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Dec20-11, 07:42 AM
P: 3,060
Quote Quote by DaleSpam View Post
No, I would not agree with that. The size of the observer is irrelevant, only it's velocity wrt the CMB determines if it is comoving or not. A cluster sized observer moving relative to the CMB would not be comoving, and a neutrino sized observer at rest wrt the CMB would be comoving.

IMO, the reason it is a very reasonable convention is because it can be applied in the identical fashion at each spatial location.
Then you'll have to agree that in GR at certain distances velocity is not well- defined (meaning it is a path dependent parameter, and if you consider the LCDM model a valid one you will have to agree that at a certain scale the universe is homogeneous in the way FRW model predicts, therefore in this context size (or scale) is not irrelevant, there can't be a hypercluster (of whatever the size it is required by the LCDM model that the universe is truly homogeneous) that is not comoving with CMBR, if you don't agree with these two facts let me know.