I I want a good reference to a discussion of the meaning of "the speed of light is constant"

[gnnmartin]

Consider the Milne universe, if you're aware of it. https://en.wikipedia.org/wiki/Milne_model

It expands, but in the region of validity of the Milne universe, t>0, there is a diffeomorphism to the standard "Minkowskii" flat space-time of special relativity which does not expand.

My not-very precise resolution of this issue is to conclude that expansion, per se, does not cause physical effects. It's basically the result of a coordinate choice, though the expanding coordinates are lacking in complete coverage of the space-time.

However, acceleration or deacceleratio of the expansion DOES have physical effects. But these effects can be analyzed in terms of the effects of the matter, dark matter, energy, or dark energy that is causing the rate of expansion to accelerate or deaccelerate in the first place. The cause can ultimately be traced back to normal and dark matter/energy distributions.

Fore example, without dark energy, "gravity" causes the expansion of the universe to de-accelerate (slow down), this gravity has physical effects. Dark energy is the only one of the four that causes an accelerating expansion.

Orodruin wrote an article on some of this recently, with much more precision than what I wrote above, but I wasn't able to locate it.

Thanks, very helpful.