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Originally Posted by battybat I would think that this would be impossible, as one or both of the galaxies would have to travel at several times the speed of light for billions of years for this to happen. |
This is one of the misconceptions of what is really moving when we say that the universe is expanding (see March 2005 issue of Scientific American for all the other misconceptions).
Globally speaking, the galaxies are really not moving away from each other at high radial velocities. Locally, there can be varities of motions between galaxies,which led to some collisions of galaxies and that one of the lingering cosmological mysteries is why do galaxies formed clusters and superclusters?
It is space itself that is expanding and the speed of the expansion is greater than the speed of light the farther away the galaxies are from each other (these superluminal speeds have been detected). This seems to imply that the size of the universe is larger than the size of its visible part.