Is there a problem in assuming the universe has a boundary?

[phinds]

I agree that the balloon analogy is too simplistic. Like you say it doesn't explain the physics driving the expansion of the universe. And it is potentially misleading because a balloon has a latex "boundary" while the universe may not have a measurable boundary.

You might find this exposition on the balloon analogy helpful:

www.phinds.com/balloonanalogy

if you read it all the way through, particularly the "FORTH: NO STRETCHING" part.

 

[twofish-quant]

But wouldn't having a boundary be in conflict with the isotropy, ie. the assumption/measurement (which is it?), that the relative velocity of matter would look the same any where in the universe and that there is no center of the universe?

Yes it would, which is why you can infer limits on what a boundary would be like by measuring the isotropy of the universe.

One thing about the universe is that it would be really, really weird if there were a boundary in the universe and we happened to be dead center. A universe which had a spherical boundary in which we were *exactly* in the middle of sphere would be indeed be hard to see.

However, either there would be something "special" about our location or else we would be slightly off center, and if we were slightly off center from the boundary of the universe or our location in space was slightly weird, we'd eventually spot something weird. Likewise if the boundary wasn't spherical, we'd spot something weird. And the papers that audioloop posted show the sort of "weird stuff" we would see (and argue that we might be seeing it).

It's also possible that the boundary is so far away that we can't see it, but you can turn that around and ask how close a boundary has to do before we would notice something, and people have come up with some numbers.

 

[twofish-quant]

I'm shooting the breeze, by the way, not proposing anything. But I have read clear statements from a few physicists about this, and unless I am misreading them, which is perfectly possible, then we cannot simply take it for granted that extension is real for an fundamental ontology, and would have to bear this in mind when considering the size of the universe and its boundaries.

Lots of things are possible. That's why you have to do observation. You will get nowhere if you just sit in a room and try to speculate about what the universe is like. What you need to do is to ask yourself "if the universe was a doughnut, what would I see?" and then point your telescope to see if you actually see it.

The papers that audioloop posted do that. Basically if the universe is a doughnut then only certain wavelengths in the gas that form the universe would be not possible, and you would see weird stuff as you look at the texture of the early universe. The papers that he/she mentioned go a bit further and say that they think we might be seeing signs of the universe being a doughnut. Looking at their interpretation, I'm not sure, but it's something that will get resolved with more data.

When we imagine we are seeing into an infinite three-dimensional space, we are falling for a fallacy in which we substitute what we actually see for an intellectual construct. This is not only a mystical vision, it is wrong.

I'm interested to understand why he thinks it's wrong.

In Leibnitz’s view, the ultimately real, something that depends on nothing else for its existence, cannot have parts. If it had parts, its existence would depend on them. But whatever has spatial extension has parts. It follows that what is ultimately real cannot have spatial extension, …

This is the type of "useless word games" that I don't think are useful. The problem is that words are tools that describe things, and the words and concepts we use are those that describe our daily life. The universe can play by very different rules, which makes trying to "figure things out" by "word games" not useful. When you study cosmology, ultimately you have to use the language of math which turns out to be able to describe things that we can't describe in our daily life.

 

[cepheid]

I agree that the balloon analogy is too simplistic. Like you say it doesn't explain the physics driving the expansion of the universe. And it is potentially misleading because a balloon has a latex "boundary" while the universe may not have a measurable boundary.

You misunderstand the balloon analogy completely if you think that it has a boundary. The balloon analogy is a 2D analogy for the universe. In other words, the 2D surface of the balloon represents the expanding universe in this model. The 2D surface, of course, has no boundaries, and no centre.

 

[Tim13]

You misunderstand the balloon analogy completely if you think that it has a boundary. The balloon analogy is a 2D analogy for the universe. In other words, the 2D surface of the balloon represents the expanding universe in this model. The 2D surface, of course, has no boundaries, and no centre.

Hmm. I apologize. I am not sure I understand your point. However I think your point may only add to mine in that the balloon analogy is potentially misleading. Obviously a real balloon is a 3D object and so if it was intended to be a 2D analogy for the universe then I did misunderstand "completely". But please tell me if I understand you correctly - do you posit that a 2D surface could never have any "boundaries"?

Plus, isn't the real universe still 3D even if it is a little flat? Hence the potential confusion when the original topic was about the real 3D universe and whether there is a problem with assuming it has a boundary?

 

[Number Nine]

do you posit that a 2D surface could never have any "boundaries"?

No, he stated that the surface of the balloon which is two dimensional surface, has no boundary, which is true. The expansion of the surface of the balloon provides the analogy for the expansion of space. The surface of the balloon happens to be embedded in three dimensional space, but it doesn't have to be. Space is not embedded in anything.

 

[Tim13]

No, he stated that the surface of the balloon which is two dimensional surface, has no boundary, which is true. The expansion of the surface of the balloon provides the analogy for the expansion of space. The surface of the balloon happens to be embedded in three dimensional space, but it doesn't have to be. Space is not embedded in anything.

That is probably what he meant. Yet if he believes that space is relatively flat but not strictly 2D (perhaps Euclidian and 3D) then it only adds more evidence that the balloon analogy is potentially misleading for numerous reasons.

Plus, my question was a little more nuanced. I was asking if he posits that a 2D surface could never have any boudaries. I wasn't limiting my question to only 2D surfaces which happen to be balloons because it is possible that he was making a general statement about 2D surfaces. And the last time I looked out my window the obsevable universe isn't shaped like a balloon.

 

[phinds]

That is probably what he meant. Yet if he believes that space is relatively flat but not strictly 2D (perhaps Euclidian and 3D) then it only adds more evidence that the balloon analogy is potentially misleading for numerous reasons.

Plus, my question was a little more nuanced. I was asking if he posits that a 2D surface could never have any boudaries. I wasn't limiting my question to only 2D surfaces which happen to be balloons because it is possible that he was making a general statement about 2D surfaces. And the last time I looked out my window the obsevable universe isn't shaped like a balloon.

I take it you did not read the exposition at the link I gave you.

 

[Tim13]

I take it you did not read the exposition at the link I gave you.

I confess that I only skimmed it and didn't fully understand the exposition at the link you gave. I just now went back and reread it. It does a much better job of explaining the potential confusion created by the analogy than what I was attempting to convey. I initially misunderstand the analogy too for some of the reasons stated in the article. Thanks.

 

[phinds]

I confess that I only skimmed it and didn't fully understand the exposition at the link you gave. I just now went back and reread it. It does a much better job of explaining the potential confusion created by the analogy than what I was attempting to convey. I initially misunderstand the analogy too for some of the reasons stated in the article. Thanks.

Glad it was helpful. Several of the members here helped me put it together for exactly this purpose.

 

[Tim13]

Glad it was helpful. Several of the members here helped me put it together for exactly this purpose.

I apologize that I didn't read it more thoroughly earlier. I would have saved myself and others from unnecessary key strokes. Is there a collection of such expositions for different topics on this website?

 

[Mark M]

I apologize that I didn't read it more thoroughly earlier. I would have saved myself and others from unnecessary key strokes. Is there a collection of such expositions for different topics on this website?

This article is frequently recommended, it was written in Scientific American:

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

Note that the first page is blank, scroll down.

And two very good FAQs that you may find helpful are these:

http://www.astro.ucla.edu/~wright/cosmology_faq.html
http://preposterousuniverse.com/writings/cosmologyprimer/faq.html

 

[PeterJ]

Lots of things are possible. That's why you have to do observation. You will get nowhere if you just sit in a room and try to speculate about what the universe is like. What you need to do is to ask yourself "if the universe was a doughnut, what would I see?" and then point your telescope to see if you actually see it.

Then you have to sit in a room and and speculate on the meaning of what the telescope has revealed. I really cannot get this idea that thinking is useless compared with experiment.

I'm interested to understand why he thinks it's wrong.

I'm afraid I can't remember the discussion. I could not comment much on it anyway. I just took his proposal to mean that physics allows for the possibility that he is right. It intrigued me that for Smolin extension is a mystical illusion while for mystcism it is a scientific one.

This is the type of "useless word games" that I don't think are useful. The problem is that words are tools that describe things, and the words and concepts we use are those that describe our daily life. The universe can play by very different rules, which makes trying to "figure things out" by "word games" not useful. When you study cosmology, ultimately you have to use the language of math which turns out to be able to describe things that we can't describe in our daily life.

This seems unfair on Leibnitz. He wasn't bad at mathematics. But the language of maths has as much trouble with fundamentals as any other language. His point was simply that if there is a fundamental, non-dependent or original phenomenon, then our reason concludes it cannot be extended. If it is extended, then the universe breaks the laws of thought and is paradoxical. He does not claim to know which it is. There is a connection with Russell's paradox so it is not an entirely non-mathematical point. As I see it, he is saying that the original phenomenon cannot be manifest for the same reason that the set-of-all-sets cannot be manifest in naive set-theory. Logic and reality would be in total accord.

I don't thing we need any experiments to form a view about this. It seems to be significant that the idea of a boundary to the universe gives rise to contradictions and does not compute.