What we know and what we don't know about the CMB spectrum

To a precision of 50 ppm, the Cosmic Microwave Background Radiation has a blackbody spectrum with a temperature of 2.725 K, at wavelengths from 0.5 to 5 mm. This measurement by the COBE satellite team confirmed the concept of an expanding universe that was extremely hot and dense when it was young. In this picture, the CMB spectrum could be slightly distorted from the blackbody form by energy release or conversion after the universe was a few months old. At that time, the interaction of photons and electrons became weak enough that photons could no longer be easily created or destroyed, so that energy added or subtracted from the CMB would result in a spectrum with a chemical potential (mu distortion). Later, Compton scattering became incapable of equilibrating the spectrum to the chemical potential form, leading to a possibility of a mixture of blackbodies at different temperatures (y distortion). So far, there are only upper limits on the y and mu distortions, setting limits on possible sources of energy release such as the dissipation of acoustic modes at small scales, the decay of WIMPs, the action of primordial black holes, etc. On the other hand, the CMB spectrum is not expected to follow a blackbody to arbitrary precision. Energy releases associated with various astrophysical processes (recombination, reionization, and structure formation) will inevitably distort the CMB spectrum to create y or mu distortions at potentially observable amplitudes. Improved instrumentation capable of detecting such distortions could open a new window into the early universe, providing new constraints on processes ranging from inflation and the nature of the first stellar objects to exotic phenomena including primordial black holes, cosmic strings, and the decay or annihilation of dark matter.