Expansion Momentum

[J.C.MacSwell]

How is the momentum of the expansion thought to be measured?

 

Assuming there is enough matter present to create a "big crunch" is it assumed that space will "shrink" (or slow in expanding, stall, then shrink) to the same beat as would be consistent locally with a gravitational collapse "in" space?

[Deified]

Does expanding space have inertia? That's a very curious thought.

 

AFAIK, the rate of expansion is measured by observing galaxies that are approximately relatively stationary.

[J.C.MacSwell]

Does expanding space have inertia? That's a very curious thought.

 

AFAIK' date=' [b']the rate of expansion is measured by observing galaxie[/b]s that are approximately relatively stationary.

 

I think this is correct.

 

So based on that rate, what inertia is the space assumed to have that must be "subdued" in order to arrest the expansion?

 

As I think (?) you were pointing out the galaxies themselves are more or less at rest wrt the expansion, at least locally.

[Deified]

I don't think that expansion of space can be said to have inertia. Of course, I'm no expert, but that just doesn't make any sense to me.

[Sisyphus]

How can something which is neither moving nor has any mass be said to have momentum? Really, I'm curious, if anyone actually knows about this stuff.

[Spyman]

If space is expanding and bringing matter with it so we can measure the speed away from us, then I think that mass would have momentum.

 

Thus a force is needed to put the breaks on the expansion.

 

I don't think J.C.MacSwell is suggesting that space itself has inertia.

[gcol]

I thought the question of whether there is such a thing as absolute space had been well chewed over previously, the concensus being that there is no such thing. If so, then whatever is in it, however small that makes it less than absolute, must surely have some momentum?

[timo]

How is the momentum of the expansion thought to be measured?

I do not really understand the question because I don´t know what the "momentum of the expansion" is supposed to be. More precisely: I can well imagine what you thought when formulating the question but I do have some doubts that one can properly define something like a momentum of the expansion. I think a mixture of Sisyphus' and Spyman's reply might be a good answer: In cosmology, all matter is supposed to be uniformly distributed and at rest. There is no movement. That´s a point where the good-old rubber-sheet analogon can trick you because seen from the outside, the points on the rubber-sheet seem to move - you are tricked because this analogon only becomes a good visualization if you watch it from outside the rubber-sheet. But usually, an "outside of the universe" point of view is not considered.

 

Assuming there is enough matter present to create a "big crunch" is it assumed that space will "shrink" (or slow in expanding, stall, then shrink) to the same beat as would be consistent locally with a gravitational collapse "in" space?

I don´t know it but I am inclined to say "no". Suppose that the universe has an infinite space-extension, completely uniformly distributed matter and sufficient energy density to recollapse. The infinite extension together with the uniform mass distribution will (in Newtonian gravity) make the whole thing stable -> no collapse.

[Spyman]

I don´t know it but I am inclined to say "no". Suppose that the universe has an infinite space-extension, completely uniformly distributed matter and sufficient energy density to recollapse. The infinite extension together with the uniform mass distribution will (in Newtonian gravity) make the whole thing stable -> no collapse.

From "A brief history of time" by Stephen W. Hawking 1988:

"If the numbers of stars was infinite and they where more or less evently distributed in an infinite space, wouldn't the universe collaps since the stars shouldn't have any center to be pulled against, according to Newton. This way of thinking is an example of errors of the mind when dealing with the infinite. The correct way to deal with this cind of problem - which people realized later - is to start with a finite solution and interpret how the situation changes when adding on more and more evently distributed stars outside this area. We can add how many stars we want and they will still collapse to one single body."

 

From a more modern point of view you have to consider the shape of the universe, with a closed universe where gravity is stronger than dark energy, expansion will slowly halt and then reverse/contract.

 

From Wikipedia "Ultimate fate of the Universe":

"The fate of the universe is determined by the density of the universe. The preponderance of evidence to date, based on measurements of the rate of expansion and the mass density, favors a universe that will not collapse."

 

But J.C.MacSwell assumes there is enough matter present to create a "big crunch".

[J.C.MacSwell]

From "A brief history of time" by Stephen W. Hawking 1988:

"If the numbers of stars was infinite and they where more or less evently distributed in an infinite space' date=' wouldn't the universe collaps since the stars shouldn't have any center to be pulled against, according to Newton. This way of thinking is an example of errors of the mind when dealing with the infinite. The correct way to deal with this cind of problem - which people realized later - is to start with a finite solution and interpret how the situation changes when adding on more and more evently distributed stars outside this area. We can add how many stars we want and they will still collapse to one single body."

 

From a more modern point of view you have to consider the shape of the universe, with a closed universe where gravity is stronger than dark energy, expansion will slowly halt and then reverse/contract.

 

From Wikipedia "Ultimate fate of the Universe":

"The fate of the universe is determined by the density of the universe. The preponderance of evidence to date, based on measurements of the rate of expansion and the mass density, favors a universe that will not collapse."

 

[b']But J.C.MacSwell assumes there is enough matter present to create a "big crunch"[/b].

 

Only to help pose the question. If you assume enough matter to slow it down the question is the same. What role does space play in the momentum of the expansion.

 

I am assuming that the expansion has momentum.

[Spyman]

Only to help pose the question.

Yes, at least thats how I interpreted your question.

 

Sorry, if I managed to get Your thread of track.

 

It is a good question and I am also curious of the answer.

[timo]

From "A brief history of time" by Stephen W. Hawking 1988:

"If the numbers of stars was infinite and they where more or less evently distributed in an infinite space' date=' wouldn't the universe collaps since the stars shouldn't have any center to be pulled against, according to Newton. This way of thinking is an example of errors of the mind when dealing with the infinite. The correct way to deal with this cind of problem - which people realized later - is to start with a finite solution and interpret how the situation changes when adding on more and more evently distributed stars outside this area. We can add how many stars we want and they will still collapse to one single body."[/quote']

Thanks for the clarification. Can you explain it? I don´t understand the reason, and I am not sure to what extend Mr. Hawking mixes up GR and Newtonian gravity without mentioning it. In Newtonian gravity (which I spoke of because otherwise the original question hadn´t made any sense) and assuming an infinite extension with uniformly distributed mass: Which point does all the matter collapse to?

 

EDIT: A possible solution would be that the matter density increases everywhere. I´ll see if I can verify that when I have the time but it still leaves the question why one approach is more correct than the other.

[Spyman]

J.C.McSwell: I apologize again, it was not my intention to bring Your thread off topic.

 

Atheist: I am not in a position to be able to explain Professor Stephen W. Hawking's thoughts, in fact I asked for an explanation myself in this forum once. The interest was low, no one refuted or explained, so I might have ended up making wrong conclusions...

 

Hawking's center of mass ? http://www.scienceforums.net/forums/showthread.php?t=10511

[timo]

Why are you repeatedly calling the question off-topic? If Newtonian gravity predicted a stable state while GR doesn´t, then that answers MacSwell's 2nd question quite easily with a "no".

[Spyman]

Why are you repeatedly calling the question off-topic? If Newtonian gravity predicted a stable state while GR doesn´t, then that answers MacSwell's 2nd question quite easily with a "no".

Assuming there is enough matter present to create a "big crunch" is it assumed that space will "shrink" (or slow in expanding, stall, then shrink) to the same beat[/b'] as would be consistent locally with a gravitational collapse "in" space?

Maybe I have misinterpret the question, J.C.MacSwell will have to clarify...

 

But I feel the question is not if a Newtonian Universe will collapse or not, the question is how fast a GR Universe will collapse if there is enough matter compared to a small locally collapse of matter in space.

 

Since the main topic is Expansion Momentum, I assume he wants to know how the current speed of expansion will counter such a collapse.

[J.C.MacSwell]

Maybe I have misinterpret the question' date=' J.C.MacSwell will have to clarify...

 

But I feel the question is not if a Newtonian Universe will collapse or not, the question is how fast a GR Universe will collapse if there is enough matter compared to a small locally collapse of matter in space.

 

Since the main topic is Expansion Momentum, I assume he wants to know how the current speed of expansion will [b']counter[/b] such a collapse.

 

Yes, counter or resist (successfully or not).

 

If I could pose a question: If you drop a ball is the universe smaller than it would have been if you had not dropped it?

[Spyman]

If I could pose a question: If you drop a ball is the universe smaller than it would have been if you had not dropped it?

Now I am confused...

 

Why would a local movement of mass affect the global size or shape of the universe ?

[J.C.MacSwell]

Now I am confused...

 

Why would a local movement of mass affect the global size or shape of the universe ?

 

Because if you dropped all of the balls in the universe you would have a big crunch.

[Spyman]

Because if you dropped all of the balls in the universe you would have a big crunch.

Well I did drop them and they are not crunching, maybe I dropped them in the wrong direction ?

[J.C.MacSwell]

Well I did drop the[/b']m and they are not crunching, maybe I dropped them in the wrong direction ?

 

I think you missed a few!

[Spyman]

Well I did drop them and they are not crunching' date=' maybe I dropped them in the wrong [b'']direction ?[/b]

I think you missed a few!

LOL !

 

BTW: I think You missed the point.

[Severian]

In Newtonian gravity (which I spoke of because otherwise the original question hadn´t made any sense) and assuming an infinite extension with uniformly distributed mass: Which point does all the matter collapse to?

 

I agree. With a completely uniform infinite (spatially) mass distribution, there would be no collapse with any classical model (Newtonian or GR).

 

However, there is always going to be some anisotropy, so even in the classical case, a small fluctuation will create a collapse. (In other words, the infinite extent isotropic mass distribution is an unstable fixed point.) And as soon as you have quantum flucuations, you are going to have a collapse.

[J.C.MacSwell]

LOL !

 

BTW: I think You missed the point.

 

I think you're right. Care to elaborate?

[Spyman]

I think you're right. Care to elaborate?

From my understanding a Big Crunch would be like the Big Bang but in reverse, any locally movement would not make any difference.

 

There is no known escape path for a powerfull spaceship, it could take off in any 3D direction but still be caught in the crunch.

 

If it would be similar to your local ball drop then the beings on the little blue ball should be able to measure in which direction they are moving and the speed caused by the expansion/collapse.

[J.C.MacSwell]

From my understanding a Big Crunch would be like the Big Bang but in reverse' date=' any locally movement would not make any difference.

 

There is no known escape path for a powerfull spaceship, it could take off in any 3D direction but still be caught in the crunch.

 

If it would be similar to your local ball drop then the beings on the little blue ball should be able to measure in which [b']direction[/b] they are moving and the speed caused by the expansion/collapse.

 

If the sum of all masses collapsing gravitationally add up to a collapse of space, would not a small insignificant gravitational "collapse" coincide with an immeasurable reduction in space. (relative to if the event had not happened and all other things being equal)

 

Edit: and if this is correct, what does it say about space?

 

Gravity seems to balance out in the 3 apparent space dimensions and all mass seems to "escape" the insignificant dropping of a ball so I'm sorry but I am still missing your point.

 

Don't give up on me though!