general relativity - Assuming that the Cosmological Principle is correct, does this imply that the universe possess an empirically privileged reference frame?

OK...before everyone blasts this with references to the relativistic invariance of the physical laws, time dilation, etc let me add some context. Also, I am an amateur with an interest in physics, so I don't know the details of the physics. However, I have read enough about the cosmological principle and cosmic time to develop this "confusion" .

Based on evidence gathered to-date, it appears that the universe is both isotropic and homogenous to a high degree, as evidenced by observations and the usefulness of the Freedman-Lemaitre-Robertson-Walker (FLRW) metric. This principle, when combined with the FLRW metric, allows space-time to be divided into non-intersecting slices and therefore establish a "cosmic time."

Now, the existance of such a time in no way suggests that it is the "right" time or a true "now", hence violating relativity -- although some presentist philosophers (e.g., W.L. Craig) have tried to make this argument, but it is not generally accepted in philosophical circles.

However, this leaves me confused about an apparent disconnect between relativity and the cosmological principle:

Isotropy only holds if we are at rest relative to the cosmic microwave background radiation (universal rest frame), o/w anisotropy is present - yet each reference frame is supposed to allow equally valid observations. How can these two be reconciled when most reference frames would lead us to conclude that the universe is NOT isotropic?

It seems that only by appealing to the idea of being at rest relative to "universal rest frame" can we explain away any discrepancies from isotropy as due to "peculiar motion". However, doesn't this give this "universal rest frame" and its associated time an empirically privileged status, even though physical laws work just fine in every reference frame?

For example (pardon any abuse of astronomy): if look off in some part of space and see only quasars, and then in another part of space and see only brown dwarfs, but I measure both as apparently the same distance from me, then can I conclude that we are in motion, since otherwise we would have an empirical contradiction (kind of like finding dinosaur and human remains in the same strata)?

Any help on where I am going wrong would be helpful. Intuitively, I don't think there should be a way to establish absolute ordering between space-like separated events, but the above reasoning suggests otherwise.

Answer

Your mistake is to assume that the cosmic microwave background constitutes a universal rest frame, because it doesn't.

In an FLRW universe there is a frame called the comoving frame or proper frame that is particularly mathematically convenient. This is the frame in which the comoving distance between all inhabitants of that frame is constant, so all the "stuff" in the universe is mutually stationary (the comoving frame factors out the Hubble expansion). Given that we expect all the "stuff" in the universe to be created in a similar way we would expect it to be approximately stationary (in a comoving sense) so the sum total of everything, matter and energy, acts as a reference point for the comoving frame.

So there's nothing special about the CMB. If you ignored the CMB and measured the Earth's velocity to all the galaxies we can see then you'd expect to get the same result as measuring the Earth's velocity relative to the CMB. The CMB occupies the same frame as everything else because it was created in basically the same way. The only special thing about the CMB is that gravitational interactions haven't given it various peculiar velocities as has happened for large aggregations of matter.