What is half a neutrino? Reviewing cosmological constraints on neutrinos and dark radiation

Abstract

Neutrinos are one of the major puzzles in modern physics. Despite measurements of mass differences, the Standard Model of particle physics describes them as exactly massless. Additionally, recent measurements from both particle physics experiments and cosmology indicate the existence of more than the three Standard Model species. Here we review the cosmological evidence and its possible interpretations.

Comments: 15 pages, 4 figures, invited review accepted for publication in PASA