Cosmic Microwave Backlight: Illuminating Large-Scale Structure with the Universe's Oldest Photons
James Colin Hill
Princeton University

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 kinematic Sunyaev-Zel'dovich effect, which refers to the Compton-scattering of CMB photons off free electrons with non-zero bulk momentum along the line-of-sight. I will show how I have used this effect to probe the abundance of ionized gas in and around modern-day galaxies and thereby resolve the long-standing "missing baryon problem". I will conclude with a look ahead to such measurements with the Advanced Atacama Cosmology Telescope and Simons Observatory, which will measure CMB secondary anisotropies -- and search for primordial gravitational waves -- with unprecedented sensitivity.