How massless neutrinos affect the cosmic microwave background damping tail

Abstract

We explore the physical origin and robustness of constraints on the energy density in relativistic species prior to and during recombination, often expressed as constraints on an effective number of neutrino species, N eff. If the primordial helium abundance, YP, follows the prediction of the big bang nucleosynthesis (BBN) theory, the constraint on Neff from current cosmic microwave background anisotropy data is almost entirely due to the impact of the neutrinos on the expansion rate, and how those changes to the expansion rate alter the ratio of the photon diffusion scale to the sound horizon scale at recombination. We demonstrate that, as long as the primordial helium abundance is derived ..