- #36
PeterDonis
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PeroK said:How would you describe the solution when it comes to the local group of galaxies and, in particular, the collision of the Milky Way and Andromeda?
The local group is a bound system surrounded by empty space (unless we include dark energy, but on this scale dark energy effects are still too small to matter). So it would be modeled as a bound system surrounded by empty space.
PeroK said:The distances here are significant if we plug in the current universe expansion rate.
You can multiply any distance you like by the Hubble constant and get a speed. That in itself doesn't mean the speed has any physical significance.
The correct criterion, as I've already said, is what matter is actually present in the region of spacetime you are talking about. The Local Group of galaxies, as above, is a bound system surrounded by empty space. There is no "expansion velocity" in such a system. The fact that the FRW model for the universe as a whole has a Hubble constant that I can multiply by, say, a million light-years to get a speed, is irrelevant, since the FRW model is simply not a correct model for a bound system surrounded by empty space.
PeroK said:That expansion rate is based on the overall universal balance of mass/vacuum.
More precisely, it's based on assuming that the universe is filled with a uniform density perfect fluid, with the density given by the average density for the universe as a whole.
PeroK said:How would the universe expansion rate get into the equations for the dynamics of the local group?
It wouldn't, since the local group is not composed of a uniform density perfect fluid.
PeroK said:should we model the local group using the specific mass/vacuum density we find in this region?
Using the specific mass/vacuum distribution we find in the region, on whatever length scales are relevant for the question we are trying to answer.
PeroK said:Can these two models be tested against the observed separation velocity?
The FRW model is obviously false for the Milky Way and the Andromeda galaxy, since they are moving towards each other whereas the FRW model says they should be moving apart.