Nobel laureate Steven Weinberg has written a new Cosmology textbook. Some folks might be interested. I don't have it yet (too stingy.) I have only second hand info. One of the things he explains is how the expansion of the universe slows down the local motion of things. In the early universe there would have been produced a hot cloud of neutrinos (particle-antiparticle pairs) all traveling very fast. The momentum of these particles would have decreased as the scalefactor increased. So at some time the neutrinos are 3000 kelvin (similar but not exactly same as CMB photons) and since that time the universe has expanded by a factor of 1000, so then the momentum of each individual is 1/1000 what it used to be. Expansion makes the neutrinos more sluggish/ (note, i am not talking about recession speeds away from some distant observer. I am talking about their speed compared with a nearby photon traveling in same direction). Galaxies would have been slowed down too, just like neutrinos, except that they were never traveling very fast (relative to CMB) in the first place. But in principle any massive object from a neutrino to a galaxy. I expect you might like to see a mathematical explanation of this effect, and there is one by Hongbao Zhang that he published because it is an improvement on Steven Weinberg's textbook method of proof. Zhang is a smart guy. I will get the link to Zhang's new paper. It is free (whereas you have to buy Weinberg's textbook.) http://arxiv.org/abs/0808.1552 Note on the thermal history of decoupled massive particles Hongbao Zhang (Submitted on 11 Aug 2008) "This note provides an alternative approach to the momentum decay and thermal evolution of decoupled massive particles. Although the ingredients in our results have been addressed in [Weinberg's new Cosmology text], the strategies employed here are simpler, and the results obtained here are more general."