AndyFin said:
what is wrong with my logic.
Nothing, essentially.
It's just that, like with many other 'obvious' concepts, like distance, speed, or age, motion in cosmology requires some extra care when using, so that we all know what we actually mean and not get confused.
I said nothing is moving for two reasons - as the space expands (or contracts), the relative positions of objects in the universe don't change (disregarding small local motions). You do get changing distances, but if something was at your 9 o'clock, it'll stay there throughout the history. Furthermore, this 'recession' (or the opposite) is a General Relativistic effect, where all objects carried away by the expanding/contracting space are in free fall, following straight lines in space-time (geodesics), and whether they approach or recede is due to how the energy content of the universe curves the geodesics (much like with regular gravity described by GR).
However, the distances between objects still change - so one could very well say, hey: that's motion! And in fact, there are many who do. You see, there is no universally accepted usage of the word here. Still, choosing not to call this 'recession' (in case of expansion) 'motion' allows you to do two things:
- avoid the inevitable confusion when you realize that recession can and does carry away objects at recession velocities in excess of the speed of light
- get used to using so called 'comoving coordinates', employed throughout modern cosmology to describe the evolution of the universe. In these coordinates, the change of distances is factored out, so everything always stays where it is (i.e., relative to each other, in those coordinates). The whole of expansion or contraction is then relegated to one function called 'scale factor', a(t), describing how to scale the distances at different times in history (e.g. now, everything is ~1090 times farther away than it was ~13.8 billion years ago).
So, if you think about contraction in the big crunch scenario, while using the comoving coordinates, you see that every observer stays where they are, but as time progresses the scale of distances between them gets shorter and shorter.
Yes, you could say that everything 'moves' closer together, but it's a special kind of motion, somewhat unlike what we're used to.
AndyFin said:
Or is it that change of distance with time for space is not the same thing as change of distance with time and space for a mass like my old body for instance,?
Right, when talking about the expansion/contraction of the universe, you are always only concerned with large-scale objects moving in relation to each other. These objects need to be sufficiently far away for local gravitational effects to lose dominance. I.e., the 'all distances change' doesn't actually include distances between molecules in your body, in the solar system, or even on the galactic scale. In other words, expansion/contraction doesn't affect local gravitationally bound systems. You need to look at superclusters to see the effects of expansion of space.
