Does the Universe have a 3D shape? If so, where is the centre?

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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 »

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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. 
There never was nothing.

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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!!! :)

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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?

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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
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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 »

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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. 
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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.

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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?
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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.
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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?

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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.
Fixation on the Einstein papers is a good definition of OCD.

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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]
Fixation on the Einstein papers is a good definition of OCD.

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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.
Observe; collate; conjecture; analyse; hypothesise; test; validate; theorise. Repeat until complete.

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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.

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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?

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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.

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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.

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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.

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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.

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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.

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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.
Fixation on the Einstein papers is a good definition of OCD.

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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?

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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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Offline dlorde

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Re: Return to the Centre of the Universe
« Reply #25 on: 12/01/2015 20:11:39 »
... 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.
Me too.

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Offline JohnDuffield

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Re: Return to the Centre of the Universe
« Reply #26 on: 12/01/2015 20:34:16 »
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?
No. Imagine you've got a vacuum, and I introduce a 10cm sphere of air at 14psi. It will expand. However if introduce the sphere of air into the room you're in, it won't. Because it's surrounded by air at 14psi. Take a look at the energy-pressure diagonal in the stress-energy tensor, and think of space as something like the ball of air:



Note though that a ball of space isn't surrounded by anything. There is no space around it or beyond it. The ball of space is all there is. This is a tricky concept. Which is why I think people talk about an infinite universe. 

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Offline chiralSPO

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Re: Return to the Centre of the Universe
« Reply #27 on: 12/01/2015 21:36:11 »
but the Universe isn't expanding into anything--it's just expanding. The space within it is growing. I don't know how good an analogy gas pressure is to "energy pressure" (but if there is a reference that explains why it is a good analogy, please direct me there--I would rather be wrong now and right later!)

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Offline JohnDuffield

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Re: Return to the Centre of the Universe
« Reply #28 on: 12/01/2015 22:04:37 »
but the Universe isn't expanding into anything--it's just expanding. The space within it is growing.
Agreed. Like I said, there is no space around it or beyond it.

I don't know how good an analogy gas pressure is to "energy pressure" (but if there is a reference that explains why it is a good analogy, please direct me there--I would rather be wrong now and right later!)
Einstein talked about it. The Einstein digital papers are now online, see this. There's a search function, though I'm not sure it picks up every instance. But anyway, see this and this for starters. The thing that people don't appreciate about the stress-energy tensor is that it "describes the density and flux of energy and momentum in spacetime". And it's got a pressure diagonal, and a shear stress term. Space is like some ghostly gin-clear compressed elastic! IMHO when the penny drops with this, it's really sad that Einstein didn't predict the expanding universe. If they'd had stress balls in his day, I imagine he would have done.
« Last Edit: 12/01/2015 22:07:22 by JohnDuffield »

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Offline chiralSPO

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Re: Return to the Centre of the Universe
« Reply #29 on: 13/01/2015 04:03:22 »
Thanks John--will read!

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Offline jeffreyH

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Re: Return to the Centre of the Universe
« Reply #30 on: 13/01/2015 05:05:19 »
OK John define the pressure gradient of a black hole. Do it with mathematics and not mumbo jumbo.
Fixation on the Einstein papers is a good definition of OCD.

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Offline Ron Maxwell

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Re: Return to the Centre of the Universe
« Reply #31 on: 14/01/2015 00:11:57 »
jeffreyH, what is your definition of infinity? Is not the result of subtracting any number still infinity? How then can you get back to a 'big bang'?

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Offline jeffreyH

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Re: Return to the Centre of the Universe
« Reply #32 on: 14/01/2015 00:37:56 »
Let me answer that another way. Not to dodge the issue but to point out an anomaly in the constant pressure argument. The great attractor, information of which can be found here:

http://en.wikipedia.org/wiki/Great_Attractor

This means that not only is the universe expanding but something else is stretching it in a particular direction. Was this mysterious source part of a big bang or was it there prior to the event? There may be many other great attractor type sources that are just too far away to detect.

Fixation on the Einstein papers is a good definition of OCD.

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Offline PmbPhy

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Re: Return to the Centre of the Universe
« Reply #33 on: 14/01/2015 01:36:39 »
Quote from: Ron Maxwell
jeffreyH, what is your definition of infinity?
Jeff is not one to create his own definitions but uses the ones which are universally defined in math and physics. I.e.

[attachment=19411]
« Last Edit: 14/01/2015 04:21:37 by PmbPhy »

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Offline petm1

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Re: Return to the Centre of the Universe
« Reply #34 on: 14/01/2015 03:21:24 »
Why do we need to return to the center of the universe?  Imme we never left the center we are all stuck in the present which is centered in time right between future and past.   

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Offline JohnDuffield

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Re: Return to the Centre of the Universe
« Reply #35 on: 14/01/2015 09:23:46 »
OK John define the pressure gradient of a black hole. Do it with mathematics and not mumbo jumbo.
 
Jeffrey: broadly speaking the pressure is proportional to the gravitational time dilation so the expression

[tex]t_0 = t_f \sqrt{1 - \frac{2GM}{rc^2}} = t_f \sqrt{1 - \frac{r_0}{r}}[/tex]

serves us adequately. The pressure increases as you approach the black hole, and the potential and coordinate speed of light decreases. The force of gravity at any location depends on the local gradient in the potential or  coordinate speed of light. However when you get to the event horizon r = r0 and the expression gives an undefined result. This corresponds to a coordinate speed of light of zero. It can't go lower than this. In similar vein the potential can't go lower and the pressure can't go higher, and there's no more gravitational gradient. What you're left with is Oppenheimer's original "frozen star" black hole. You don't hear much about this, but see The Formation and Growth of Black Holes where Kevin Brown says it's one of two alternative interpretations. He doesn't favour it, but I think it's right. Here's a depiction:
 
« Last Edit: 16/01/2015 10:04:35 by evan_au »

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Offline dlorde

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Re: Return to the Centre of the Universe
« Reply #36 on: 14/01/2015 10:45:13 »
jeffreyH, what is your definition of infinity? Is not the result of subtracting any number still infinity? How then can you get back to a 'big bang'?
The idea is that the big bang may have been an expansion of an infinite expanse of very hot dense stuff. This seems to potentially allow for a version of a multiverse (as it's hard to see how an infinite expanse can evolve uniformly).

As I understand it, there's no clear evidence to rule out the infinite hypothesis, so it must remain on the table (unless some new analysis has ruled it out).

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Offline JohnDuffield

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Re: Return to the Centre of the Universe
« Reply #37 on: 14/01/2015 11:09:05 »
As I understand it, there's no clear evidence to rule out the infinite hypothesis, so it must remain on the table (unless some new analysis has ruled it out).
I think the clear evidence that rules it out is the universe is expanding. An infinite universe can't expand because at every location the spatial pressure is counterbalanced on all sides. What surprises me is that people dismiss that, even though the stress-energy tensor has an energy-pressure diagonal. They don't say what's that all about? even when they've got a physics PhD. And they never seem to notice that there's one hell of a problem reconciling big bang cosmology with a universe that's always been infinite. Or that saying a flat universe is infinite is a total non-sequitur. Or that the infinite universe is just a turtles all the way down non-answer.

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Offline dlorde

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Re: Return to the Centre of the Universe
« Reply #38 on: 14/01/2015 16:44:45 »
As I understand it, there's no clear evidence to rule out the infinite hypothesis, so it must remain on the table (unless some new analysis has ruled it out).
I think the clear evidence that rules it out is the universe is expanding. An infinite universe can't expand because at every location the spatial pressure is counterbalanced on all sides. What surprises me is that people dismiss that, even though the stress-energy tensor has an energy-pressure diagonal. They don't say what's that all about? even when they've got a physics PhD.
I can only assume that they don't agree that your pressure argument is valid.

Quote
And they never seem to notice that there's one hell of a problem reconciling big bang cosmology with a universe that's always been infinite.
What problem is that?

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Or that saying a flat universe is infinite is a total non-sequitur. Or that the infinite universe is just a turtles all the way down non-answer.
It may not be intended as a sequitur; i.e. the logic may be that if the universe is flat, it could also be infinite (implying that flatness is necessary). I don't think it's intended as an answer, it's an option that, in their opinion, hasn't yet been ruled out. At present, all current origin options seem unsatisfactory, whether turtles-all-the-way-down, creation ex-nihilo, or 'it just is'. 

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Offline PmbPhy

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Re: Return to the Centre of the Universe
« Reply #39 on: 14/01/2015 17:27:18 »
Quote from: jccc
If the universe has mass, then there is a center of the total mass.
No. That is incorrect. Not only is it wrong but there is no justification in any sense for it to be right. That this is so follows from the fact that an infinitely large universe does not have a finite total mass. Consider an infinitely large sheet which has a uniform mass density. Such a sheet wouldn't have a center of mass.

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Offline PmbPhy

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Re: Return to the Centre of the Universe
« Reply #40 on: 14/01/2015 17:30:56 »
Quote from: JohnDuffield
An infinite universe can't expand because at every location the spatial pressure is counterbalanced on all sides.
What in the name of God do you mean by this? What justification do you have that the pressure is always balanced and that because of this the universe can't expand? The equations of motion for an expanding universe certainly does hold true for our universe, that's for sure.

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Offline jccc

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Re: Return to the Centre of the Universe
« Reply #41 on: 14/01/2015 20:13:02 »
Quote from: jccc
If the universe has mass, then there is a center of the total mass.
No. That is incorrect. Not only is it wrong but there is no justification in any sense for it to be right. That this is so follows from the fact that an infinitely large universe does not have a finite total mass. Consider an infinitely large sheet which has a uniform mass density. Such a sheet wouldn't have a center of mass.
What do you want to bet Pete?

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Offline CliffordK

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Re: Return to the Centre of the Universe
« Reply #42 on: 14/01/2015 22:26:43 »
Quote from: CliffordK
Shape of the visible universe: spherical, 3 dimensional.
Radius: About 13 billion lightyears.
Center: EARTH
I assume this is a joke?
No joke--the Earth is very close to the center of the OBSERVABLE universe.

Yes & No.

The original question was about the observable universe, I.E.  What we can see, which is roughly a sphere around Earth, with the most distant objects that we can see being about 13-14 billion light years away. 

I'm not saying that there isn't more Universe out there that we can't observe (and undoubtedly the most distant stars have already changed from what we're seeing today), and we are likely far from the middle of the universe as a whole, assuming a middle or edge exists.

However, we are in the middle of what we can observe.  This is one of the few geocentric things left in Astronomy.  Even if humanity manages to visit more stars in the Milkyway, it is unlikely we'll ever exit from the Milkyway (unless Andromeda does a close flyby, and we can spread to that galaxy in a few billion years.  We should probably start planning the jump soon.    So, over time we may extend the range of our telescopes, or stars from which the light hasn't reached us may come into view (unless they are going in the wrong direction too fast).  But, or observable universe will still remain more or less a sphere around Earth and the Milkyway.

In a sense, it is like looking at a single cell in the Human body and trying to figure out what the rest of the body looks like. 

What I will say is that there are some things one can learn from what is observable about stuff that we either don't see, or can't see.  For example, Neptune was actually predicted before it was found.  Perhaps order will be found within the universe that will predict the form of what lies beyond.  Unfortunately, we may never be able to confirm or deny anything about the universe beyond the sphere which we can see.

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Offline jeffreyH

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Re: Return to the Centre of the Universe
« Reply #43 on: 14/01/2015 23:22:56 »
OK John define the pressure gradient of a black hole. Do it with mathematics and not mumbo jumbo.
 
Jeffrey: broadly speaking the pressure is proportional to the gravitational time dilation so the expression

[tex]t_0 = t_f \sqrt{1 - \frac{2GM}{rc^2}} = t_f \sqrt{1 - \frac{r_0}{r}}[/tex]

serves us adequately. The pressure increases as you approach the black hole, and the potential and coordinate speed of light decreases. The force of gravity at any location depends on the local gradient in the potential or  coordinate speed of light. However when you get to the event horizon r = r0 and the expression gives an undefined result. This corresponds to a coordinate speed of light of zero. It can't go lower than this. In similar vein the potential can't go lower and the pressure can't go higher, and there's no more gravitational gradient. What you're left with is Oppenheimer's original "frozen star" black hole. You don't hear much about this, but see The Formation and Growth of Black Holes where Kevin Brown says it's one of two alternative interpretations. He doesn't favour it, but I think it's right. Here's a depiction:
 

Well John I would like to know the path you took in deriving your equations. What steps did you go through?
« Last Edit: 16/01/2015 10:08:42 by evan_au »
Fixation on the Einstein papers is a good definition of OCD.

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Offline PmbPhy

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Re: Return to the Centre of the Universe
« Reply #44 on: 15/01/2015 04:36:39 »
Quote from: jccc
What do you want to bet Pete?
I don't make bets on principle. State your argument and I'll show you where your error is.

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Offline jccc

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Re: Return to the Centre of the Universe
« Reply #45 on: 15/01/2015 04:54:09 »
Quote from: jccc
What do you want to bet Pete?
I don't make bets on principle. State your argument and I'll show you where your error is.
I bet there is a center of the mass of the universe. Just for a dollar. 

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Offline PmbPhy

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Re: Return to the Centre of the Universe
« Reply #46 on: 15/01/2015 05:11:24 »
Quote from: jccc
I bet there is a center of the mass of the universe. Just for a dollar.
Nope. No bets. One of the problems with bets like this is that you'll post what you think is a proof and then when I've proved it to be flawed you'll be unable to understand what I've said and refuse to pay.

The fact is that I really don't care what you have to say in a rebuttal because I know as a fact that it'd be wrong. I've been studying this for years. What kind of shmuck studies cosmology for years and misses basic well-known facts of the field? That fact is that the universe is unbounded and an unbounded universe has no center. That's even true for a universe with a finite amount of mass.
« Last Edit: 15/01/2015 05:18:08 by PmbPhy »

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Offline jccc

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Re: Return to the Centre of the Universe
« Reply #47 on: 15/01/2015 05:27:46 »
Quantum center? Maybe? Sweet night Pete.

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Offline CliffordK

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Re: Return to the Centre of the Universe
« Reply #48 on: 15/01/2015 06:56:07 »
To say that one bets there is a center of the universe is pretty pointless. 

Any center may well like outside of the sphere we are able to visualize.  One may not ever be able to disprove a "center", and proving it may be quick, or it could take centuries or millennia of observations and calculations.

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Offline dlorde

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Re: Return to the Centre of the Universe
« Reply #49 on: 15/01/2015 10:38:14 »
I don't know what distortion of the usual meaning of 'centre' would allow an unbounded topology to have one; does a loop or circle have a centre? or the surface of a sphere? or an unbounded volume? if so, how does one calculate it?