Planck constraints on neutrino isocurvature density perturbations

TL;DR

This paper uses Planck and other cosmological data to constrain neutrino isocurvature density perturbations and the effective number of relativistic species, finding no evidence for isocurvature modes and tightening bounds on N_eff.

Contribution

It provides new observational constraints on neutrino isocurvature perturbations and the effective number of neutrino species using recent cosmological data.

Findings

No significant neutrino isocurvature component detected.

Constraints on N_eff are tightened, favoring standard values.

Non-standard N_eff indications are further relaxed.

Abstract

The recent Cosmic Microwave Background data from the Planck satellite experiment, when combined with HST determinations of the Hubble constant, are compatible with a larger, non-standard, number of relativistic degrees of freedom at recombination, parametrized by the neutrino effective number $N_{eff}$. In the curvaton scenario, a larger value for $N_{eff}$ could arise from a non-zero neutrino chemical potential connected to residual neutrino isocurvature density (NID) perturbations after the decay of the curvaton field, parametrized by the amplitude $\alpha^{NID}$. Here we present new constraints on $N_{eff}$ and $\alpha^{NID}$ from an analysis of recent cosmological data. We found that the Planck+WP dataset does not show any indication for a neutrino isocurvature component, severly constraining its amplitude, and that current indications for a non-standard $N_{eff}$ are further relaxed.