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Author Topic: Does the Universe have a 3D shape? If so, where is the centre?  (Read 21817 times)

Offline Ron Maxwell

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I know.  Thats why I put 'return', yes, we've been here before.  In an expanding universe there is no centre.  (Got it, thanks).  Lets forget about expansion, just look at what we have got.  Can we estimate the shape and dimensions of the visible universe?  Does it have a three dimensional form?  If so, where is the centre?
« Last Edit: 29/12/2015 08:44:19 by chris »


 

Offline Bill S

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Re: Return to the Centre of the Universe
« Reply #1 on: 13/08/2013 20:25:35 »
Hi Ron,

The reason I tend to leap in early in threads like this is not because I necessarily think I have the answer; rather, it is in the hope of testing my ideas.  Unfortunately, it does not always work, which I assume is either because I am absolutely right, or my idea is so silly it does not merit a response.

Anyway, once again, this is my understanding. 

Currently, the majority view is that the Universe is flat (or as near as makes no real difference), and is infinite.  If this is right, a hypothetical craft could travel for ever without visiting the same point twice.  It would also mean that there was no centre to the Universe.  This is not just because we canít find one, but because there is no centre, in the same way that there is no central number between zero and infinity.

If you have been following other threads you will be aware that I have as serious problem with the suggestion that something that is finite can become infinite.  The main reason why I distinguish between the Universe and the cosmos is that the Universe could be just that part of the cosmos we can detect, so it could be the cosmos that is infinite. 
The reason I tend to leap in early in threads like this is not because I necessarily think I have the answer; rather, it is in the hope of testing my ideas.  Unfortunately, it does not always work, which I assume is either because I am absolutely right, or my idea is so silly it does not merit a response.

Anyway, once again, this is my understanding. 

Currently, the majority view is that the Universe is flat (or as near as makes no real difference), and is infinite.  If this is right, a hypothetical craft could travel for ever without visiting the same point twice.  It would also mean that there was no centre to the Universe.  This is not just because we canít find one, but because there is no centre, in the same way that there is no central number between zero and infinity.

If you have been following other threads you will be aware that I have as serious problem with the suggestion that something that is finite can become infinite.  The main reason why I distinguish between the Universe and the cosmos is that the Universe could be just that part of the cosmos we can detect, so it could be the cosmos that is infinite. 
 

Offline Pmb

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Re: Return to the Centre of the Universe
« Reply #2 on: 14/08/2013 01:42:56 »
There are three dimensions to space. You can think of the spatial geometry of the universe in one of three ways; a sphere, a flat sheet or shaped like an infinitely large saddle. In all cases there is no center.

I agree with Bill in that I too think the cosmos as being different than the universe. I think of the cosmos of all that exists while the universe is the set of all places which can be connected to each other by a continuous set of points. Nice thinking Bill!!! :)
 

Offline Ron Maxwell

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Re: Return to the Centre of the Universe
« Reply #3 on: 15/08/2013 12:18:26 »
You will note I specified 'visible' and I really don't understand it when you say the universe could be a sphere with no centre.  My question isn't just trivia, it arises from other things I've been thinking about.  Particularly, is there a body anywhere in the universe that is truly at rest, that is, not part of a system that is in motion?  We seemingly have a super massive black hole at the centre of our galaxy.  Might there be a primordial black hole around which everything else is in motion?  I have heard of something called the great attractor (I think!); might this be it?
 

Offline Bill S

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Re: Return to the Centre of the Universe
« Reply #4 on: 15/08/2013 15:51:27 »
Quote from: Pmb
  You can think of the spatial geometry of the universe in one of three ways; a sphere, a flat sheet or shaped like an infinitely large saddle. In all cases there is no center.

Letís take the spherical universe as an example, because itís the easiest Ė I think.

If it is finite and spherical, how can it not have a centre?

If it is infinite, how could anyone possibly know it is a sphere?  I accept that if one can discover that it is positively curved, one can consider that it must be spherical, and that that sphere might be considered to be unbounded, in which case it can extend in every direction without limit, but infinite implies that it has already done that.  If it is infinite, it must already be infinity, and must always have been infinite.  It has no centre, but in what sense can it be said to be a sphere?

Wow! Post 666 - "the Number of the Beast". :D
 

Offline Pmb

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Re: Return to the Centre of the Universe
« Reply #5 on: 15/08/2013 17:30:32 »
Quote from: Bill S
If it is finite and spherical, how can it not have a centre?
Because we're talking only about the geometry of the surface. Only the space consisting of the surface of the sphere is part of the "space" that we're talking about. Remember that this is an analogy and not a physical representation of the space. An analogy is defined as something which is similar in some ways but not in all ways. In this case the analogy does not apply to two dimensional surface of the sphere being "embedded" in three dimensional space. Two dimensional beings who live on the surface could travel everywhere on the surface but none of them would ever run into a place which they could call the center since all places on the surface are identical to all other places.

If the geometry of the universe is closed then it's finite, not infinite. The spherical universe is bounded.
« Last Edit: 15/08/2013 17:32:23 by Pmb »
 

Offline Bill S

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Re: Return to the Centre of the Universe
« Reply #6 on: 15/08/2013 23:01:59 »
Pete, as you rightly point out, the surface of a sphere has no central point.  You also point out that this is an analogy and not a physical representation of the space. 

Would I be right in thinking that in order to apply this analogy to physical, 3D, space, the sphere would actually have to be a 4D hypersphere?  If so, this is where I run into difficulty.  I cannot visualise a hypersphere, much less its 3D surface.  As a result, useful though the analogy is in some respects, it fails me when I try to connect the dots to apply it to our Universe.  I end up seeing a spherical (3D) Universe bounded by a (2D) surface. 
 

Offline Pmb

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Re: Return to the Centre of the Universe
« Reply #7 on: 15/08/2013 23:28:04 »
Pete, as you rightly point out, the surface of a sphere has no central point.  You also point out that this is an analogy and not a physical representation of the space. 

Would I be right in thinking that in order to apply this analogy to physical, 3D, space, the sphere would actually have to be a 4D hypersphere?
I dthink so.
Pete, as you rightly point out, the surface of a sphere has no central point.  You also point out that this is an analogy and not a physical representation of the space. 

Would I be right in thinking that in order to apply this analogy to physical, 3D, space, the sphere would actually have to be a 4D hypersphere?  If so, this is where I run into difficulty.  I cannot visualise a hypersphere, much less its 3D surface.  As a result, useful though the analogy is in some respects, it fails me when I try to connect the dots to apply it to our Universe.  I end up seeing a spherical (3D) Universe bounded by a (2D) surface. 

Think about it like this. In 3-space which has the geometry of a closed universe, if you set rulers end to end in a line parallel to each other each starting off in a straight line they will end up leading right back too where they started out from but not coming back parallel but crossing each other. This happens no matter which direction they lead out from. Now appl;y this to the surface of a sphere and you'll see that the same thing happens. That's how to use the analogy.
 

Offline Bill S

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Re: Return to the Centre of the Universe
« Reply #8 on: 16/08/2013 19:03:25 »
OK, but where do lines of latitude fit into that picture?
 

Offline yor_on

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Re: Return to the Centre of the Universe
« Reply #9 on: 22/08/2013 21:40:39 »
Well, the center should be there :)

Just compress.
 

Offline Ron Maxwell

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Re: Return to the Centre of the Universe
« Reply #10 on: 12/01/2015 04:04:29 »
With (hopefully) the intelligence to follow the arguments but not the education to understand the Maths, I go a bit lost with some of this. If the Big Bang theory is correct then surely the universe cannot be infinite. Is Pmb correct that if it is infinite, it must always have been so? It seems so to me... The replies here seem to suggest the universe (or cosmos) is located on the edge of an expanding shape whether a sphere or not. I had imagined, wrongly it seems, that it was of a 3D form. It's like the analogy to a balloon. Is the space within akin to the universe or is it the balloon?
 

Offline jeffreyH

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Re: Return to the Centre of the Universe
« Reply #11 on: 12/01/2015 05:37:02 »
The question should be is the universe expanding out into something else. Consider two big bangs happening simultaneously in neighboring regions of a larger cosmos. They are both expanding and at some point the expansions will impinge upon one another. Since at the hubble horizon of each space will be expanding at the speed of light the impact of one on the other is likely to end up in an implosion where the forces of gravity will become intense. This could likely lead to what has been termed a big crunch. This would only occur in those potions of the expansion that overlap. This is unlikely to ever be verified as heat death may proceed it and no observers will be left to witness the event.
 

Offline CliffordK

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Re: Return to the Centre of the Universe
« Reply #12 on: 12/01/2015 05:47:30 »
Can we estimate the shape and dimensions of the visible universe?  Does it have a three dimensional form?  If so, where is the centre?
Shape of the visible universe: spherical, 3 dimensional.
Radius: About 13 billion lightyears.
Center: EARTH.

We don't know how big the unseen portion of the Universe is, and don't have a good idea to its form and shape.  But, the visible portion is roughly a sphere with Earth at the center.
 
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Offline jeffreyH

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Re: Return to the Centre of the Universe
« Reply #13 on: 12/01/2015 06:25:35 »
Can we estimate the shape and dimensions of the visible universe?  Does it have a three dimensional form?  If so, where is the centre?
Shape of the visible universe: spherical, 3 dimensional.
Radius: About 13 billion lightyears.
Center: EARTH.

We don't know how big the unseen portion of the Universe is, and don't have a good idea to its form and shape.  But, the visible portion is roughly a sphere with Earth at the center.

You can't argue with that.  :D
 

Offline Ophiolite

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Re: Return to the Centre of the Universe
« Reply #14 on: 12/01/2015 12:35:08 »
Can we estimate the shape and dimensions of the visible universe?  Does it have a three dimensional form?  If so, where is the centre?
But, the visible portion is roughly a sphere with Earth at the center.
I've done some checking and it seems to be centered on me, although it is influenced by which way I turn my head.
 

Offline dlorde

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Re: Return to the Centre of the Universe
« Reply #15 on: 12/01/2015 12:58:44 »
When I first tried to comprehend the topology of the universe as a potentially finite but non-bounded volume, I found it helped to think of the old arcade game of Asteroids, where whatever disappeared off the screen in one direction, reappeared on the opposite side, moving in the same direction - a finite but unbounded 2D surface.

As I understand it, there is - in principle - no reason why even an expanding universe could not be infinitely large - if it started off infinitely large. An infinite extent can expand, as Hilbert's Hotel illustrates. I'm told that this is the default assumption of standard cosmology.

I've been told that inflation theory gives an estimate for the size of the universe beyond the observable horizon (very large), but I'm waiting for a reference on this.
 

Offline PmbPhy

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Re: Return to the Centre of the Universe
« Reply #16 on: 12/01/2015 16:34:15 »
Quote from: CliffordK
Shape of the visible universe: spherical, 3 dimensional.
Radius: About 13 billion lightyears.
Center: EARTH
I assume this is a joke?
 

Offline chiralSPO

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Re: Return to the Centre of the Universe
« Reply #17 on: 12/01/2015 18:21:12 »
No joke--the Earth is very close to the center of the OBSERVABLE universe.
 

Offline PmbPhy

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Re: Return to the Centre of the Universe
« Reply #18 on: 12/01/2015 19:37:09 »
No joke--the Earth is very close to the center of the OBSERVABLE universe.
Ah! I see what you mean now. Thanks.
 

Offline PmbPhy

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Re: Return to the Centre of the Universe
« Reply #19 on: 12/01/2015 19:38:29 »
When I first tried to comprehend the topology of the universe as a potentially finite but non-bounded volume, I found it helped to think of the old arcade game of Asteroids, where whatever disappeared off the screen in one direction, reappeared on the opposite side, moving in the same direction - a finite but unbounded 2D surface.
Right. It had the topology of a cylinder.

Here's something interesting. The space around a cosmic string is flat. The geometry is that of a cone.
 

Offline jccc

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Re: Return to the Centre of the Universe
« Reply #20 on: 12/01/2015 19:48:05 »
If the universe has mass, then there is a center of the total mass.
 

Offline JohnDuffield

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Re: Return to the Centre of the Universe
« Reply #21 on: 12/01/2015 19:54:14 »
With (hopefully) the intelligence to follow the arguments but not the education to understand the Maths, I go a bit lost with some of this. If the Big Bang theory is correct then surely the universe cannot be infinite.
Correct. There's this huge non-sequitur wherein people say WMAP found that the universe is "flat" and therefore the universe must be infinite. Here's a NASA website for example: shape of the universe. But it just doesn't follow. And it doesn't tie in with big bang cosmology. When you challenge the people who say the universe is infinite, they duck and dive and say the universe has always been infinite. It's a cop-out.

Is Pmb correct that if it is infinite, it must always have been so?
Yes. But he also says the universe is infinite. Only it can't be infinite if it's expanding. If it was infinite the energy-pressure* would be counterbalanced at all locations. It couldn't expand.

The replies here seem to suggest the universe (or cosmos) is located on the edge of an expanding shape whether a sphere or not. I had imagined, wrongly it seems, that it was of a 3D form.
Wrongly? There is no evidence of any "higher dimensions". There's no evidence that the universe is located on the surface of some shape. The only evidence we have is that the universe is an expanding ball of space dotted with galaxies as per the raisin-cake analogy.

It's like the analogy to a balloon. Is the space within akin to the universe or is it the balloon?
The space within is akin to the universe because we live in 3D space. People will tell you the universe is like the surface of the balloon, but there's absolutely no scientific evidence that supports that assertion. None whatsoever. Zip. Zero. Zilch.

* See the energy-pressure diagonal in the stress-energy-momentum tensor.
 

Offline jeffreyH

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Re: Return to the Centre of the Universe
« Reply #22 on: 12/01/2015 20:04:09 »
Is Pmb correct that if it is infinite, it must always have been so?
Yes. But he also says the universe is infinite. Only it can't be infinite if it's expanding. If it was infinite the energy-pressure* would be counterbalanced at all locations. It couldn't expand.

I disagree.
 

Offline chiralSPO

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Re: Return to the Centre of the Universe
« Reply #23 on: 12/01/2015 20:08:17 »
Is Pmb correct that if it is infinite, it must always have been so?
Yes. But he also says the universe is infinite. Only it can't be infinite if it's expanding. If it was infinite the energy-pressure* would be counterbalanced at all locations. It couldn't expand.

I disagree.

I don't think that having an infinite universe would preclude expansion. Using JohnDuffield's logic, wouldn't the energy pressure also "cancel out" if the universe were finite and unbounded?
 

Offline dlorde

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Re: Return to the Centre of the Universe
« Reply #24 on: 12/01/2015 20:10:05 »
When I first tried to comprehend the topology of the universe as a potentially finite but non-bounded volume, I found it helped to think of the old arcade game of Asteroids, where whatever disappeared off the screen in one direction, reappeared on the opposite side, moving in the same direction - a finite but unbounded 2D surface.
Right. It had the topology of a cylinder.
More of a sphere - top and bottom were also contiguous; perhaps I should have said 'edge' rather than 'side'...

Quote
Here's something interesting. The space around a cosmic string is flat. The geometry is that of a cone.
Ooh-er, I don't think I can visualise a flat cone...
 

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Re: Return to the Centre of the Universe
« Reply #24 on: 12/01/2015 20:10:05 »

 

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