A preference for a non-zero neutrino mass from cosmological data

We present results from the analysis of cosmic microwave background (CMB), large scale structure (galaxy redshift survey) and X-ray galaxy cluster (baryon fraction and X-ray luminosity function) data, assuming a geometrically flat cosmological model and allowing for tensor components and a non-negligible neutrino mass. From a combined analysis of all data, assuming three degenerate neutrinos species, we measure a contribution of neutrinos to the energy density of the universe, Omega_nu h^2=0.0059^{+0.0033}_{-0.0027} (68 per cent confidence limits), with zero falling on the 99 per cent confidence limit. This corresponds to ~4 per cent of the total mass density of the Universe and implies a species-summed neutrino mass \sum_i m_i =0.56^{+0.30}_{-0.26} eV, or m_nu~0.2 eV per neutrino. We examine possible sources of systematic uncertainty in the results. Combining the CMB, large scale structure and cluster baryon fraction data, we measure an amplitude of mass fluctuations on 8h^{-1} Mpc scales of sigma_8=0.74^{+0.12}_{-0.07}, which is consistent with measurements based on the X-ray luminosity function and other studies of the number density and evolution of galaxy clusters. This value is lower than that obtained when fixing a negligible neutrino mass (sigma_8=0.86^{+0.08}_{-0.07}). The combination of CMB, large scale structure and cluster baryon fraction data also leads to remarkably tight constraints on the Hubble constant, H_0=68.4^{+2.0}_{-1.4} km/s/Mpc, mean matter density, Omega_m =0.31\pm0.02 and physical baryon density, Omega_b h^2=0.024\pm0.001, of the Universe.