# Neutrinos and Other Dark Lights: Cosmological Constraints

A number of lines of evidence, both laboratory and astrophysical, suggest that there are extra (sterile) neutrino species, or otherwise extra light and dark degrees of freedom (‘dark lights’) than exist in the standard model of particle physics. I will review the physical origin of cosmological constraints on $N_{\rm eff}$ which is the energy density in dark lights expressed in units of the contribution from one active and massless neutrino species. I will also review the current constraints from light element abundances, high-resolution cosmic microwave background data, distance vs. redshift measurements, and galaxy cluster abundances.

This talk is part of the Institute of Astronomy Seminars series.