- Feb 25, 2016
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OK, this is how I see it: a galaxy would only have proper motion through space relative to other galaxies if some force was, or is, acting on it. The only significant force that is effective over galactic and supra-galactic scales is gravity, which is attractive - it's why galaxies congregate into (super)clusters in the first place.
So the galaxies, overall, will tend to be moving through space towards large centres of mass by the mutual attraction of gravity (and yes, there will be galaxies moving away from each other due to 'overshoot' or because they're attracted to different centres of mass). But there will be no preferred direction of movement overall; galaxies will be gravitationally attracted towards superclusters from all directions. So we should expect a roughly equal number of red-shifted and blue-shifted galaxies - and all shades in-between - overall due to this motion; although if we are in a supercluster, we might expect a few more blue-shifted galaxies close to our supercluster (as they will be more likely to be moving toward us).
The expansion of space is analogous to a repulsive force, tending to increase the separation between galaxies or reduce their relative closing speeds. So it will tend to counter the proper motion due to gravitational attraction rather than reinforce it.
Since the overall proper motion of galaxies through space has no preferred direction (being the result of gravitational attraction), and has no relation to the distance from any observer over cosmological scales, it will not be relevant to calculations of the Hubble red-shift if a reasonably sized sample is taken.
Cool
Yes, I know that gravity is what causes movement within galactic clusters super clusters, and between or among galaxies comprising such clusters. One example within our local cluster involves our Milky Way and Andromeda Galaxy which are approaching each other and are predicted to temporarily bypass each other and then pull themselves together via gravitational attraction. Other galaxies within our local group are satellite galaxies such as the Magellanic Clouds which orbit the Milky way and the Sagittarius dwarf Galaxy which is presently merging with ours.
These motions are proper motions either away from or towards each other and very often part of orbits. It is these motions that I am referring to in relation to the expansion of Space that supposedly takes place within the voids.
Now here is the hypothetical scenario, if we were to take a position within that void and measured the red shift of a super cluster that includes all these proper motions, how exactly would we be able to tell the difference between the two redshifts-the one caused by proper motion and the one being caused by space expansion. That is my question.
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