Non-Gaussian isocurvature perturbations in dark radiation

TL;DR

This paper investigates the non-Gaussian features of isocurvature perturbations in dark radiation, deriving bispectra expressions, analyzing CMB signals, and exploring particle physics models that could produce detectable non-Gaussianities.

Contribution

It provides the first detailed derivation of bispectra for mixed curvature and dark radiation isocurvature perturbations and forecasts their detectability in CMB observations.

Findings

Derived bispectra expressions for mixed perturbations.

Forecasted CMB constraints on nonlinearity parameters.

Presented models generating large isocurvature non-Gaussianities.

Abstract

We study non-Gaussian properties of the isocurvature perturbations in the dark radiation, which consists of the active neutrinos and extra light species, if exist. We first derive expressions for the bispectra of primordial perturbations which are mixtures of curvature and dark radiation isocurvature perturbations. We also discuss CMB bispectra produced in our model and forecast CMB constraints on the nonlinearity parameters based on the Fisher matrix analysis. Some concrete particle physics motivated models are presented in which large isocurvature perturbations in extra light species and/or the neutrino density isocurvature perturbations as well as their non-Gaussianities may be generated. Thus detections of non-Gaussianity in the dark radiation isocurvature perturbation will give us an opportunity to identify the origin of extra light species and lepton asymmetry.