- #1
cephron
- 124
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I was just reading on wikipedia about the expansion of space. ( http://en.wikipedia.org/wiki/Metric_expansion_of_space )
It explains that the expansion of space can be treated as if it were a repulsive force between particles proportional to the distance between them, ie the further apart they are, the more strongly they are pushed apart.
But then it says: "However this does not cause the objects [talking mainly about galaxies here] to grow steadily or to disintegrate; unless they are very weakly bound, they will simply settle into an equilibrium state which is slightly (undetectably) larger than it would otherwise have been."
This doesn't make sense to me - if the "equilibrium state" is slightly larger, then the force of gravity is reduced and the "force" of expansion is increased. This would tend to push the objects even further apart. I don't see any kind of negative feedback here to keep the cosmological expansion from pushing any orbiting bodies away from each other.
Another way of looking at it -
Imagine an empty universe with two equal-mass objects orbiting each other in a perfectly circular path. If there were no expansion of space, they would orbit each other infinitely without getting any closer (Right?). Now add small repulsive force between them, proportional to their distance from each other. What can happen except that they get pushed slightly further apart by it? And now that they're slightly further apart, the attraction is weaker and the repulsion is stronger. What can happen except they again get pushed slightly further apart? I must be missing something...
It explains that the expansion of space can be treated as if it were a repulsive force between particles proportional to the distance between them, ie the further apart they are, the more strongly they are pushed apart.
But then it says: "However this does not cause the objects [talking mainly about galaxies here] to grow steadily or to disintegrate; unless they are very weakly bound, they will simply settle into an equilibrium state which is slightly (undetectably) larger than it would otherwise have been."
This doesn't make sense to me - if the "equilibrium state" is slightly larger, then the force of gravity is reduced and the "force" of expansion is increased. This would tend to push the objects even further apart. I don't see any kind of negative feedback here to keep the cosmological expansion from pushing any orbiting bodies away from each other.
Another way of looking at it -
Imagine an empty universe with two equal-mass objects orbiting each other in a perfectly circular path. If there were no expansion of space, they would orbit each other infinitely without getting any closer (Right?). Now add small repulsive force between them, proportional to their distance from each other. What can happen except that they get pushed slightly further apart by it? And now that they're slightly further apart, the attraction is weaker and the repulsion is stronger. What can happen except they again get pushed slightly further apart? I must be missing something...