Simultaneous constraints on the number and mass of relativistic species

TL;DR

This paper combines multiple cosmological observations to simultaneously constrain the effective number of relativistic species and neutrino mass, providing the strongest current bounds and indicating a mild preference for extra neutrino species.

Contribution

It presents the first combined analysis of neutrino number and mass constraints using diverse cosmological data, accounting for parameter correlations.

Findings

N_eff=3.58(+0.15/-0.16) at 68% CL

Neutrino mass sum < 0.60 eV at 95% CL

Preference for N_eff > 3 at 2sigma level

Abstract

Recent indications from both particle physics and cosmology suggest the existence of more than three neutrino species. In cosmological analyses the effects of neutrino mass and number of species can in principle be disentangled for fixed cosmological parameters. However, since we do not have perfect measurements of the standard Lambda Cold Dark Matter model parameters some correlation remains between the neutrino mass and number of species, and both parameters should be included in the analysis. Combining the newest observations of several cosmological probes (cosmic microwave background, large scale structure, expansion rate) we obtain N_eff=3.58(+0.15/-0.16 at 68% CL) (+0.55/-0.53 at 95% CL) and a sum of neutrino masses of less than 0.60 eV (95 CL), which are currently the strongest constraints on N_eff and M_nu from an analysis including both parameters. The preference for N_eff >3 is now at a 2sigma level.