Universe center of mass?

  • #1

Main Question or Discussion Point

We are learning about centers of mass now and it made me wonder, does the universe have a center of mass? If so , what is its meaning? Is it place where "big bang" occurred?

Also, on a related note, could the fact that the universe is expanding at an accelerated rate be explained by a net force acting on our universe?
 

Answers and Replies

  • #2
phinds
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Since the universe has no center, it is not possible for it have a center of mass

Universal expansion IS explained by a net force (or SOMETHING) which is called "dark energy".
 
  • #3
Since the universe has no center, it is not possible for it have a center of mass

Universal expansion IS explained by a net force (or SOMETHING) which is called "dark energy".
ohhh so thats what dark energy is :D

And how do we know that the universe doesn't have center? If the universe is finite in volume, and if the amount of mass is finite, then there must be a center of mass?
 
  • #4
phinds
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And how do we know that the universe doesn't have center? If the universe is finite in volume, and if the amount of mass is finite, then there must be a center of mass?
We don't know that the amount of mass is finite or that the universe is finite. If the universe IS finite, that does not imply a center.

The current theory of cosmology says the universe, whether finite or infinite, has is homogenous and isotropic and therefore has no center and no boundary (no "edge").

You would likely find it informative to read the FAQ section in cosmology.
 
  • #5
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but if our universe evolved from the big bang (which consisted only of internal forces), then shouldn't the centre of mass lie at the place where the big bang occurred?
 
  • #6
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but if our universe evolved from the big bang (which consisted only of internal forces), then shouldn't the centre of mass lie at the place where the big bang occurred?
In one sense, the big bang occurred everywhere in space. There is nowhere you can point to. Check out the balloon analogy. If the entire universe is the surface of the balloon, you cannot point to a place on that universe (again, just the surface) where the expansion started.
 
  • #7
phinds
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but if our universe evolved from the big bang (which consisted only of internal forces), then shouldn't the centre of mass lie at the place where the big bang occurred?
Jack21222 has explained it correctly ... you would do well to read the FAQ in the Cosmology section ... since you don't yet have a handle on the basics of cosmology, you'll find it very helpful.
 
  • #8
Now I think about it though, I does seem odd that it doesn't have a centre of mass. I get the no centre of the universe, due to space curving in a circle, but newton's laws rely on something having a centre of mass, as far as I can remember. If you subject the universe to a force or something, how will it react?
 
  • #9
Chronos
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The big bang did not occur in some vast ocean of empty space. It created space as well as matter. The cosmic microwave background [relic radiation from the big bang] is equidistant is all directions, so either we just happen to be smack in the center of the observable universe [improbable], or, the CMB is equidistant from every point in the universe [the accepted explanation].
 
  • #10
phinds
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Now I think about it though, I does seem odd that it doesn't have a centre of mass. I get the no centre of the universe, due to space curving in a circle, but newton's laws rely on something having a centre of mass, as far as I can remember. If you subject the universe to a force or something, how will it react?
Uh ... "subject your universe to a force" Now THAT would be a good trick

You can't apply Newton's law to the universe as a whole because it has no edge, no boundard, no OUTSIDE, so you can't subject it to a force.

Space does not necessarily curve and if it does, it's not in a circle. It might just go on forever or it might wrap back on itself in some way but a circle is a 2D object, so impossible
 
  • #11
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You could still find the centre of mass of the universe, if there was a finite amount of matter in it, and space itself wasn't so curved as to make it ambiguous. There are of course practical problems, one of them being that you only know the position of other galaxies in the past, and can only see to the horizon.
But another problem is that a centre of mass is less meaningful in general relativity, because mass is equivalent to energy... so you'd want to sum the energy of the universe... but then I think that the total energy of the universe is 0:
 
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  • #12
DaveC426913
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You could still find the centre of mass of the universe,
No you could not, even with your premises.

The most accurate thing to say is that everywhere is the centre of the universe.
 
  • #13
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OK, I'll take your word since you didn't elaborate. Maybe you're referring to the universe appearing differently in different reference frames.
 
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  • #14
DaveC426913
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OK, I'll take your word since you didn't elaborate.
If you were to choose a point and try to measure the distance to all matter, you would determine yourself to be at the centre. If you were to fly a billion light years west and do the same experiment, you would once again determine yourself to be at the centre.

No matter where you are, you're at the centre.
 
  • #15
Chronos
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Let's be fair, you are also at the temporal edge [most ancient region] of the universe no matter where you are. This often creates confusion. A galaxy a billion years distant was also a billion years younger when the light we now observe was emitted. That means their CMB was a billion years younger at that time. For observers in that galaxy, the universe 'now' is a billion years more ancient than the one we currently observe.
 
  • #16
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In one sense, the big bang occurred everywhere in space. There is nowhere you can point to. Check out the balloon analogy. If the entire universe is the surface of the balloon, you cannot point to a place on that universe (again, just the surface) where the expansion started.
Of course, it would be impossible to tell, if that hole was patched up and so smooth it resembled a sphere. But that doesn't mean there wasn't a place that started it.
 
  • #17
DaveC426913
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Of course, it would be impossible to tell, if that hole was patched up and so smooth it resembled a sphere.
Are you stumbling over the idea that a balloon has a spigot? Are you unable to make the leap to a balloon that has no spigot?

OK. Then imagine a soap bubble instead. Pretend it's being warmed by the sun so that the air inside it expands, causing hte soap bubble to expand without any netry point.

Better?

But that doesn't mean there wasn't a place that started it.
Yes. It does.
 
  • #18
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OK. Then imagine a soap bubble instead. Pretend it's being warmed by the sun so that the air inside it expands, causing hte soap bubble to expand without any netry point.

Better?
That's only valid if the soap bubble always existed as a bubble. We're talking about t = 0 here, just before the balloon of space and time was created. Obviously, the universe didn't always exist as a "bubble" or whatever shape; there was an entry point purportedly caused by the Big Bang, the so called spigot.
 
  • #19
phinds
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That's only valid if the soap bubble always existed as a bubble. We're talking about t = 0 here, just before the balloon of space and time was created. Obviously, the universe didn't always exist as a "bubble" or whatever shape; there was an entry point purportedly caused by the Big Bang, the so called spigot.
Thinking about it that way leads you to conclude that there was a point-origin for the big bang, but there wasn't.

You also talk about t=0, but there is no current model that gives the slightest clue what was going on at t=0 and that makes your proposal personal opinion and overly speculative according to the rules of the forum.

If you come up with a justifiable and falsifiable theory of what happened at t=0, you would make one hell of a big splash in physics, but your current proposal of a personal theory isn't going to do it.
 
  • #20
DaveC426913
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That's only valid if the soap bubble always existed as a bubble. We're talking about t = 0 here, just before the balloon of space and time was created. Obviously, the universe didn't always exist as a "bubble" or whatever shape; there was an entry point purportedly caused by the Big Bang, the so called spigot.
"Entry point"? What makes you think there was an entry point?
 
  • #21
Uh ... "subject your universe to a force" Now THAT would be a good trick

You can't apply Newton's law to the universe as a whole because it has no edge, no boundard, no OUTSIDE, so you can't subject it to a force.

Space does not necessarily curve and if it does, it's not in a circle. It might just go on forever or it might wrap back on itself in some way but a circle is a 2D object, so impossible
But I read somewhere the univese might be spinning, and found another article http://www.dailygalaxy.com/my_weblog/2011/07/-is-the-universe-spinning-new-research-says-yes.html" [Broken]. If it is spinning, then there must be a centrapetal force.

Yeah, I knew about that, but if you go off in one direction, you can come back in a circle, so that was the way I was thinking about it. If it doesn't curve, surely it must have a centre?
 
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  • #22
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But I read somewhere the univese might be spinning, and found another article http://www.dailygalaxy.com/my_weblog/2011/07/-is-the-universe-spinning-new-research-says-yes.html" [Broken]. If it is spinning, then there must be a centrapetal force.

Yeah, I knew about that, but if you go off in one direction, you can come back in a circle, so that was the way I was thinking about it. If it doesn't curve, surely it must have a centre?
The research quoted in that article is interesting, because it's the same exact research done by Galaxy Zoo. Galaxy Zoo found no preference for the rotation of galaxies, but five undergrads found an excess of 7%.

http://uk.arxiv.org/abs/0803.3247

The paper addresses why some studies might find an anisotropy in galaxy rotations.

And, if the universe doesn't curve, why must it have a center? It's quite possible that the universe is infinite in extent, that's why it's more correct to talk about the "scale factor" of the universe rather than the "size." In this case, no matter where you were in the universe, you'd conclude you were at the "center."
 
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  • #23
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Yeah, I knew about that, but if you go off in one direction, you can come back in a circle, so that was the way I was thinking about it. If it doesn't curve, surely it must have a centre?
If the universe is not curved but is spatially flat in the euclidean sense, then it cannot possibly have a center - As all models which dictate a euclidean flat isotropic topology all have to assume spatial infinity.

You cannot have a center to infinity, it is like asking the middle number between 0-infinity, the question becomes a moot question.

I hope this helps your understanding.
 
  • #24
phinds
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If it doesn't curve, surely it must have a centre?
No, if it doesn't curve it can't possibly have a center unless it is finite and the big bang occurred at only one point, and that combination of events is absolutely contrary to current understanding. Instead of stateing what "must" be true, you'd do better to figure out what IS true and if it doesn't make sense, ask questions.
 
  • #25
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One more reason it dose not and can not have a center (geometrical, mass, gravity, whatever) is that if it did, you would automatically have a preferred, selected reference frame or coordinate system attached to it. Bye bye goes relativity. And unless you have something better to replace it with, that doesn't sound like a good idea right now.
 

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