Apologies if this sounds like a dumb question, I am mostly self-taught when it comes to physics.
I am trying to understand an answer I read to the question "why should we not think of the expansion of space-time as a cosmological force acting on the galaxies, pushing/pulling them apart?" The answer explained that any cosmological forces acting on comoving observers would violate the Equivalence Principle because the laws of physics must be the same in all inertial frames. The answer assumes that comoving observers are free-falling i.e there are no forces acting other than gravity, which ends up looking like a circular argument to me.
My questions are
- Why are comoving observers in a free-fall/unaccelerated/inertial frame?
- Why does a cosmological force that pulls everything away from everything else violate the Equivalence Principle? Isn't it doing the same thing for each comoving observer? Why/How is it a violation of the laws of physics?