Constraints on dark radiation from cosmological probes.

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