Probing neutrino interactions and dark radiation with gravitational waves

TL;DR

This paper investigates how nonstandard neutrino interactions and other light relics in the early Universe affect the propagation of primordial gravitational waves, with implications for cosmic microwave background polarization and high-frequency observations.

Contribution

It introduces semianalytic models for gravitational wave damping due to free-streaming particles with nonstandard interactions, extending understanding beyond standard neutrino physics.

Findings

Neutrino interactions modify gravitational wave damping signatures.

Imprints on CMB B-mode polarization can reveal nonstandard neutrino physics.

Potential detection of effects at higher frequencies discussed.

Abstract

After their generation, cosmological backgrounds of gravitational waves propagate nearly freely but for the expansion of the Universe and the anisotropic stress of free-streaming particles. Primordial signals -- both that from inflation and the infrared spectrum associated to subhorizon production mechanisms -- would carry clean information about the cosmological history of these effects. We study the modulation of the standard damping of gravitational waves by free-streaming radiation due to the decoupling (or recoupling) of interactions. We focus on nonstandard neutrino interactions in effect after the decoupling of weak interactions as well as more general scenarios in the early Universe involving other light relics. We develop semianalytic results in fully free-streaming scenarios to provide intuition for numerical results that incorporate interaction rates with a variety of temerpature dependencies. Finally, we compute the imprint of neutrino interactions on the $B$-mode polarization of the cosmic microwave background, and we comment on other means to infer the presence of such effects at higher frequencies.