Is the reference frame defined by the average motion of massive particles in the universe the same as that defined by the CMBR?

Question

Title.

Also, related question: It seems easiest to measure the velocity of stars, but does the average star velocity actually match the overall average mass velocity, including the interstellar and intergalactic media?

Answer 1

The assumption is yes - i.e., the CMB dipole is entirely kinematic in origin. However, this has not been shown for certain (e.g. see Cosmic flow from 2MASS redshift survey: The origin of CMB dipole and implications for LCDM cosmology).

The conventional assumption is that we see a dipole in the CMB entirely due to the Doppler effect due to the Solar System's motion relative to the CMB. This is rooted in the fact that we assume our universe to be homogeneous and isotropic (the cosmological principle), thus we can assume that there is some the "CMB rest frame" in which the CMB dipole would be zero. If not, then there would be some preferred direction given by the bulk motion of e.g. galaxies or large scale structures.

Measuring the Solar System motion relative to the CMB dipole also implies a certain velocity for the Galaxy and the Local Group of galaxies relative to the CMB, with some uncertainty in the velocity of the Solar System relative to the Local Group. The Earth's velocity relative to the Local Group of galaxies and the Local Group’s motion relative to the CMB rest frame are usually calibrated against for cosmological studies.

Addendum to address comments: It sounds there was an expectation of a definitive answer, but this is an open question. Further links to the literature with data are https://arxiv.org/abs/1503.05422, https://arxiv.org/abs/1912.09383.

Different groups have data and interpretations ranging from the mainstream ("broadly supports the assumption of isotropy") to rejecting isotropy.