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bigsplit
2008-Apr-19, 09:50 PM
As space expands around us, what allows it to move mass with it? For the rising cake analogy with the fruit moving apart, we know that the bread between is expanding and we have mass on mass from an energy effect. What transfers the energy from the expanding space to the masses throughout the Universe or why doesn't space just "blow" right by us?

Steve Limpus
2008-Apr-20, 02:10 AM
Some of the more knowledgeable posters around here refer to the 'metric' expansion of space which is related to the 'coordinate' system. I know that doesn't really answer your question, but I suspect it's at least somewhat on the right track. Hopefully others will chip in, I'm interested in this question too. :)

Steve Limpus
2008-Apr-20, 02:20 AM
See what you make of this (http://arxiv.org/PS_cache/arxiv/pdf/0707/0707.0380v1.pdf) Bigsplit - I'm trolling through it now.

loglo
2008-Apr-20, 05:06 AM
As space expands around us, what allows it to move mass with it? For the rising cake analogy with the fruit moving apart, we know that the bread between is expanding and we have mass on mass from an energy effect. What transfers the energy from the expanding space to the masses throughout the Universe or why doesn't space just "blow" right by us?

I think it is a matter of definition. We say space expands because we see the mass moving apart. How could we measure it any other way? Ascribing any more properties to space isn't necessary, well wasn't until Dark Energy came along. Even there it is not clear.

Jeff Root
2008-Apr-20, 05:31 AM
bigsplit,

It really isn't known whether "expanding space" applies a force to the
matter in it. The question changed ten years ago, when the acceleration
of the expansion was discovered. That makes it seem more likely that
something about space itself pushes or pulls matter around. But it still
isn't a sure thing.

Before the acceleration was discovered, the expansion of galaxies could
be very accurately described as ballistic, slowed by gravity. That is, the
galaxies are flying apart on ballistic trajectories as if from an initial
explosion, and their gravity slows their motion away from one another.
Many sources will tell you that there was no initial explosion, but there
really is no good reason to say that. Before the acceleration was known,
there was no good reason to think that "expanding space" pushes or
drags matter with it. Now there is some reason, but it still is not
compelling.

-- Jeff, in Minneapolis

loglo
2008-Apr-20, 05:43 AM
That was an excellent link by the way Steve. This quote directly answers the OP I think. my bold

A clearer explanation is simply that on the scales of galaxies the cosmological
principle does not hold, even approximately, and the FRW metric is not valid. The metric of spacetime in the region of a galaxy (if it could be calculated) would look much more Schwarzchildian than FRWlike, though the true metric would be some kind of chimera of both. There is no expansion for the galaxy to overcome, since the metric of the local universe has already been altered by the presence of the mass of the galaxy. Treating gravity as a four-force and something that warps spacetime in the one conceptual model is bound to cause student more trouble than the explanation is worth. The expansion of space is global but not universal, since we know the FRW metric is only a large scale approximation.

So the mass changes the expanding metric locally to one that isn't expanding. Between the clusters the metric remains expanding. Saying that the property of mass itself "anchors" it to space sounds reasonable.

bigsplit
2008-Apr-20, 04:40 PM
bigsplit,

It really isn't known whether "expanding space" applies a force to the
matter in it. The question changed ten years ago, when the acceleration
of the expansion was discovered. That makes it seem more likely that
something about space itself pushes or pulls matter around. But it still
isn't a sure thing.

Before the acceleration was discovered, the expansion of galaxies could
be very accurately described as ballistic, slowed by gravity. That is, the
galaxies are flying apart on ballistic trajectories as if from an initial
explosion, and their gravity slows their motion away from one another.
Many sources will tell you that there was no initial explosion, but there
really is no good reason to say that. Before the acceleration was known,
there was no good reason to think that "expanding space" pushes or
drags matter with it. Now there is some reason, but it still is not
compelling.

-- Jeff, in Minneapolis

Thanks Jeff.