Neutrino Physics during the epoch of Recombination

In this talk, I will discuss Cosmic Microwave Background observables of interest to neutrino physics. Specifically, I address two such observables: N_eff, normally used as a parameter in the radiation energy density; and Σm_ν, the sum of the active neutrino masses. There exists a surprising and counter-intuitive result for how a non-zero Σm_νaffects N_eff. Instead of treating N_effas a parameter in the radiation energy density, I look at how the sound horizon and especially the diffusion length change with the addition of extra energy density. The increase in energy density due to non-zero Σm_νleads to a larger free-electron fraction, thereby changing the diffusion length. This leads to a different recombination history of the universe which lowers N_eff, as opposed to the expectation of an increase in N_eff from extra energy density. Furthermore, this counter-intuitive result is present in other exotic cosmologies, such as non-zero lepton number or the presence of light sterile neutrinos. This work is part of a long-term project on investigating early-universe observables in beyond- standard-model physics.