The Physics of Light Relics

Understand

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.