We present joint constraints on the number of effective neutrino species N _ { eff } and the sum of neutrino masses \sum 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 \sum 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 \pm 0.20 ( 95 % CL ) and \sum m _ { \nu } < 0.14 eV ( 95 % CL ) when we further add baryon acoustic oscillations .
Our results provide strong evidence for the Cosmic Neutrino Background from N _ { eff } \sim 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 } \simeq 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 \Lambda CDM model .