Self-interacting Dark Radiation

TL;DR

This paper explores models where self-interacting dark radiation, such as hidden gauge bosons or chiral fermions, affects the CMB spectrum and could explain excess neutrino species without free streaming.

Contribution

It introduces a class of models with self-interacting dark radiation that impacts CMB anisotropies and accounts for extra neutrino-like species.

Findings

Dark radiation with self-interactions alters CMB anisotropies.

Models can produce ff 0.29 consistent with observations.

Self-interacting dark radiation leaves a distinct signature on the CMB spectrum.

Abstract

We consider a simple class of models where dark radiation has self-interactions and therefore does not free stream. Such dark radiation has no anisotropic stress (or viscosity), leaving a distinct signature on the CMB angular power spectrum. Specifically we study a possibility that hidden gauge bosons and/or chiral fermions account for the excess of the effective number of neutrino species. They have gauge interactions and remain light due to the unbroken hidden gauge symmetry, leading to \Delta N_{\rm eff} \simeq 0.29 in some case.