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

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I hear about the balloon analogy, and that there is no need to say that the universe has a boundary, but is that the only reason or would it be problematic to assume that space-time has a volume and a boundary?
 

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  • #2
Drakkith
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When we get down to it the theory and models don't care whether there is a boundary or not for the most part. (If there is it MUST be significantly further away than we can see, otherwise it would be noticeable)

IF the universe is finite in size, then it is very likely that it is "boundless", which just means that you can keep going forever in a direction, but you will keep passing by the same places, like going off the screen on an old arcade game only to come back in from the other side.
 
  • #3
thanks for the quick reply :)
 
  • #4
HallsofIvy
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An interesting question would be- what would the boundary be made of?
 
  • #5
phinds
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I hear about the balloon analogy, and that there is no need to say that the universe has a boundary, but is that the only reason or would it be problematic to assume that space-time has a volume and a boundary?
What would happen to physics as we know it at the boundary? How could our physics possibly deal with an "edge" to the universe? That of course does not PROVE the lack of a boundary, but you asked if it would be problematic. How about a total breakdown of all of our known cosmology? You lose isomorphism, you lose isotropy ... basically you pretty much lose your mind trying to figure out how do model a boundary.
 
  • #6
bcrowell
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Since we don't see any evidence for a boundary, we have no evidence on which to base any theory of how physics would work at a boundary.
 
  • #7
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I hear about the balloon analogy, and that there is no need to say that the universe has a boundary, but is that the only reason or would it be problematic to assume that space-time has a volume and a boundary?
It would be problematic because we don't see anything observationally that looks like a boundary. If there were a boundary then it would be so far away that we wouldn't see any evidence for it, and do for the purpose of doing calculations, it's assumed that the boundary is irrelevant.
 
  • #8
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What would happen to physics as we know it at the boundary? How could our physics possibly deal with an "edge" to the universe?
As a theorist, I don't think it would be that difficult to come up with something. If we actually did come up with something that looked like a boundary to the universe, you'd just look at it and see what happens there.
 
  • #9
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There could be some kind of a boundary, but we can't see it. Once we have an orbiting telescope that pushes farther into the IR, I expect that we will see further "back" in terms of BB cosmology, but I don't expect to see an edge. Any boundary at this point would probably be an artifact of our inability to detect fainter and fainter objects. I could be wrong about this, but I doubt it.
 
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  • #10
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?
 
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Drakkith
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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?
If it does, and we find a boundary, then we'd have to re-examine our ideas about the universe.
 
  • #12
basically you pretty much lose your mind trying to figure out how do model a boundary.
we may lose our minds but it'll sure be exciting! :biggrin:
 
  • #13
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The expansion of space causes a natural boundary.

Any objects separated in space by a distance that the expansion between them exceeds speed c then they become causally disconnected. In short our visible universe is surrounded by a collapsing event horizon beyond which the known laws of nature cannot communicate.
 
  • #14
phinds
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The expansion of space causes a natural boundary.

Any objects separated in space by a distance that the expansion between them exceeds speed c then they become causally disconnected. In short our visible universe is surrounded by a collapsing event horizon beyond which the known laws of nature cannot communicate.
I don't think that's what the OP was asking about at all. I'm not arguing w/ your statement, just saying that it is irrelevant to this discussion.
 
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bcrowell
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  • #16
phinds
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Yes, "boundary" has a technical definition here http://en.wikipedia.org/wiki/Manifold_with_boundary#Manifold_with_boundary , and whether the OP knew it or not, #1 was using the term in a way that was consistent with that definition.
Yes, that definition is consistent w/ how I thought about the term, but I don't see how the event horizon of our observable universe can be what the OP had in mind, as QuantumHop seems to think.

Again, I was not arguing at all w/ what QuantumHop was saying, just about the relevance to the OPs question, which I took to be a question about the "whole" universe, not the observable universe.
 
  • #17
rbj
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An interesting question would be- what would the boundary be made of?
stretchy latex.
 
  • #18
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I hear about the analogy, and that there is no need to say that the universe has a boundary, but is that the only reason or would it be problematic to assume that space-time has a volume and a boundary?

Friedmann–Lemaître–Robertson–Walker cosmology requieres a connected universe hence a non-trivial topology.
 
  • #19
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Would I be right in thinking that a boundary to the universe is an absolute, and that physics does not deal with absolutes.
 
  • #20
When people refer to the balloon analogy, they are actually making a dimensional analogy. The surface of the balloon is a two dimensional plane expanding in a three dimensional way, which is meant to represent the three dimensional space expanding in a four dimensional way, if that makes any sense.
 
  • #21
Drakkith
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Would I be right in thinking that a boundary to the universe is an absolute, and that physics does not deal with absolutes.
Any boundary, if it were observable, would be dealt with by science. *Breaks out the shotgun* I'm going to deal with it right now actually.
 
  • #22
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So what would this boundary be like? The only thing I can think of is that it would be like some kind of cosmic event horizon. Once you cross over you'd cease to exist. It would be like falling into a black hole.
 
  • #23
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Any boundary, if it were observable, would be dealt with by science. *Breaks out the shotgun* I'm going to deal with it right now actually.
Okay. But I'm assuming it wouldn't be observable for reasons already given.
 
  • #24
well, yes, my question is about the whole universe or put in a better way all that came out of the big bang in this universe. When we say the universe has no boundary, I think it means we talk about a n-1 submanifold of a n dimensional manifold, so if we say we live in a 3D universe and it has no boundary then we live on the surface of a 4D universe. Or is there another concept behind "no boundary"?
 
  • #25
Friedmann–Lemaître–Robertson–Walker cosmology requieres a connected universe hence a non-trivial topology.
thnx didn't see there is already a second site for the answers, gotta look into that..
 

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