Learning Cosmology from the First Stars and Reionization

I will describe how to use current and upcoming astrophysical measurements in the redshift range z=4-20 to learn new cosmological information. I will first describe the 21-cm line during the cosmic-dawn era, which saw the formation of the first stars. The radiation from these stars excited neutral hydrogen and allowed it to absorb 21-cm photons from the cosmic microwave background (CMB). I will illustrate how the timing of this signal, which is dictated by the formation of the small minihalos that hosted the first stars, can be used to tightly constrain the small-scale behavior of dark matter, and determine if it's cold, warm, or self interacting. Then, I will explain how the acoustic physics of recombination become imprinted onto the 21-cm fluctuations, resulting in robust velocity-induced acoustic oscillations (VAOs) in the expected signal. These act as a new standard ruler during cosmic dawn, bridging the gap in measurements of the expansion rate of our universe between us and the CMB, which are presently at tension with each other. Finally, I will show how current measurements of UV luminosity functions during reionization can be used to probe primordial non-Gaussianity at smaller scales than accessible by the CMB.