Neutrino Anisotropies after Planck

TL;DR

This paper uses Planck data to constrain neutrino properties, finding hints of non-standard neutrino behavior that may explain anomalies in lensing amplitude and connecting neutrino physics with cosmological observations.

Contribution

It provides new constraints on neutrino anisotropic stress and sound speed using Planck data, exploring their impact on cosmological parameters and anomalies.

Findings

Hints of higher neutrino viscosity parameter c_{vis}^2 around 0.60

Lower effective sound speed c_{eff}^2 near 0.30

Potential link between neutrino properties and lensing amplitude A_L

Abstract

We present new constraints on the rest-frame sound speed, c_{eff}^2, and the viscosity parameter, c_{vis}^2, of the Cosmic Neutrino Background from the recent measurements of the Cosmic Microwave Background anisotropies provided by the Planck satellite. While broadly consistent with the expectations of c_{eff}^2=c_{vis}^2=1/3 in the standard scenario, the Planck dataset hints at a higher value of the viscosity parameter, with c_{vis}^2=0.60+/-0.18 at 68% c.l., and a lower value of the sound speed, with c_{eff}^2=0.304+/-0.013 at 68% c.l.. We find a correlation between the neutrino parameters and the lensing amplitude of the temperature power spectrum A_L. When the latter parameter is allowed to vary, we find a better consistency with the standard model with c_{vis}^2=0.51+/-0.22, c_{eff}^2=0.311+/-0.019 and A_L=1.08+/-0.18 at 68% c.l.. This result indicates that the anomalous large value of A_L measured by Planck could be connected to non-standard neutrino properties. Including additional datasets from Baryon Acoustic Oscillation surveys and the Hubble Space Telescope constraint on the Hubble constant, we obtain c_{vis}^2=0.40+/-0.19, c_{eff}^2=0.319+/-0.019, and A_{L}=1.15+/-0.17 at 68% c.l.; including the lensing power spectrum, we obtain c_{vis}^2=0.50+/-0.19, c_{eff}^2=0.314+/-0.015, and A_L=1.025+/-0.076 at 68% c.l.. Finally, we investigate further degeneracies between the clustering parameters and other cosmological parameters.