Chalnoth

Yes, but that doesn't mean it doesn't match reality. This just means that the torus would have to be significantly larger than the observable universe.

18. Jul 27, 2015

### Finny

As already noted, to the best accuracy we can measure, the portion of the universe we can observe seems flat or close to it...that is, angles of a triangle add to very close to 180 degrees. We assume we live in a typical region of the universe and what we observe locally likely extends further.

Enough has been posted that this Wikipedia article will explain things in a way that is understandable.

Here is a quick synopsis:
"The shape of the global universe can be broken into three categories:[1]

1. Finite or infinite
2. Flat (no curvature), open (negative curvature) or closed (positive curvature)
3. Connectivity, how the universe is put together, i.e., simply connected space or multiply connected.
One should note that any combination of these can occur, that is, a flat universe can be finite or infinite, or any combination.

The exact shape is still a matter of debate in physical cosmology, however, experimental data ...... confirm that the universe is flat with only a 0.4% margin of error

https://en.wikipedia.org/wiki/Shape_of_the_universe

What we cannot observe, what we cannot detect experimentally, are regions of space from which we cannot receive light signals. That's because information is limited to the speed of light and if we can't get any confirming measurement [observation] information, all we have is theory.

19. Jul 27, 2015

### Orodruin

Staff Emeritus
True, but impossible to falsify experimentally (as is the infinite Universe) and therefore the distinction s more philosophy than science.

20. Jul 27, 2015

### stedwards

Given the FRW metric, how is the curvature derived from the values of $\Lambda$ and the trace components of the stress-energy tensor, $T_{\mu\mu}$?