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PeterDonis
#36
Dec20-11, 10:42 PM
Physics
Sci Advisor
PF Gold
P: 6,034
Quote Quote by my_wan View Post
So I ask, how do you empirically justify that only a particular set of observers see the universe as homogeneous and isotropic?
*We* don't see the universe as homogeneous and isotropic, and we are observers. I think that counts as empirical.

Only observers who are at rest in the "comoving" frame used in the FRW models see the universe as homogeneous and isotropic. Observers who are not at rest in that frame don't. Earth is not at rest in that frame, because we see a dipole anisotropy in the CMBR. We don't have reports from astronomers in other galaxies, but based on what we can see of their motions, it appears that at least some of the nearby ones are not at rest in the "comoving" frame either (that is, their observed relative velocity to us is different from what it would need to be to cancel the dipole anisotropy we see in the CMBR). AFAIK it gets harder to tell as you go farther out.