CMB dipole

The CMB dipole is the CMB's most prominent anisotropy, presumed to be caused by the velocity of the solar system in relation to the average velocity of the material producing the CMB many years ago (i.e., at the surface of last scattering). The dipole consists of continuous variation with a maximum and minimum in opposing directions, suggesting a blue shift in one of the directions and a redshift in the opposite, one determination (Planck 2018 data analysis) indicating a speed of about 370 km/s roughly in the direction of right ascension 11h11m46s, declination -06deg56m39s. This fact is of interest, but is irrelevant to much cosmology research, and analysis of the CMB anisotropies often begins by removing the effects of the dipole from the data. In theory, this dipole should match an analogous dipole (suggesting our velocity) inherent in galaxy counts and radial velocities from redshift surveys: only partial agreement has been found and the disagreement is of research interest.

This can be thought of as the velocity of our motion through the universe. In the 1800s, the Michelson-Morley experiment attempted to determine our motion through measurement of the speed of light; their experiment did not show any movement and later, Einstein's theory of relativity showed how physics (such as the propagation of light) does not depend upon anything we are moving through, such as any medium in which the waves that constitute light (the term ether had been devised for whatever that substance might be). The velocity indicated by the CMB dipole could be considered the best we know of our velocity as compared to the universe, but unlike the velocity searched for by Michelson and Morley, our velocity compared to the CMB does not effect the physics we experience here; it only affects our view of very distant phenomena.