Planck constraints on neutrino isocurvature density perturbations

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 $N_{\text{eff}}$. In the curvaton scenario, a larger value for $N_{\text{eff}}$ 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 ${\alpha }^{\mathrm{NID}}$. Here we present new constraints on $N_{\text{eff}}$ and ${\alpha }^{\mathrm{NID}}$ from an analysis of recent cosmological data. We find that the $\text{Planck}+\mathrm{WMAP}$ polarization data set does not show any indication of a NID component (severely constraining its amplitude), and that current indications for a nonstandard $N_{\text{eff}}$ are further relaxed.

Figure 1

68% and 95% C.L. likelihood contours for $\text{Planck}+\mathrm{WP}$ and $\text{Planck}+\mathrm{WP}+\mathrm{HST}$ in the $N_{\text{eff}}$ vs ${\alpha }^{\mathrm{NID}}$ plane. Note the small correlation between the two parameters.

Figure 2

68% and 95% C.L. likelihood contours for $\text{Planck}+\mathrm{WP}$ and $\text{Planck}+\mathrm{WP}+\mathrm{HST}$.