Putting this here to help passers-by. My basic confusion appears to have been caused by not realizing that the universe is stretching. Thus, we aren’t in an inertial frame with respect to other galaxies.
Inertial frame is a local notion, as well, so “inertial frame with respect to other galaxies” is not a meaningful statement. In GR inertial frame is generalized to a geodesic. There is also the concept of comoving frame, that in which the expansion rate looks the same in all directions.
Putting this here to help passers-by. My basic confusion appears to have been caused by not realizing that the universe is stretching. Thus, we aren’t in an inertial frame with respect to other galaxies.
Inertial frame is a local notion, as well, so “inertial frame with respect to other galaxies” is not a meaningful statement. In GR inertial frame is generalized to a geodesic. There is also the concept of comoving frame, that in which the expansion rate looks the same in all directions.