B How can the Universe grow if it is infinite?

[PeroK]

Hi PeroK. It's nice to know somebody knows what infinity represents.

There are a few of us about!

 

[Comeback City]

Lol... I can't argue with that, Comeback city.

Why are you misquoting me now? Not sure where that middle part even came from

 

[Comeback City]

Infinity is not a number.

Yes, I know.

 

[Daisyroots]

Why are you misquoting me now? Not sure where that middle part even came from

Sorry, I'm not aware of having added or removed anything. I was just saying that I couldn't argue with your observation. Ignore me. :)

 

[Comeback City]

Sorry, I'm not aware of having added or removed anything. I was just saying that I couldn't argue with your observation. Ignore me. :)

Comeback City said: ↑
What do you mean by it has to be specific? As in it has to be a defined number?
Good question. And very interesting: because I do indeed mean that it must consist in a particular quantity; but at the same time, as I think you point out, any specified number is an irrelevance.

If my understanding is correct, then the whole point of Hilbert's hotel is to show that adding one to infinity still gives you infinity.
----------------
That was the portion (in bold) I did not write. Was this your answer to the question?

 

[phinds]

But does it 'describe the expansion'? The thread question was 'How can the universe grow if it's infinite?' In your view (I ask genuinely) has our little conversation around the question moved us on at all, as yet?

I recommend the link in my signature.

 

[russ_watters]

Ever hear of the balloon analogy?

I'm not following how the balloon analogy applies; I thought it was describing a finite yet boundless universe?

 

[phinds]

I'm not following how the balloon analogy applies; I thought it was describing a finite yet boundless universe?

The balloon analogy describes the expansion without regard to whether finite/infinite. It doesn't explain it in the sense of saying what dark energy is or whether the universe is finite/infinite. You can think of it as describing what goes on in the observable universe.

 

[Comeback City]

I'm not following how the balloon analogy applies; I thought it was describing a finite yet boundless universe?

Exactly what @phinds said above. It describes the distance between galaxies getting larger and larger, regardless of the universe being infinite or finite.

 

[Daisyroots]

Comeback City said: ↑
What do you mean by it has to be specific? As in it has to be a defined number?
Good question. And very interesting: because I do indeed mean that it must consist in a particular quantity; but at the same time, as I think you point out, any specified number is an irrelevance.

If my understanding is correct, then the whole point of Hilbert's hotel is to show that adding one to infinity still gives you infinity.
----------------
That was the portion (in bold) I did not write. Was this your answer to the question?

Yes, Comeback, that was my answer to the question. Sorry. I obviously pressed something wrong. I'm not used to the system having only just joined the forum. It's lively chat!

 

[Comeback City]

Yes, Comeback, that was my answer to the question. Sorry. I obviously pressed something wrong. I'm not used to the system having only just joined the forum. It's lively chat!

No problem. And welcome to PF!

 

[Daisyroots]

I recommend the link in my signature.

Thanks for that. Very helpful.

 

[Ken G]

So basically the prevailing view is that the universe has an infinite amount of space AND an infinite amount of matter that is fairly evenly distributed throughout the universe. I think the infinite matter aspect is often not emphasized strongly enough. This should help dispel the misconception among laymen of infinite space but with all the matter concentrated in a region of the universe and expanding outward into the empty space surrounding it.

We should be careful though, no scientist is claiming we have evidence either that the universe is infinite, or that it contains an infinite amount of matter. All we can responsibly say is we have no evidence that it is finite and no evidence the matter shows a boundary, so we can successfully apply an infinite model without encountering any problems. We should always recognize the crucial difference between the attributes of a successful model, and statements about what is, because the latter requires the assumption that the model will never need to be changed. Certainly the history of science has taught us not to do that, unless we have a strong urge to have future generations condescend to our naive notions! And even if humanity goes extinct without ever improving the model, that is still no guarantee that we simply had not the time or technology to take the next step.

 

[russ_watters]

The balloon analogy describes the expansion without regard to whether finite/infinite. It doesn't explain it in the sense of saying what dark energy is or whether the universe is finite/infinite. You can think of it as describing what goes on in the observable universe.

Exactly what @phinds said above. It describes the distance between galaxies getting larger and larger, regardless of the universe being infinite or finite.

Fair enough guys/whatever helps a layman, but can you explain how? I'm not seeing the connection made, so my admittedly layman brain immediately tries to visualize an infinitely large sphere, causing a stack overflow. I can't get past it.

To be a little more technical, I was under the impression that the rubber sheet and balloon analogies were mutually exclusive: that curved=finite and flat=infinite. Is that not the case/can the opposites be true? Indeed, I have always favored the balloon analogy, but I'm getting the perception that scientists are leaning toward flat/infinite, which would imo make the rubber sheet analogy superior.

Looking at marcus's thread, he doesn't list being able to describe an infinite universe as one of the features of the balloon analogy, but he does list "finite but boundless".

Also, the Insight article (which I admittedly hadn't looked at before...) seems problematic and maybe contradictory:

The analogy should ONLY consider a portion of the balloon’s surface — it does not make any statements about the size or shape of the universe (other than it is getting bigger). Forget that the surface of the balloon is curved. That’s NOT intended to be representative of the actual universe. It is actually more reasonable to think of a flat sheet of rubber that is being stretched equally in all directions. That would be a better analogy, but you’d have to confine the analogy to only a section of the sheet. Edges would NOT be part of the analogy. The analogy is not intended to comment in any way on the shape of the universe, whether it is open or closed, flat or curved, or ANY of those things. Those are NOT part of the analogy. The universe not only has no center, it has no edge, but that does not imply that it is necessarily infinite, it could be finite but unbounded (like the surface of the Earth, for example).

Much of that seems contradictory:
1. How can we see it as finite if we aren't supposed to visualize the whole balloon?
2. How is the curved shape not a part of it/differentiate it from the rubber sheet? If we're only supposed to think of the expanding balloon as a flat rubber sheet, why bother with it at all?

Heck, I have an easier time visualizing an infinitely large flat rubber sheet than I do an infinitely large balloon!

 

[phinds]

Russ, have you read my article on it? Link in my signature.

 

[Comeback City]

Fair enough guys/whatever helps a layman, but can you explain how? I'm not seeing the connection made, so my admittedly layman brain immediately tries to visualize an infinitely large sphere, causing a stack overflow. I can't get past it.

To be a little more technical, I was under the impression that the rubber sheet and balloon analogies were mutually exclusive: that curved=finite and flat=infinite. Is that not the case/can the opposites be true? Indeed, I have always favored the balloon analogy, but I'm getting the perception that scientists are leaning toward flat/infinite, which would imo make the rubber sheet analogy superior.

Looking at marcus's thread, he doesn't list being able to describe an infinite universe as one of the features of the balloon analogy, but he does list "finite but boundless".

Also, the Insight article (which I admittedly hadn't looked at before...) seems problematic and maybe contradictory:

Much of that seems contradictory:
1. How can we see it as finite if we aren't supposed to visualize the whole balloon?
2. How is the curved shape not a part of it/differentiate it from the rubber sheet? If we're only supposed to think of the expanding balloon as a flat rubber sheet, why bother with it at all?

Heck, I have an easier time visualizing an infinitely large flat rubber sheet than I do an infinitely large balloon!

Just keep in mind, this whole dark energy/ universal expanding concept came from the observation that galaxies were getting farther and farther away from each other. Yes, the rubber sheet model works just as fine of an analogy as does the balloon model. The whole point behind it, as phinds mentions in his insight, is that galaxies are getting farther apart from each other. Maybe you are overthinking it a little bit.

As for the shape of the universe, I agree with what Marcus wrote: you cannot count out a finite/curved universe. But if this were the case, the universe would have to be INCREDIBLY large so that the overall curve of spacetime is not noticeable to us (since most observations show our universe is mostly flat).

 

[russ_watters]

Russ, have you read my article on it? Link in my signature.

Unless I'm missing something, the link in your signature doesn't contain an article, it only contains a link to the Insight article you helped write (and other 3rd party articles) -- as I said (and quoted!), I read that article. Is there more content I'm missing?

 

[russ_watters]

Yes, the rubber sheet model works just as fine of an analogy as does the balloon model.

So you are saying the two analogies are equivalent?

The whole point behind it, as phinds mentions in his insight, is that galaxies are getting farther apart from each other.

There is definitely more to these analogies than just that galaxies are getting further apart.

Maybe you are overthinking it a little bit.

Maybe I am, but I asked a few specific questions - I'm really interested in hearing the answers:

As for the shape of the universe, I agree with what Marcus wrote: you cannot count out a finite/curved universe. But if this were the case, the universe would have to be INCREDIBLY large so that the overall curve of spacetime is not noticeable to us (since most observations show our universe is mostly flat).

So again; how do those different geometries relate to the analogies?

 

[Comeback City]

So you are saying the two analogies are equivalent?

There is definitely more to these analogies than just that galaxies are getting further apart.

Maybe I am, but I asked a few specific questions - I'm really interested in hearing the answers:

So again; how do those different geometries relate to the analogies?

Just to clarify: are you thinking of the two analogies in this way...
Balloon = curved/finite
Rubber Sheet = flat/infinite
?

 

[phinds]

Unless I'm missing something, the link in your signature doesn't contain an article, it only contains a link to the Insight article you helped write (and other 3rd party articles) -- as I said (and quoted!), I read that article. Is there more content I'm missing?

Sorry, I missed that you had read it. Yes, I meant the Insights article. If that doesn't explain to you how really simple the balloon analogy is, really, then I can't add anything. You seem to want to be taking the analogy to places it wasn't designed to go.

 

[russ_watters]

Just to clarify: are you thinking of the two analogies in this way...
Balloon = curved/finite
Rubber Sheet = flat/infinite

Yes.

 

[russ_watters]

Sorry, I missed that you had read it. Yes, I meant the Insights article. If that doesn't explain to you how really simple the balloon analogy is, really, then I can't add anything.

Really? You can't even answer a yes or no question? Or respond to what looks to me like contradictions in the descriptions?

You seem to want to be taking the analogy to places it wasn't designed to go.

Can you respond to what marcus says in his sticky thread about one of the things the analogy helps us visualize?:

3. to understand that something can be finite (finite area if 2D or finite volume if it's 3D) without having any boundary.

To make it easy, can you answer yes or no that you agree or disagree?

or with this one:

It might be infinite, an infinite radius of curvature is equivalent to zero curvature, complete flatness.

 

[Comeback City]

Yes.

Maybe this is the problem then. For example, the balloon model is still used to describe expansion of an flat/infinite universe.

There is definitely more to these analogies than just that galaxies are getting further apart.

In your opinion, what more is there to it?

 

[phinds]

Really? You can't even answer a yes or no question? Or respond to what looks to me like contradictions in the descriptions?

I'm sorry, Russ, I'm not trying to avoid any question, I've just lost track of what's what in this thread. To me the balloon analogy is nothing more than a simple description of how galaxies move apart metrically with no center and no edge. It is, as I say in the article, agnostic about infinite/finite, flat/curved, and so forth.

 

[russ_watters]

Maybe this is the problem then. For example, the balloon model is still used to describe expansion of an flat/infinite universe.

How?

In your opinion, what more is there to it?

See here:

1. to picture how distances can increase between stationary objects

2. to picture distances increasing at a percentagewise rate. Like one percent per minute.
So the longer the distance the faster (inches per minute) it increases. This is Hubble Law.

3. to understand that something can be finite (finite area if 2D or finite volume if it's 3D) without having any boundary

4. to understand that something can be curved without there being an extra dimension---part of the mental exercise is to picture the balloon surface as all there is, there is no inside the balloon and there is no outside---only the balloon surface exists.
I haven't talked about this part yet.

5. to picture light traveling between stationary points, as wrigglers traveling across the balloon surface at a fixed speed of one inch per minute----and to understand how the distance from a wriggler's starting point can increase faster than one inch per minute even tho the wriggler is always only traveling at one inch per minute.

https://www.physicsforums.com/threads/effort-to-get-us-all-on-the-same-page-balloon-analogy.261161/

 

[russ_watters]

I'm sorry, Russ, I'm not trying to avoid any question, I've just lost track of what's what in this thread.

? Are you ok, phinds? We're only talking about one post! (#44)

To me the balloon analogy is nothing more than a simple description of how galaxies move apart metrically with no center and no edge.

Others, including well-respected physicists, have said it has more to offer than just that.

 

[phinds]

? Are you ok, phinds? We're only talking about one post! (#44)

I took the heart of that post to be asking how the analogy explains the cause of the expansion and I responded that it does not. It describes how the expansion looks, not what causes it.

Your questions

1. How can we see it as finite if we aren't supposed to visualize the whole balloon?
2. How is the curved shape not a part of it/differentiate it from the rubber sheet? If we're only supposed to think of the expanding balloon as a flat rubber sheet, why bother with it at all?

I thought were addressed in the article and I still think that.

Others, including well-respected physicists, have said it has more to offer than just that.

OK, then I'm taking an overly simplistic view of the analogy. I would add, however, that the article was contributed to and proofed by several of the senior staff in the cosmology section and no one mentioned that I was missing anything significant regarding the analogy.

 

[russ_watters]

I would add, however, that the article was contributed to and proofed by several of the senior staff in the cosmology section and no one mentioned that I was missing anything significant regarding the analogy.

Ok, well I guess I'll wait and see if any are willing to assist.

 

[Comeback City]

See here:
https://www.physicsforums.com/threads/effort-to-get-us-all-on-the-same-page-balloon-analogy.261161/

I am going to start here if it is alright with you...
1) (already covered)
2) also covered; expansion, but with different units
3) This is just referring to a curved/finite universe (which refers to positive or negative curvature just for mental note) and how it needs no boundary
4) This is a little bit tricky, but this seems to be referring to the fact that the universe isn't expanding into anything (ie there is no outside to the universe that the universe is moving into)
5) I may be wrong on this one, but it seems to be describing the acceleration of the expansion of the universe

 

[russ_watters]

I am going to start here if it is alright with you...
1) (already covered)
2) also covered; expansion, but with different units
3) This is just referring to a curved/finite universe (which refers to positive or negative curvature just for mental note) and how it needs no boundary
4) This is a little bit tricky, but this seems to be referring to the fact that the universe isn't expanding into anything (ie there is no outside to the universe that the universe is moving into)
5) I may be wrong on this one, but it seems to be describing the acceleration of the expansion of the universe

Right, so I am of course referring to #3...