The microwave sky and the dark universe

In today’s era of precision cosmology, we can go beyond cosmic accounting to actually testing various ideas for the identity of the dark matter and dark energy. One such idea is that the dark matter or even the dark energy is an ultra-light particle like an axion, the Goldstone boson of a new high-energy symmetry, motivated by the mystery of why charge-parity symmetry violation in the strong sector is so small. After reviewing the properties of the cosmic microwave background, I will discuss how we have tested the ultra-light axion idea using both cosmic microwave background fluctuations and the large-scale clustering of galaxies, putting constraints on ~7 orders of magnitude in axion mass. I will then discuss the prospects for future cosmological probes of axions and other light particles. On small scales, microwave background fluctuations are damped by diffusion of photons out of wavefronts. This damping heats the plasma after it can perfectly thermalize, leading to minute deviations from a perfect blackbody. These deviations could be detected in the next ten years. I will close by touching on how these deviations probe the cosmic thermal history at extremely high redshifts and could possibly probe the dark universe.​