Institute for Advanced Study Informal Astrophysics Seminar

ABSTRACT: A detailed and precise understanding of the formation of neutral atomic hydrogen is necessary to predict the spectrum of microwave anisotropies. New experiments like Planck and ACT will gain considerable leverage on the cosmological parameters (including the properties of the primordial density fluctuation spectrum) by using data at small angular scales, precisely where the details of atomic physics matter. After reviewing past work, I will discuss recent results that include many more Rydberg states, higher-order transitions, corrections to a simplified picture of radiative transfer, and the possibility of a cosmological maser. If time permits, I will present preliminary results on how fluctuations in the baryon-dark ratio would affect microwave observables.