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

TL;DR

The paper addresses whether cosmological data require non-zero neutrino masses by performing a joint analysis of CMB, large-scale structure, and X-ray cluster constraints within a flat cosmology that permits tensor modes. Using CosmoMC/CAMB to explore a nine-parameter space with three degenerate neutrinos, and applying X-ray cluster priors via importance sampling, the authors find a neutrino density of $\Omega_\nu h^2 = 0.0059^{+0.0033}_{-0.0027}$ (68% CL), implying $\sum_i m_i = 0.56^{+0.30}_{-0.26}$ eV, i.e., about $0.2$ eV per neutrino, with zero excluded at 99% CL. This neutrino contribution lowers the present-day amplitude of mass fluctuations to $\sigma_8 = 0.74^{+0.12}_{-0.07}$ and yields precise constraints on $H_0$, $\Omega_m$, and $\Omega_b h^2$ (e.g., $H_0 = 68.4^{+1.5}_{-1.6}$ km s$^{-1}$ Mpc$^{-1}$, $\Omega_m = 0.301 \pm 0.024$, $\Omega_b h^2 = 0.0236 \pm 0.0012$). The results remain broadly robust to subsets of data and to the inclusion of tensors, though they depend on XLF systematics related to the mass–luminosity relation. Overall, the study provides evidence for a non-zero cosmic neutrino mass and tight cosmological parameter constraints from a multi-probe analysis.

Abstract

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.

Figures (6)

• Figure 1: The enclosed baryon fraction relative to the universal value as a function of radius, in units of the virial radius $r_{\rm vir}$, from the simulations of Eke et al. (1998). The simulated clusters have similar masses and temperatures to the clusters used here. The results for the most dynamically active system in the simulations have been excluded. Beyond a radius $r > 0.2 r_{\rm vir}$, the simulated clusters exhibit consistent, relatively flat $b$ profiles. At $r=0.25 r_{\rm vir}$, a radius comparable to the measurement radius for the Chandra observations, the simulations give $b=0.824\pm0.033$.
• Figure 2: Marginalized probability distributions for the cosmological parameters from the analysis of the CMB+2dF (dotted) CMB+2dF+$f_{\rm gas}$ (dark solid) and CMB+2dF+$f_{\rm gas}$+XLF (grey solid) data using the nine-parameter model, allowing for the presence of tensor components and massive neutrinos.
• Figure 3: Joint 68.3 and 95.4 per cent confidence limits on $\sigma_8$ and $\Omega_{\rm m}$ from the analysis of the CMB+2dF+$f_{\rm gas}$ data, allowing (solid curve) and excluding (dashed curve) a contribution to the energy density of the universe from massive neutrinos. The dotted curve shows the 95.4 per cent confidence limit from the analysis of the local cluster XLF from Allen et al. 2003
• Figure 4: (a) Marginalized probability distribution for $\Omega_{\nu}h^2$ from the analysis of the CMB+2dF+$f_{\rm gas}$ (dotted curve), CMB+$f_{\rm gas}$+XLF data (grey curve) and CMB+2dF+$f_{\rm gas}$+XLF (dark, solid curve) data. In all cases we have allowed for the presence of tensor components (b) The dashed curve shows the results from the CMB+2dF+$f_{\rm gas}$+XLF data from the analysis without tensor components. The dark, solid curve is the same as in (a).
• Figure 5: The joint 68.3 and 95.4 per cent confidence limits on $H_0$ and $\Omega_{\rm m}$ from the analysis of the CMB+2dF (dotted curve) and CMB+2dF+$f_{\rm gas}$ (solid curve) data. We have allowed for the presence of tensor components and massive neutrinos in the analysis.