# B How is the Big Bang compatible with an infinite universe?

1. Aug 11, 2015

### Scheuerf

I have very little understanding of the Big Bang, but it seems like it would require a finite universe even though there seems to be a scientific consensus that an infinite universe is a strong possibility. How are these ideas compatible? If space started expanding from a small point at a finite speed, how can the universe be infinite in size, a finite amount of time later?

2. Aug 11, 2015

### phinds

It did NOT start as a "small point". This is a canard that you see everywhere in popular science presentations because reality is a bit more complex and they don't want to be bothered (or they don't actually know what they are talking about)

It DID start as a "singularity" but that does not mean "point" it just means " the place where our math model, if extrapolated backwards, gives unphysical results and we don't know what is happening". The big bang happened everywhere at once.

3. Aug 11, 2015

### rootone

You also have to bear in mind that the originally very much denser and smaller Universe refers to the observable Universe.
We don't know if there is more to the Universe beyond that which is observable, and if there is it might be infinite or it might not.

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4. Aug 11, 2015

### H Smith 94

It's also worth noting that it is not physically possible to picture the big bang from outside of the universe, since there was nothing that can exist where there is no universe. Instead, the big bang happened from within the universe, with space itself expanding rapidly -- not with the universe expanding from a finite 'point' to a bigger finite area. In this way, the single point idea often misleads people into thinking of it from the outside and thus, believing erroneously that a universe in which the big bang happened cannot be infinite.

Out of interest, can anyone recall the evidence that the universe may be infinite? I've read it in lots of places but can't remember exactly why it's an acceptable assumption.

5. Aug 11, 2015

### Jorrie

We observe large scale space to be flat, or very nearly so. Couple this to homogeneity and infinite space becomes a real possibility.

6. Aug 11, 2015

### PAllen

An analogy might help to conceive this. Imagine an infinite unstretched rubber sheet. Now, imagine stretching this uniformly everywhere at once, so any patch doubles in size every second. A patch, followed back in time to infinite density is singular in the sense Phinds described. However, any any moment of existence (the singularity might better be thought of as 'before existence', and anyway may not occur in a wide range of models with quantum gravity), the universe is infinite. Infinite is peculiar - you can keep cutting it in half forever but it is still infinite.

7. Aug 11, 2015

### Chronos

Yes, that initial small 'point' occupied the entire universe [keeping in mind there is no place outside the universe, by definition] so it is not possible to assign it dimensiona in any classical sense. That implies it could have been infinite from the git to. If the universe is perfectly 'flat' as existing data suggests, it should be infinite. But, there is some amount of error in any possible measurement - so while it is possible to prove it is finite, to the limits of measurement accuracy, it is impossible to prove it is infinite.

8. Aug 11, 2015

### stedwards

If the universe is a deSitter space, it can be embedded in a higher dimensional space (I think 5 is sufficient) where it may be "pictured" from outside in the sense that the observer does not reside on the $R^4$ submanifold occcupied by the universe.
https://en.wikipedia.org/wiki/De_Sitter_space#Static_coordinates

Last edited: Aug 11, 2015
9. Aug 11, 2015

### stedwards

I don't believe "singularity" (at some given time, as you imply) and a "point" are mutually exclusive. Take a manifold consisting of the surface of a half cone, for instance. The apex of the cone is both a point on the manifold and singular; differentiation fails.

From a quantum physics point of view, it's considered that, at the beginning of the universe, the radius could not be zero, but some minimum value around the Planck length. With this idea in mind, none of the mathematical machinery of relativity is expected to work. Calling it a singularity is like saying, "this thing that isn't a topological set is also singular". No one seems to have a good name for it beyond "beginning of time".

Last edited: Aug 11, 2015
10. Aug 11, 2015

### phinds

I see no problem with using "singularity" as a placeholder so that we don't have to always say "the place where the math model breaks down if you extrapolate it backwards to where its time value is zero". That's too many sylables. "Singularity" is simpler and since we pretty much all agree that that's what it means, it's a perfectly good placeholder.

11. Aug 11, 2015

### stedwards

Sounds like slang to me; like saying "electricity flows in wires".

12. Aug 11, 2015

### rootone

I think the idea of the 'placeholder name' is not unreasonable in itself.
After all we refer to 'dark matter' and 'dark energy' to describe phenomena which are seen to exist, but which we know very little about at present.
The problem with 'the singularity' is that (some of. not all) pop-science repeatedly conveys the wrong impression that we know exactly what is happening when we simply don't,
in the worst cases with overtones of creationism being implied.

13. Aug 11, 2015

### phinds

Another problem with "singularity" is that since we call the "start" of the big bang epoch a singularity AND well call the thing at the center of a black hole a singularity, pop-science sometime conflates the two when actually they have nothing to do with each other except for the shared placeholder name. Nonetheless, scientists seem to have no problem with the convention, just pop-science readers.

14. Aug 13, 2015

### Scheuerf

Would it be correct to say that the universe could be both finite or infinite, and that of its finite the Big Bang occurred at a single point. If it's infinite, it occurred everywhere at once?

15. Aug 13, 2015

### phinds

No it would not. The big bang did not happen at a single point, regardless of whether the universe is finite or infinite. It happened everywhere at once.

16. Aug 13, 2015

### Scheuerf

Maybe my problem is just that it's very counter intuitive. It seemed to me like an infinite universe would be required if the Big Bang happend everywhere at once. What exactly does it mean for the Big Bang to happen everywhere at once?

www.youtube.com/watch?v=XBr4GkRnY04 at 4:10 is where some of my confusion comes from.

Last edited: Aug 13, 2015
17. Aug 13, 2015

### phinds

I don't know how else to describe "everywhere at once" but perhaps it would help you to consider a topology that is finite but unbounded, which is what we would probably have if the universe were finite. Finite and bounded presents serious problems in terms of known physics because there just isn't any way to deal with the "edge". Finite but unbounded is where things "wrap around". For example the surface of a sphere is finite but unbounded.

EDIT: Oh, and by the way, yes it IS non-intuitive, as are many things in cosmology and quantum mechanics.

18. Aug 13, 2015

### jerromyjon

Just as a layman's analogy, a bomb has a location so when it "explodes" you can see how it expands from its initial location. When the BB is considered everything expands from everything else without any "central point of reference".

19. Aug 13, 2015

### Mordred

There is no outside perspective, our observable portion of the universe is finite, but that's just our observable portion. If you extrapolate backwards in time, this portion would have a miniscule point like volume. However we have no idea on the volume of the entire universe. It could be finite or infinite.

Even an Infinite universe can expand the same way a finite universe can expand.

As Phinds mentioned there is no edge

20. Aug 13, 2015

### phinds

Exactly. "Everywhere at once" means the big band WAS the entire universe, whether it was finite or infinite, and a center would imply a preferred frame of reference, which experiments say does not exist.

21. Aug 13, 2015

### jerromyjon

This is probably the number one argument for an infinite universe: if there was some type of boundary the expansion would not be expected to be so homogeneous across the entire observable universe. The "wrap-around" concept of finite space could alleviate that symptom, but then you might expect to see galaxies very far away from us moving towards us, which we don't.

22. Aug 13, 2015

### phinds

No, the most current hypotheses regarding a "wrap around universe" are SO huge that there is no possibility we would actually SEE the evidence.

23. Aug 13, 2015

### stedwards

Hmm. See phind's post #17. See the FRW universe, where, for a given time > zero (universal time), space is unbounded and finite.

This entire thread is nearly impossible to make sense of. "Infinite universe" is an ambiguous term. Infinite time? infinite space?

24. Aug 13, 2015

### phinds

The clear consensus in modern cosmology is that time is finite (so far, but just wait around ). It is space that might be infinite. Certainly the Big Bang Theory says that time is finite and specifically the amount that has passed so far is about 13+ billion years. If this is correct, it will always be finite because you can't get from finite to infinite. There's always tomorrow but there are a limited number of yesterdays.

25. Aug 14, 2015

### Chronos

What part of everywhere is difficult; left right up or down?