CarstenDierks said:
OK. But does it mean that a "constant drag" of the expansion exists which tries to "push" Earth and sun apart?
And if the gravitational force of the sun (or better: the gravitational force of sun and Earth together) is large enough to act against this force,..
Carsten, Powell is already engaged in this thread and he knows the stuff thoroughly so I would normally not jump in. He'll probably get back sometime today.
What your question is basically about is how to intuitively picture the expansion process, how to "feel" it with your acquired physics hunches, what familiar physical analogies to use. In a sense the mathematical reality is the same for everybody but different people may prefer different words and images.
Since you wrote me a PM asking about this, maybe I'll jump in, but hopefully Powell will still give his description of how to look at it. It's good to get several different takes and see which one is clearest for you.
My view is that galaxies experience only small accelerations due to forces around them and (compared with light) have only trivial or negligible speeds in their surroundings. You talk in terms of forces, but in my view of expansion force plays no role. It is negligible. Very large scale distances expand because geometry is dynamic, vintage 1915 Gen Rel is a story about dynamic geometry, how distances change. We have no right to expect that distances will not. But at present the overall expansion rate is only 1/140 of one percent per million years. And only concerns widely separated points or observers which are not gravitationally bound (it doesn't involve clusters of stars or clusters of galaxies because they are bound by gravity in stable configurations.)
What is basic in the way I approach this, intuitively, is the ancient light of the cosmic microwave background. You know you are stationary relative to Background if you have no doppler hotspot. If you are moving you will see a hotspot ahead and a coldspot astern.
As far as we can tell, galaxies are all approximately at rest relative Background. The deviations that have been detected are quite small. (on order 1/1000 of c).
What the Hubble law talks about is the rate of increase of distances between observers which are at rest relative CMB. It describes a simple feature of the dynamic geometry we happen to be in. It says v = Hd where d is a distance and v is the rate that distances is increasing. Distances are measured between stationary observers, ideally. In practice, between widely separated galaxies considered to be approximately stationary.
Maybe other people will give you some different intuitive takes.
In Gen Rel, what is called the "gravitational field" is actually a distance function which describes a geometry. And which is dynamic, it changes on its own and in response to matter, according to the 1915 GR equation. So our best (essentially our only) way to understand gravity at present requires us to chuck out Euclid and a bunch of static-geometry-type expectations. That's the part that takes getting used to.
