Finding eV-scale Light Relics with Cosmological Observables

TL;DR

This paper forecasts the potential of current and future cosmological surveys to detect light relic particles with eV-scale masses, which could reveal new physics beyond the Standard Model through their effects on cosmic structure and radiation.

Contribution

It provides the first comprehensive forecast of how well upcoming cosmological data can detect and constrain light relics with masses around eV, extending the search for new physics beyond previous limits.

Findings

Upcoming surveys can significantly improve detection sensitivity for light relics.

Large parameter space coverage will enable exploration of new physics beyond the Standard Model.

Light relics with different masses and temperatures leave distinct imprints on cosmological observables.

Abstract

Cosmological data provide a powerful tool in the search for physics beyond the Standard Model (SM). An interesting target are light relics, new degrees of freedom which decoupled from the SM while relativistic. Nearly massless relics contribute to the radiation energy budget, and are commonly parametrized as variations in the effective number $N_{\rm eff}$ of neutrino species. Additionally, relics with masses greater than $10^{-4}$ eV become non-relativistic before today, and thus behave as matter instead of radiation. This leaves an imprint in the clustering of the large-scale structure of the universe, as light relics have important streaming motions, mirroring the case of massive neutrinos. Here we forecast how well current and upcoming cosmological surveys can probe light massive relics (LiMRs). We consider minimal extensions to the SM by both fermionic and bosonic relic degrees of freedom. By combining current and upcoming cosmic-microwave-background and large-scale-structure surveys, we forecast the significance at which each LiMR, with different masses and temperatures, can be detected. We find that a very large coverage of parameter space will be attainable by upcoming experiments, opening the possibility of exploring uncharted territory for new physics beyond the SM.