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Message: <blockquote data-quote="briggart" data-source="post: 9814117" data-attributes="member: 6805135">Again, not sure I follow, given it's the basis of modern cosmology. Now, you, me, the Earth, etc. are proof that the cosmological principle cannot hold exactly. If that is your point, we agree, but I don't think it significantly changes the discussion.I feel it is more of a disagreement about what you and consider a "big deal", rather than misunderstanding. SR tell us that we can not look at the form of the laws of physics to a priori pick a privileged reference system from a family of inertial frames, but that does not prevent us from a posteriori picking one on the basis of empirical evidence.  "Meaningful", after all,  it's human concept.If you continuously observe zero kinematic dipole in the CMB, you are in a locally inertial frame which is at rest with the overall distribution of matter in the observable universe (in some average sense: distant galaxies and such are still moving away). This is the closest equivalent we can get to the intuitive concept of "absolutely at rest".Rereading what you were replying to, I think I could have been clearer. My point was not about the significance, or lack thereof, of the universe being homogeneous and isotropic, only that observing the CMB allows us to infer how we are moving (in an average sense) with respect to the overall distribution of matter. Also, while on human timescales our motion with respect to the CMB can be approximated a constant linear motion, properly speaking acceleration needs to be accounted for. Even after correcting for Earth's rotation and orbit around the Sun, our dipole is due to a combination of the Sun motion around the Galactic center and the Milky Way infall toward the Local Attractor. In general, while we speak of deviations from the Hubble flow in term of peculiar velocities, on cosmological timescales we should properly talk of peculiar accelerations."Instantaneous" is important because we can be at rest with each other at most for a single instant. We'll be in relative acceleration both before and after that instant, specifically because movement due to cosmological expansion is actual movement.</blockquote>

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