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Now , in standard cosmology, there is a set of so-called "fundamental or comoving observers" that are supposed to move with the "cosmological Hubble flow" and that act as observers that wouldn't register motion wrt the CMBR. This is a fundamental premise(a.k.a. Weyl's postulate) for the construction and use of FRW metric. These fundamental observers define a common temporal order of events for everyone that uses this metric regardless of how distant apart they are or their speeds.

It would seem at first sight that if the relativity of simultaneity,(due to the finiteness of c and equivalence of inertial frames for physical laws), forbids that an omniscient observer be able to define the order of events in different frames, then the fundamental observers of cosmology shouldn't be allowed since that is exactly what they do.

It' looks like if we were subtly introducing an absolute rest frame or aether with the fundamental observers comoving with the CMBR , but this set of observers is needed to build the hypersurfaces or homogenous density slices defined by the FRW metric and to interpret redshift as expansion.

There seems to be an arbitrariness here, is the SR relativity of simultaneity no longer valid in GR and cosmology? or is there any other explanation to clear this up?

any comments?