New Information in Ancient Photons: Novel Approaches to CMB Secondary Anisotropies
Studies of the cosmic microwave background (CMB) radiation have driven the current era of precision cosmology. The tightest cosmological constraints to date have been derived from the primary CMB anisotropies, which predominantly probe the universe in its infancy. However, CMB experiments have recently entered a new regime in which constraints derived from the secondary anisotropies — sourced by effects between our vantage point and the surface of last scattering — substantially improve upon those derived from the primary anisotropies alone. Moreover, the secondary anisotropies contain valuable astrophysical information about the distribution of baryons and dark matter at late times. I will describe new approaches to extract information from these signals, focusing in particular on the thermal and kinematic Sunyaev-Zel’dovich effects, which refer to the Compton-scattering of CMB photons off ionized gas with high temperature or non-zero bulk momentum, respectively. I will show how these effects can be used to obtain percent-level constraints on the amplitude of matter density fluctuations, as well as to probe the abundance of ionized gas in and around modern-day galaxies, thus resolving the long-standing "missing baryon problem”. I will conclude with a look ahead to such measurements with the Advanced Atacama Cosmology Telescope and the recently-proposed Primordial Inflation Explorer.