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7.5. Dipole

To a very good approximation the CMB is a flat featureless blackbody; there are no anisotropies and the temperature is a constant To = 2.725 K in every direction. When we remove this mean value, the next largest feature visible at 1000 times smaller amplitude is the kinetic dipole. Just as the 17 satellites of the Global Positioning System (GPS) provide a reference frame to establish positions and velocities on the Earth. The CMB gives all the inhabitants of the Universe a special common rest frame with respect to which all velocities can be measured - the comoving frame in which the observers see no CMB dipole. People who enjoy special relativity but not general relativity often baulk at this concept. A profound question that may make sense is: Where did the rest frame of the CMB come from? How was it chosen? Was there a mechanism for a choice of frame, analogous to the choice of vacuum during spontaneous symmetry breaking?