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Author Topic: Does the Universe have a 3D shape? If so, where is the centre?  (Read 21773 times)

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.
 

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. 
 

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!)
 

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 »
 

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!
 

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.
 

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'?
 

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.

 

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.

« Last Edit: 14/01/2015 04:21:37 by PmbPhy »
 

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.   
 

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

6a4e334625b4d1928d97f653734b8a73.gif

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 »
 

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).
 

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.
 

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?

Quote
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'. 
 

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.
 

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.
 

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?
 

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.
 

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

6a4e334625b4d1928d97f653734b8a73.gif

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 »
 

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.
 

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. 
 

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 »
 

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.
 

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.
 

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?
 

The Naked Scientists Forum

Re: Return to the Centre of the Universe
« Reply #49 on: 15/01/2015 10:38:14 »

 

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