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

TL;DR

This paper reviews cosmological evidence for neutrinos and dark radiation, discussing their mass, number of species, and implications for physics beyond the Standard Model.

Contribution

It provides a comprehensive review of cosmological constraints on neutrino properties and the existence of dark radiation, highlighting recent experimental and observational findings.

Findings

Cosmological data suggest more than three neutrino species.

Evidence indicates neutrinos may have non-zero mass.

Dark radiation could account for additional relativistic particles.

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.