The cosmic microwave background (CMB) is a nearly perfect blackbody with a temperature of about 2.7K . Since we observe the CMB (and the entire astronomical sky) from a moving reference frame, every observation is subject to a small Doppler boost. This motion results from several factors: the motion of the experiment (e.g. on the spinning Earth or satellite); Earth’s orbit around the Sun (the so-called orbital dipole); the Sun’s orbit around the Galactic centre; and the motion of our Galaxy within the local group. The Local Group itself may also be moving relative to the rest frame of the CMB. This Doppler boost affects the observed positions, brightness, and frequencies of our observations of objects in the Universe.
Several exotic theories suggest the existence of an intrinsic dipole in the Universe, which would violate the assumption of isotropy and hint at new physics on the largest scales, originating from very early in cosmic history. Even if the dipole is simply the result of a boost, several higher-order effects are often overlooked. Moreover, the CMB dipole has interesting historical significance for understanding our place in the cosmos
I will begin by reviewing the history of the CMB dipole, including its discovery, early measurements, and the controversy surrounding its origin. I will then outline the first-, second-, and higher-order effects of a boost on the CMB, briefly covering topics such as relativistic thermodynamics, aberration and modulation, and frequency-dependent effects. I will lastly provide an overview of some current tensions in the field, focusing on the dipole discrepancies in radio and quasar observations.
