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Sep15-11, 11:45 AM
Sci Advisor
P: 1,682
Yes, a perfect fluid has a comoving frame. The CMB, ignoring its inhomogeneities, is well approximated by a perfect fluid. Reread Marcus's first post -- the rest frame of the CMB is the frame in which the fluid has "no average overall motion", i.e. it's the comoving reference frame of the fluid.