Expansion Momentum

[Sisyphus]

If you drop a ball is the universe smaller than it would have been if you had not dropped it?

 

I think this is the source of the confusion. The answer is no, since the Earth-ball system has not become any closer to everything else. There is still the same overall density, despite this gravitational collapse, since there is just more empty space around it.

 

Analogously, if I move all the furniture in this room into one corner in a big pile, I've greatly increased the furniture density in that corner. However, I haven't made the room any smaller, and I haven't affected the overall furniture density, since its the same amount of stuff in the same space.

[J.C.MacSwell]

I think this is the source of the confusion. The answer is no' date=' since the Earth-ball system has not become any closer to everything else[/b']. There is still the same overall density, despite this gravitational collapse, since there is just more empty space around it.

 

Analogously, if I move all the furniture in this room into one corner in a big pile, I've greatly increased the furniture density in that corner. However, I haven't made the room any smaller, and I haven't affected the overall furniture density, since its the same amount of stuff in the same space.

 

Newtonian/Euclidean this is obviously right.

[Sisyphus]

Newtonian/Euclidean this is obviously right.

 

It should hold for general relativity, too, unless I'm being stupid. Moving stuff around xyz-coordinates shouldn't change the metric.

[J.C.MacSwell]

It should hold for general relativity, too, unless I'm being stupid. Moving stuff [/b']around xyz-coordinates shouldn't change the metric.

 

Then why would space collapse when all the stuff gets moved together?

 

I think GR predicts this whereas Newtonian physics does not. It is like the potential energy of mass displacements supports what space "is".

 

So the one small ball drop (and Earth "dropping" immeasurably toward the ball) is one small "tuck" (much much smaller than the immeasurable Earth displacement) in the fabric of space.

[Sisyphus]

Because the collapse is not all stuff moving together. Technically, nothing is actually "moving" at all. It's just getting closer, since space itself is shrinking.

[J.C.MacSwell]

Because the collapse is not all stuff moving together. Technically, nothing is actually "moving" at all. It's just getting closer,[/i'] since space itself is shrinking.

 

It's getting closer because it is yielding to gravity.

 

I think I see what you are getting at with regards to direction though by your "nothing is moving at all" comment. You are only considering the effects of nondirectional yielding to gravity to affect (or coincide with) a shrinking of space. (Am I reading you correctly?)

[Sisyphus]

Yes, that's more or less how I understand it. I could be wrong, but I don't think so.

[J.C.MacSwell]

Yes, that's more or less how I understand it. I could be wrong, but I don't think so.

 

Good, I think that was Spyman's point also. More on this later (anyone know definitively what GR would say on this?) but I like your view of the expansion or contraction as nothing actually moving (local movement aside) which is also how I picture it from an "outside" POV.

 

This brings us back toward my original question about the momentum of the expansion.

 

Since "nothing is actually moving" what keeps the expansion going? What tells space to keep expanding at the same rate?

 

Assuming gravity can only change the rate incrementally over time from the present rate the main factor that determines tomorrows rate is today's rate, the present "momentum" of the expansion.

[Spyman]

Good, I think that was Spyman's point also. More on this later (anyone know definitively what GR would say on this?) but I like your view of the expansion or contraction as nothing actually moving (local movement aside) which is also how I picture it from an "outside" POV.

Close enough, just not sure what would happen to space outside a universal sized black hole, if you actually could move all mass and energy together, (locally).

 

This brings us back toward my original question about the momentum of the expansion.

 

Since "nothing is actually moving" what keeps the expansion going? What tells space to keep expanding at the same rate?

 

Assuming gravity can only change the rate incrementally over time from the present rate the main factor that determines tomorrows rate is today's rate' date=' the present "momentum" of the expansion.[/quote']

You could argue that the momentum of expansion would keep the expansion going and that gravity would slowly change the rate until it eventually stops and starts to collapse. And I think that was at least part of the mainstream view until they discovered that there is another player on the field

- Dark Energy, which is currently accelerating the expansion.

 

"the universe appears to be expanding at an accelerating rate"

Wikipedia Dark Energy http://en.wikipedia.org/wiki/Dark_energy

 

And some other links you might find interesting to read:

 

Wikipedia Shape of the Universe http://en.wikipedia.org/wiki/Shape_of_the_Universe

 

Scientific American "Misconceptions about the Big Bang" http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147

 

Ned Wright's Balloon Analogy http://www.astro.ucla.edu/~wright/balloon0.html