Suppose in the universe, there are inertial frames in the vicinity of galaxies. Suppose also that these frames rotate slightly with respect to each other - that the universe is not quite a 'mill pond'.
If this were true, our Galaxy would have its own inertial frame, rotating wrt distant galaxies.
The rotation of our galaxy's frame would affect astronomical observations.
Does anyone know the upper bound on the rotation rate of our galaxy's inertial frame which is consistent with observations?