The Cosmic Microwave Background: Spectral Distortions

The cosmic microwave background (CMB) traveled the cosmos long before it reached our telescopes today. Consequently, it is one of the best probes of fundamental processes in the early Universe that we could hope to observe. The cosmological information is encoded in two distinct ways. First, we can investigate how the CMB photons in one sky-direction are distributed across energy by focusing on information carried by the CMB frequency spectrum. Second, we can compare the flux of CMB photons that we receive from different directions, this time at a fixed frequency, to study the CMB anisotropies. In the past six decades since the serendipitous discovery of the CMB in 1965, cosmologists have advanced both frontiers in terms of theory and observation. In this chapter, I will give a broad-brush overview about how the CMB spectrum forms and evolves throughout cosmic history, mentioning CMB anisotropies only on the side. I will attempt to highlight some of the key theoretical ingredients that allowed us to establish the detailed picture of the Universe we have today. With this, I hope to convince you that, beyond the impressive past successes, the CMB still holds many treasures for us, and will keep generations of scientists busy for the decades to come.