Constraints on dark radiation from cosmological probes

Abstract

We present joint constraints on the number of effective neutrino species N_eff and the sum of neutrino masses ∑m_ν, 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 N_eff=2.9 1_-0.22^+0.21 (95% C.L.) and ∑m_ν<0.15 eV (95% C.L.) when we combine BOSS Lyman-α forest data with CMB (Planck+ACT +SPT +WMAP polarization) measurements, and N_eff=2.88 ±0.20 (95% C.L.) and ∑m_ν<0.14 eV (95% C.L.) when we further add baryon acoustic oscillations. Our results provide strong evidence for the cosmic neutrino background from N_eff∼3 (N_eff=0 is rejected at more than 14 σ ), 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 Δ N_eff≃1 —at a significance of over 5 σ , the strongest bound to date, implying that there is no need for exotic neutrino physics in the concordance Λ CDM model.