High School Is the Universe Finite or Infinite?

[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?

 

[phinds]

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?

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.

 

[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.

 

[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.

 

[phinds]

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.
.

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

Paul

 

[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?

 

[Drakkith]

The universe- from our understanding, is expanding, thus the regions (for lack of a better word) particles have not yet reached do not exist.

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.

 

[phinds]

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.

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

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

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

 

[phinds]

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

 

[Drakkith]

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.

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.

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

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.

 

[newjerseyrunner]

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.

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.

 

[Drakkith]

Can you have time being a negative value?

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

 

[Drakkith]

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.

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.

 

[newrd]

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.

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.

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?

 

[phinds]

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?

Did you read the article in the link in my signature?

 

[newrd]

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

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!

 

[Drakkith]

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?

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

 

[newrd]

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

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?!

 

[newjerseyrunner]

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?!

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.

 

[phinds]

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?!

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.

 

[newjerseyrunner]

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.

I think he was stating that from the fish's point of view, the space has expanded beyond what it can see so it would appear infinite.

 

[newrd]

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.

The blather was a reply to the Matrix reference! But that being said, how has it come to be thought that we are on the surface of the expanding balloon and not actually inside the balloon (or fishtank!)? I really must read the balloon analogy before I ask anymore questions..

 

[phinds]

The blather was a reply to the Matrix reference! But that being said, how has it come to be thought that we are on the surface of the expanding balloon and not actually inside the balloon (or fishtank!)? I really must read the balloon analogy before I ask anymore questions..

What an excellent idea.

 

[Drakkith]

The blather was a reply to the Matrix reference! But that being said, how has it come to be thought that we are on the surface of the expanding balloon and not actually inside the balloon (or fishtank!)? I really must read the balloon analogy before I ask anymore questions..

The key is that the balloon analogy is just that. An analogy. The real universe has three spatial dimensions (not two like the surface of the balloon) yet the behavior of objects within space is similar to imaginary objects on the surface of this imaginary expanding balloon. But that's it. A similarity.

 

[newjerseyrunner]

But that being said, how has it come to be thought that we are on the surface of the expanding balloon and not actually inside the balloon (or fishtank!)? I really must read the balloon analogy before I ask anymore questions..

I prefer the baking raisin bread analogy. It's already in 3D.

 

[Drakkith]

I prefer the baking raisin bread analogy. It's already in 3D.

Yeah but it tastes funny and has all those nasty edges!

 

[Chronos]

Your grandma cut the crust off your grilled cheese sandwiches, didnt she.

 

[Bandersnatch]

The concept of the BB happening everywhere at the same time was proposed around 1999.

I consider the concept of the BB occurring everywhere at the same time a proposal. As far as I know its not a theory...it has no postulates..it makes no predictions.

Where does this come from? It was there from the inception. I think you're somehow confusing it with the discovery of dark energy, which happened around the time you indicated.
It is also an integral part of the BB cosmology, and any predictions BB makes are necessarily linked to it happening everywhere.

Consider the development steps of the theory:
- You start with General Relativity and the observation that the universe you see is homogeneous and isotropic (i.e., 'cosmological principle').
- Application of the cosmological principle to GR equations allows you to obtain a solution describing the metric of the universe (the FLRW metric)
- The FLRW metric describes how the universe evolves in time depending on its curvature and contents, which are determined from observations, and gives predictions as to the redshifts, form of the CMBR, age of the universe, abundances of elements.
- The phrase 'Big Bang' is used to describe either the whole theory, or the early, dense, hot stage, or the singularity popping up when you extrapolate the evolution sufficiently backwards in time.

You assume there is no center when you decide to apply the cosmological principle, and you decide to do that based on observations. That using this assumption to solve the equations of an extremely well-tested theory (GR) gets you predictions that match observations is a clear indication that it was a good assumption. You don't get to keep the predictions while ditching the no-center assumption that got you those.

The FLRW metric includes the curvature parameter, which determines whether the universe is finite or infinite. The parameter is not time dependent, so whichever the universe really is - it stays that way throughout its evolution.

For an overview of inflation and where it fits into the BB see this paper by A.Liddle:
An introduction to cosmological inflation
If maths is a problem, you can still get some understanding from the descriptive parts, but you'd probably be better off reading Guth's own popularization book: 'The Inflationary Universe'.

 

[newjerseyrunner]

The concept that the big bang happened everywhere was not introduced, that's completely backwards. It was determined that the universe was denser in the past than it is now. That density, if you rolled the clock backwards approached infinity. The "size" of space has been increasing too, things aren't simply flying apart from each other like shrapnel from a bomb, it's the space between them that's stretching. But if you roll the clock back on this, you still have an infinite amount of space at the beginning, not a point.

 

[phinds]

The concept that the big bang happened everywhere was not introduced, that's completely backwards. It was determined that the universe was denser in the past than it is now. That density, if you rolled the clock backwards approached infinity. The "size" of space has been increasing too, things aren't simply flying apart from each other like shrapnel from a bomb, it's the space between them that's stretching. But if you roll the clock back on this, you still have an infinite amount of space at the beginning, not a point.

It's not known that there was an infinite amount of space at the beginning so you should not state that as fact. It is possible that the universe is and was finite but unbounded. Yes, the consensus is that it is and was infinite but that is not a known fact.