Isocurvature perturbations in extra radiation

TL;DR

This paper investigates the possibility that the extra radiation component in the universe exhibits isocurvature fluctuations, providing a formalism within a mixed inflaton-curvaton framework and deriving observational constraints.

Contribution

It introduces a general formalism for evaluating isocurvature perturbations in extra radiation within a mixed inflaton-curvaton model and applies observational constraints to particle physics models.

Findings

Current data favors extra radiation existence but not its isocurvature nature.

Constraints on the abundance of extra radiation are derived.

Future detection of isocurvature perturbations could reveal the origin of extra radiation.

Abstract

Recent cosmological observations, including measurements of the CMB anisotropy and the primordial helium abundance, indicate the existence of an extra radiation component in the Universe beyond the standard three neutrino species. In this paper we explore the possibility that the extra radiation has isocurvatrue fluctuations. A general formalism to evaluate isocurvature perturbations in the extra radiation is provided in the mixed inflaton-curvaton system, where the extra radiation is produced by the decay of both scalar fields. We also derive constraints on the abundance of the extra radiation and the amount of its isocurvature perturbation. Current observational data favors the existence of an extra radiation component, but does not indicate its having isocurvature perturbation. These constraints are applied to some particle physics motivated models. If future observations detect isocurvature perturbations in the extra radiation, it will give us a hint to the origin of the extra radiation.