Why does a cosmological force violate the Equivalence Principle?

Question

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

1. Why are comoving observers in a free-fall/unaccelerated/inertial frame?
2. 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?

Answer 1

There is no gravitational force causing expansion: it is simply how a uniform fluid density evolves under gravitational influence.

1) In a uniformly expanding universe, observers at rest with respect to the expansion are comoving. They are inertial because their worldlines, $x(t) \propto a(t)$, have no local acceleration.

2) The equivalence principal states that inertial and gravitational masses are equal; that is, a frame free-falling in a uniform gravitational field is inertial (in effect, one can "turn off" gravity by free-falling in the field). If there is an additional force than this relationship is disrupted, since free-falling under the influence of gravity is insufficient to remove all forces acting on the frame.

Answer 2

Comoving observers are inertial because that is part of the definition of comoving coordinates.

I don't know what question or answer you read, but the cosmological constant introduces a gravitational "force" pushing everything apart. I use inverted commas because gravitational forces are inertial, or "fictional". They are the product of a coordinate choice. In comoving coordinates, inertial, comoving, observers remain stationary - there is no active force in this frame.