B Is the Universe finite?

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1. Jul 13, 2016

newrd

The universe- from our understanding, is expanding, thus the regions (for lack of a better word) particles have not yet reached do not exist. How far our universe can/ will expand is unknown, it may be infinite, but we can conclude at this time, as it is still expanding, that it is finite. True or False?

2. Jul 13, 2016

phinds

False. We cannot conclude anything about the size/shape of the universe based on our current knowledge, other than that it is likely to be FAR bigger than the Observable Universe. It could be infinite or it could be finite but unbounded.

3. Jul 13, 2016

newjerseyrunner

Expanding has nothing to do with being finite or infinite. Take a line (not a segment.) It's infinite right? Now take two points on the line, doesn't matter where they are. Now expand the line so that the distance between the two points is doubled. The line is still infinite, but has expanded considerably.

4. Jul 13, 2016

BvU

Yes, indeed. True or False. Finite and unbounded. (*) Which Google. Just Google around for plenty good and bad stuff. Wonder and learn. And don't ask the wrong questions (like 'True or False?')
Have you already stumbled upon the balloon analogy ? Check out Peter Hinds and especially his signature.

(*) I bow to phinds' reply - which crossed my slow typing.

5. Jul 13, 2016

phinds

Damn. I usually get "Phil" as the wrong name. "Peter" is a new one.

Paul

6. Jul 13, 2016

newjerseyrunner

To be clear, is there any evidence to support the idea that the universe is unbounded? The lack of a mathematical equation to describe a boundary condition does not eliminate the possibility.

If you mean the universe to include all of spacetime, isn't there a boundary where time = 0?

7. Jul 13, 2016

Drakkith

Staff Emeritus
That's not how expansion works. Expansion causes all objects to get further away from all other objects not bound by gravity or some other force. Particles are not sent flying off into unoccupied space, they are simply getting further away from everything else.

8. Jul 13, 2016

phinds

You make a good point but I think the general thinking is that a boundary just really messes up known physics.

Another good point, but I think we were discussing space, not space-time (at least I was).

9. Jul 13, 2016

phinds

@newrd I second BvU's advice that you check out the link in my signature. It may clearly up some of your misconceptions.

10. Jul 13, 2016

Drakkith

Staff Emeritus
There is approximately the same amount of evidence supporting the idea that the universe is unbounded as there is supporting the idea that it is bounded.

No, there's a mathematical singularity at t=0, not a boundary.The density everywhere in the universe goes to infinity as t approaches 0, so at no point could you say that there is a boundary.

11. Jul 13, 2016

newjerseyrunner

Can you have time being a negative value? I understand that space could have still been unbounded at time = 0, but isn't it generally accepted that at that point, there was nothing before it, and therefore it's a boundary condition at the beginning of time? I'm referring to spacetime, not space.

12. Jul 13, 2016

Drakkith

Staff Emeritus
I'm not certain. Time is linked to the scale factor of the universe, so a negative value would be... problematic.

13. Jul 13, 2016

Drakkith

Staff Emeritus
It is unknown whether or not something existed before t=0. That point in time could be a true "point of creation" for the universe, or it could simply be a misunderstanding based on an incomplete knowledge of physics. We really don't know.

14. Jul 13, 2016

newrd

Ahh ok, so there is no particle horizon so to speak, there is only the expanding balloon analogy again. I think I understand. So does infinity come down to the fact of whether we live on the surface of the balloon, or inside it?

15. Jul 13, 2016

phinds

16. Jul 13, 2016

newrd

Thankyou to both of you, I have bookmarked it and put it on tomorrows reading list :) All of your replies remind me of the saying- "There is no such thing as a stupid question"- we are all here to learn and we all have to start somewhere!

17. Jul 13, 2016

Drakkith

Staff Emeritus
The surface. There is no inside of the balloon. Just like there is no spoon!

18. Jul 13, 2016

newrd

Could we not be the fish inside a 1ft x 1ft fish tank, unaware of anything outside our little tank which we can circumnavigate with ease, until one day, surprisingly, our little fish tank has expanded so much we think it may be endless?!

19. Jul 13, 2016

newjerseyrunner

We could be. It's a perfectly logical conclusion, it's just not mathematical. If we don't know for sure, we can't make assumptions one way or the other, we can simply say what we think is most likely.

20. Jul 13, 2016

phinds

A fun concept for sci-fic but just blather for actual science. If it is endless (infinite) then it has always been infinite. You can't get from finite to infinite physically.