Cosmic Neutrinos and Other Light Relics

TL;DR

Cosmological observations of radiation density, including primordial element abundances and CMB measurements, serve as powerful tools to test and constrain physics beyond the standard model and explore the early universe.

Contribution

This paper reviews how current and future cosmological measurements can probe light relics and new physics in the early universe.

Findings

Primordial element abundances constrain new physics models.

Upcoming CMB measurements will significantly improve radiation density estimates.

Enhanced measurements will either reveal new physics or tighten existing constraints.

Abstract

Cosmological measurements of the radiation density in the early universe can be used as a sensitive probe of physics beyond the standard model. Observations of primordial light element abundances have long been used to place non-trivial constraints on models of new physics and to inform our understanding of the thermal history to the first few minutes of our present phase of expansion. Precision measurements of the angular power spectrum of the cosmic microwave background temperature and polarization will drastically improve our measurement of the cosmic radiation density over the next decade. These improved measurements will either uncover new physics or place much more stringent constraints on physics beyond the standard model, while pushing our understanding of the early universe to much earlier times.