The Physics of Light Relics

TL;DR

This paper reviews how light relic particles from early universe extensions of the Standard Model influence cosmological observations and how future measurements can reveal new physics or constrain existing theories.

Contribution

It provides a detailed analysis of the physics of cosmological light relics and discusses how upcoming measurements can probe physics beyond the Standard Model.

Findings

Light relics affect cosmic microwave background and large-scale structure.

Upcoming surveys can measure relic density and mass with high precision.

Results can either reveal new physics or constrain theoretical models.

Abstract

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.