Planck constraints on neutrino isocurvature density perturbations

Abstract

The recent cosmic microwave background data from the Planck satellite experiment, when combined with Hubble Space Telescope determinations of the Hubble constant, are compatible with a larger, nonstandard 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 nonzero neutrino chemical potential connected to residual neutrino isocurvature density (NID) perturbations after the decay of the curvaton field, the component of which is parametrized by the amplitude α^NID . Here we present new constraints on N_eff and α^NID from an analysis of recent cosmological data. We find that the Planck+WMAP polarization data set does not show any indication of a NID component (severely constraining its amplitude), and that current indications for a nonstandard N_eff are further relaxed.