Bill6; I recall saying that I would return to your comment about the convergence of the Milky Way and Andromeda. Trouble is, I can’t remember now what the point of discussion was. Time is too short at present to do a lot of hunting through past threads, so I shall just post a few questions and answers, not necessarily correct answers :), in the hope of getting near the topic. I thought that might be better than just letting the whole thing slip into oblivion.

In an expanding Universe, why are Andromeda and the Milky Way moving closer together?

The expansion of the Universe is causing the galaxy groups to separate. These two galaxies are in the same group, so the major influence comes from gravity, rather than the force of expansion. The expansion of the universe is not enough on the scale of our local galaxy group to overcome the attraction of gravity.

Considering the relativity of motion; is it possible to say if either galaxy is moving towards the other?

At first sight it would seem that the easiest way to tackle this question would be to regard the Local Group as a static frame of reference and decide which galaxy is moving relative to the LG. However, even that is not straightforward. Considering the situation more precisely; if you consider an inertial frame that is stationary relative to the centre of mass of the LG both galaxies will appear to be moving towards each other. There is a school of thought that says that to claim things are moving towards or away from each other is a contradiction in itself, and that instead we should say they are moving towards a state of equilibrium. However, in this instance, that seems not to be very helpful.

Is there any other, larger, thing in the Universe that could be taken as a static F of R?

Possibly, a supercluster can be regarded as a larger static unit, but the only universal thing I can think of is the Cosmic Microwave Background, which seems to approach every point in the Universe, at the same speed, from every angle. Presumably movement relative to the CMB is as near as we could get to “absolute movement”.

Do we know why Andromeda and the Milky Way are moving closer?

Gravitational attraction within the LG has to be the reason. At first glance it might seem that an equilibrium exists between gravity and the force of expansion which keeps the group together, but does not permit it to collapse. However, the collective velocity of the LG is estimated at around 270 kps towards the Virgo cluster (Virgocentric infall rate). It would seem that the LG is collapsing, but Andromeda and the Milky Way must be sufficiently close for their mutual gravity to provide movement in a secondary direction. The trouble seems to be that the closer one tries to look at the situation, the more one finds different movements. For example, both the LG and Virgo Cluster are moving at roughly 600kps (relative to the CMB) towards Hydra Centaurus and converging at the rate 270kps. Where does astronomy end and cosmology begin? Or is it the other way round?

There never was nothing.