  Many well-motivated extensions of the Standard Model predict the existence of
new light species that may have been produced in the early universe. Prominent
examples include axions, sterile neutrinos, gravitinos, dark photons, and more.
The gravitational influence of light relics leaves imprints in the cosmic
microwave background fluctuations, the large-scale structure of the universe
and the primordial element abundances. In this paper, we detail the physics of
cosmological light relics, and describe how measurements of their relic density
and mass serve as probes of physics beyond the Standard Model. A measurement of
the light relic density at the precision of upcoming cosmological surveys will
point the way toward new physics or severely constrain the range of viable
extensions to the Standard Model.
