Is the universe finite or infinite?

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  • #101
phinds
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If so and if the magnitude of this effect could be measured, it could theoretically be possible to calculate the size of the universe beyond our visible bubble with the assumption that the universe has a roughly equal mass-energy density at very large scales.
If it were possible/meaningful to do so, don't your reckon someone would have DONE it by now?
 
  • #102
Drakkith
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On a related note: can Hawking radiation cross the "boundary" from what's beyond it into our visible bubble? Afaik the phase velocity of the Schrödinger wave isn't limited to c.
What boundary? Hawking radiation is emitted from our side of the event horizon of a black hole. And what does the Schrodinger wave function have anything to do with this?

If it were possible/meaningful to do so, don't your reckon someone would have DONE it by now?
Phinds, down boy! Down! *waves a steak at Phinds* Go get the steak!
 
  • #103
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there's probably some element of "space" that is expanding from the Big Bang, but the void which that space resides in (i.e., which it is expanding into) must be infinite.

Eric
Please allow me to make a clarification, I realize my wording sounds like nonsense but when I said "space" I meant "anything created by the big bang" and the farthest known element from that bang would demarc "space" and the "void" but I realize the word space is confusing :-)
 
  • #104
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Please allow me to make a clarification, I realize my wording sounds like nonsense but when I said "space" I meant "anything created by the big bang" and the farthest known element from that bang would demarc "space" and the "void" but I realize the word space is confusing :-)
There is no void in any sense that space is expanding into. The Big Bang happened everywhere.
 
  • #105
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There is no void in any sense that space is expanding into. The Big Bang happened everywhere.
Everywhere? In basic terms, it was a central node that exploded right? Then the elements of that explosion expanded outward (and is still doing so). What is 1 inch beyond the boundary of that ever expanding explosion?
 
  • #106
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In basic terms, it was a central node that exploded right?
Nope. Again, it sort of occurred everywhere. In basic terms, it was an incredibly dense stuff everywhere, and space sort of started expanding, decreasing the density of this incredibly dense stuff and bringing any two points further apart. Sorry if this doesn't sound too rigorous, it's an imperfect wording, but suffices.
 
  • #107
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Eric, you must not understand the expansion as an outward movement within three dimensions. If so we would observe empty space (no stars or galaxies) "inwards" and "outwards" and the galaxies would lie in a plane or disc in the other directions. This is not the case the universe is evenly filled with galaxies in all directions. The expansion is an ever increasing distance between non-gravitationally bound objects. There is no spatial point an inch beyond the big bang at any time. At least not located in what we usually define as the universe.
 
  • #108
Drakkith
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Everywhere? In basic terms, it was a central node that exploded right? Then the elements of that explosion expanded outward (and is still doing so). What is 1 inch beyond the boundary of that ever expanding explosion?
This is why I hate the term "Big Bang". It just screams "explosion in space". Instead of an explosion, imagine the universe running backwards in time. Everything gets closer to everything else until the entire universe, ALL of it, is extremely dense and extremely hot. This is the earliest period that we can make good theories about. At this point in time, the universe was still infinite in size and there is still no center. As time passed the universe expanded, meaning that the distance between all objects not bound increased with time, until we get to our present day universe. Have you ever heard of the raisin bread analogy?
 
  • #109
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This is why I hate the term "Big Bang". It just screams "explosion in space". Instead of an explosion, imagine the universe running backwards in time.
I actually like the term. A "bang" is a loud noise and much of the data we are looking at involves essentially looking at the "sound of the universe".

One term that I've used in talking about the CMB which I think is accurate is the "wall of fire".

Everything gets closer to everything else until the entire universe, ALL of it, is extremely dense and extremely hot.
The other thing that I think is useful is not to think about the entire universe. One thing to imagine is to take a camera and "zoom in" to our part observable universe and then just think of that. There is a part of the universe that you can't see, but it helps not to think of that.

One other point is that much of what we think of as the big bang, isn't that dense and isn't that hot. One reason I like talking about the "wall of fire" is that the temperatures and density at CMB separation are typical of those that we see when we deal with ordinary fire.

At this point in time, the universe was still infinite in size and there is still no center.
This is why the "zoom lens" picture makes sense to me. Imagine a picture with you at the center that goes out several tens of billions of light years. By thinking about *part* of the universe rather than the whole thing, I'm thinking about a finite bit that my mind can comprehend.
 
  • #111
phinds
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... At this point in time, the universe was still infinite in size
No, it was only infinite in size THEN if it is infinite in size NOW and we don't know that it is.
 
  • #112
My personal answer will be like this, the universe is neither finite nor infinite.
 
  • #113
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Can you explain more please? What do you mean?
 
  • #114
phinds
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My personal answer will be like this, the universe is neither finite nor infinite.
Which is nonsensical.
 
  • #115
Drakkith
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No, it was only infinite in size THEN if it is infinite in size NOW and we don't know that it is.
Of course my slobbery friend!
 
  • #116
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Of course my slobbery friend!
Gad, you are slowing down. Two weeks to kibitz ?
 
  • #117
BruceW
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they haven't ruled out an FLRW universe with non-zero curvature yet! In other words, the universe is not necessarily infinite. It is simply large enough that we have a 'fairly flat' universe. 'fairly flat' meaning that most cosmological models predict that the most likely curved universes would be very curved compared to ours. Also, our universe is within experimental error of zero curvature. So in this sense, the universe is 'flat'. There are also chaotic inflation models that do not require a big bang. I guess these are less standard.
 
  • #118
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Gad, you are slowing down. Two weeks to kibitz ?
Quiet you, or I'll stuff you back in your kennel!
 
  • #119
Chronos
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In theory, if the universe is exactly flat, it is probably spatially infinite. Observationally, it is so close to being exactly flat it is impossible to draw any conclusion - other than it is huge [what a shock]. I doubt we will ever be able to conclusively prove one way or another.
 

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