Improved cosmological parameter constraints from CMB and H(z) data

We discuss the cosmological degeneracies among the Hubble parameter H(z), the age of the Universe and cosmological parameters describing simple variations from the minimal ΛCDM (CDM: cold dark matter) model. We show that independent determinations of the Hubble parameter H(z) such as those recently obtained from ages of passively evolving galaxies, combined with cosmic microwave background data (Wilkinson Microwave Anisotropy Probe five-year data), provide stringent constraints on possible deviations from the ΛCDM model. In particular we find that this data combination constrains at the 68% (95%) confidence limit the following parameters: the sum of the neutrino masses  eV, the number of relativistic neutrino species N_rel = 4.1_−0.9^+0.4(_−1.5^+1.1), the dark energy equation of state parameter w = −0.95 ± 0.17(± 0.32), and the curvature Ω_k = 0.002 ± 0.006(± 0.014), in excellent agreement with data set combinations involving the cosmic microwave background, supernovae and baryon acoustic oscillations. This offers a valuable consistency check for systematic errors.