Precision effective number of neutrinos $N_\textrm{eff}$

Abstract

Many beyond-Standard-Model theories predict light and feebly interacting states that can be efficiently produced in the early universe. From the cosmological perspective, these states behave like Standard-Model neutrinos and can impact on a variety of cosmological observables including the light element abundances from primordial nucleosynthesis and the cosmic microwave background (CMB) anisotropies. It is common to quantify the energy density residing in these light states in terms of its contribution to the effective number of thermalised neutrino species, $N_\textrm{eff}$, where the Standard-Model value (accounting for the energy density in 3 families of Standard-Model neutrinos) is $N_\textrm{eff} = 3.044$. In this talk, I will discuss first of all the signatures of additional light species in cosmological observables, current observational constraints on the $N_\textrm{eff}$ parameter and prospects for improving these constraints with future CMB experiments. I will then report on a recent precision theoretical calculation of the Standard-Model expectation for the $N_\textrm{eff}$ parameter, performed by two independent groups, both of which found to five-significant-digit agreement $N_\textrm{eff}^\textrm{SM} = 3.0440 \pm 0.0002$.