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