Constraints on dark radiation from cosmological probes

TL;DR

This study uses cosmological data, including the Lyman-Alpha forest and CMB measurements, to tightly constrain the effective number of neutrino species and their total mass, supporting the standard neutrino background and ruling out additional relativistic relics.

Contribution

It provides the most stringent constraints to date on N_eff and neutrino masses using a combined analysis of Lyman-Alpha forest and other cosmological probes.

Findings

N_eff is approximately 3, consistent with the Cosmic Neutrino Background.

Sterile neutrinos with N_eff=4 are strongly disfavored.

No evidence for additional relativistic relics beyond standard neutrinos.

Abstract

We present joint constraints on the number of effective neutrino species N_eff and the sum of neutrino masses M_nu, based on a technique which exploits the full information contained in the one-dimensional Lyman-Alpha forest flux power spectrum, complemented by additional cosmological probes. In particular, we obtain N_eff=2.91(+0.21)(-0.22) (95% CL) and M_nu<0.15 eV (95% CL) when we combine BOSS Lyman-Alpha forest data with CMB (Planck+ACT+SPT+WMAP polarization) measurements, and N_eff=2.88(+0.20)(-0.20) (95% CL) and M_nu<0.14 eV (95% CL) when we further add baryon acoustic oscillations. Our results provide evidence for the Cosmic Neutrino Background from N_eff~3 (N_eff=0 is rejected at more than 14 sigma), and rule out the possibility of a sterile neutrino thermalized with active neutrinos (i.e., N_eff=4) - or more generally any decoupled relativistic relic with Delta N_eff ~ 1 - at a significance of over 5 sigma, the strongest bound to date, implying that there is no need for exotic neutrino physics in the concordance LCDM model.