Is it guaranteed that there exists a universal time on which all comoving observers will agree?
Special relativity teaches us that inertial observers in relative motion do not agree on the time they observe. Now in standard cosmology, the physical distance between the comoving observers changes with time because of the expansion of the Universe even though they sit at fixed coordinates in the comoving coordinate system.
Does this mean they do not agree on the cosmic time $t$? I'm not talking about the conformal time.
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