Electron-Positron Annihilation Freeze-Out in the Early Universe

Electron-positron annihilation is the final particle-antiparticle annihilation process in the history of the early universe. It largely occurs after the neutrinos fall out of thermal equilibrium (weak decoupling) and during the Big Bang Nucleosynthesis (BBN) epoch. This annihilation occurs predominantly in local thermal and chemical equilibrium, and its effects can be seen in BBN yields as well as the relativistic degrees of freedom. This work looks to self-consistently calculate annihilation rates to determine when this process falls out of equilibrium, and to estimate the magnitude of this out-of-equilibrium effect. Connecting precision cosmological measurements to tests of beyond standard model physics models requires full consideration of the standard cosmological model, including these oft-overlooked out-of-equilibrium effects.