Simulating the Thermal Evolution of Dark Matter During an Early Matter-Dominated Era

The period between inflation and the onset of Big Bang nucleosynthesis is, in many ways, poorly understood and poorly constrained. In particular, the ramifications of including an early matter-dominated era (EMDE) in this period have not been fully explored. One possible probe of this era is the evolution of dark matter. The precise way in which WIMP dark matter decouples from the relativistic bath has a profound effect on the smallest scale (k_cut) of structure formation, which then influences annihilation signatures that can be observed by Fermi-LAT. There has already been work connecting k_cut to the annihilation signatures, but there is a missing link between fundamental particle physics and k_cut itself. The goal of this work is to begin to close that gap, by connecting dark matter microphysics to k_cut. The thermal evolution of dark matter during radiation domination can be solved analytically, but the inclusion of an EMDE violates key assumptions made in the analytical solution. To understand the effect of an EMDE on the velocity distribution of dark matter, we model the collisions between individual dark matter particles and relativistic leptons. In this work, we use UNC's Longleaf Cluster to evolve an ensemble of 15,000 dark matter particles through both the EMDE and the subsequent transition to radiation domination. We find that the shape of the velocity distribution deviates significantly from the analytical solution and that the new distribution is not well described by the dark matter temperature. This represents a breakdown of the analytical solution for the momentum distribution when decoupling deep in an EMDE, making this numerical simulation necessary in order to calculate k_cut and connect a particular dark matter candidate to observable annihilation signatures.Bachelor of Scienc