# Size of the Universe just after Big Bang

1. Apr 22, 2012

### Steely Dan

If the universe is finite, what would its total volume have been at $t = 1$ s?

2. Apr 22, 2012

### phinds

unknowable since we do not know, among other things, the current size

3. Apr 22, 2012

### Some Slacker

I would posit that the universe is not finite, though our view of it is limited by the speed of light and of course time. Though we can look back through time to see what things looked like 12 billion years ago, we cannot see what a current galaxy looks like at 12 billion light years distant. (or beyond).

at T=1 second? Who knows? I would posit that the universe was infinite at any time after inflation which is a factor of billions sooner that 1 second. The reason I would posit this is that simply expansion happened at faster than light making everything infinitely far apart...

4. Apr 22, 2012

### phinds

I also tend to the belief (and I recognize that it is ONLY a belief with no basis in fact) that the universe is likely infinite, but your post doesn't make any sense. If the universe was finite at ANY time after the singularity, as you seem to believe, then it is of necessity finite now.

Inflation most certainly did NOT make everything infinitely far apart; that's just ridiculous.

5. Apr 22, 2012

### Some Slacker

Yes I should have perhaps said, infinite to any possible observer in the inflationary universe.

And yes I would also posit that if the universe came from a singularity that too would be infinite! HAH! (to any possible observer from inside the singularity)

6. Apr 22, 2012

### phinds

I have no idea what you are talking about.

7. Apr 23, 2012

### Staff: Mentor

I don't understand this post, nor your previous one. They don't seem to make any sense. Are you sure you adequately got your thoughts down correctly?

8. Apr 23, 2012

### Some Slacker

Yes, perhaps I took too much of a flippant attitude towards this question, but with only theory to base any possible answer, I took the low road instead and for this I apologize.

But I will try again (why I don't know, I'm just Some Slacker with too much time on his hands, and I like to think that I know things)

Okay, we are at t=0 and the universe is a singularity?
Yes? (if no, then I have no answer)
Not to plunge back into darkness but... how long was the singularity around?
Since t=0 must not have been there for long?
Yes? No? Doesn't really matter, my point here is that time (in our universe?) begins with a singularity, there is nothing else. To any observer from within the singularity the universe to me would still appear infinite (though very boring) as the singularity has no space, (or light, or matter or anything else) so if there were some way for you sense anything from within the singularity it would appear the same in all directions and go on forever, since nothing can escape the singularity anyway and there is nothing out there to see.

So from the start I see the universe as infinite and then during inflation I would also see the universe as infinite since expansion is happening at faster than light speed after that the universe ends up as infinite because I want it to be (yes back to flippancy so shoot me). Infinity +/- 1 makes no sense so I'm stickin with infinite.

And you are right if the universe was less than infinite at any time in the past then it is also true today as not enough time has passed to get to infinity.

So for these reasons my answer is that at t=1s the universe was infinite (or it wasn't) and I'm stickin to it (or not).

And I hope you don't take me too seriously as I am just Some Slacker.

9. Apr 23, 2012

### manojr

After the Big Bang, volume of universe increased by factor of at least 10^78 within 10^-32 seconds. This is described as Inflation (http://en.wikipedia.org/wiki/Cosmic_inflation). After inflation, the universe continued its expansion at lower rate.
This Wikipedia article gives timelines of the Big Bang (http://en.wikipedia.org/wiki/Timeline_of_the_Big_Bang), but there is no mention of sizes at various stages. In my opinion, since Big Bang did not happen at a particular point, it happened everywhere, it probably makes no sense of talking of sizes.
But I think there is a valid question. Since universe is expanding, it must have been smaller back in time, and therefore, what was the size of our current observable universe say 1 billion years back, 10 billions years back and just 1 second after the Big Bang?

10. Apr 23, 2012

### manojr

11. Apr 23, 2012

### phinds

Slacker, besides being oblivious to actual physics in general, you seem to have the notion that the "singularity" was some known, specific event. Actually "singularity" just means "the place where our models fail and we have no idea what was going on".

12. Apr 23, 2012

### Cosmo Novice

This is a tough question with only two possible outcomes:

1. If the Universe is infinite then at t=1s the Universe is still infinite
2. If the universe is finite then at t=1s the Universe is still finite

Now I suppose we could look at possible lower bounds for estimated finite size, but we are having to make a pretty big assumption the Universe is flat in order to do this. As we have no idea on the overall curvature of the Universe then we can only guess at a possible size of a finite U.

I think when talking about expansion "size" is not a particularly useful term.

I am with phinds and previous posters in my OPINION that the Universe is, will continue to be and always has been infinite in its nature.

13. Apr 23, 2012

### Steely Dan

This is exactly why I'm asking the question. It's easier to discuss the expanding universe in the infinite universe case, because if someone asks what does the "size" of the Universe mean in that model, you can just say that the size of the observable universe is increasing with time, or that the average distance between distant galaxies is increasing with time. You can then also say that if the Big Bang is a singularity in density, it's not because the volume of the universe shrunk to zero, but because the average space between galaxies went to zero. But in the finite universe model, it also seems to actually mean that the volume of the universe is increasing with time. So I want to get a better intuitive feel for what that universe would have looked like in early times.

14. Apr 23, 2012

### phinds

Read The First Three Minutes by Weinberg

15. Apr 23, 2012

### Steely Dan

I understand the basic concepts of the early universe. What I am looking for is a discussion on what the subtle differences would be, if any, between the infinite and finite universes when it comes to the very early universe. Does Weinberg address this?

16. Apr 23, 2012

### phinds

Not as I recall.

17. Apr 23, 2012

### Mark M

No, not smaller, but denser as you go closer the big bang. And that is not an opinion that the big bang happened everywhere, you are correct, it is a fact.

18. Apr 23, 2012

### Staff: Mentor

It is unknown what existed at the point in time that we define as t=0.
Actually the conditions prior to the Big Bang could have existed for any amount of time before inflation started.
In addition to not knowing what happened at this point in time, there would NOT be any way to sense anything from within any said singularity. We cannot just say that we can and ask what happens. It is likely that if the singularity was real, there is absolutely no way for you or I to even imagine it, let alone attempt to describe it.
You would never see the universe as infinite, as there is a maximum speed limit in the universe, c. You can only see as far as the conditions allow. In our case today, we can see as far as 50ish billion light years in radius thanks to expansion. Anything beyond this is not observable due to the finite speed of light.

19. Apr 23, 2012

### Naty1

possible if you mean 'observe'...but Hawking-Hartle might disagree:

Hartle–Hawking state
http://en.wikipedia.org/wiki/Hartle%...3Hawking_state [Broken]

Last edited by a moderator: May 5, 2017
20. Apr 23, 2012

### Staff: Mentor

I don't think they would disagree.

Last edited by a moderator: May 5, 2017