New constraints on cosmological parameters and neutrino properties using the expansion rate of the Universe toz∼ 1.75

We have assembled a compilation of observational Hubble parameter measurements estimated with the differential evolution of cosmic chronometers, in the redshift range 0 < z < 1.75. This sample has been used, in combination with CMB data and with the most recent estimate of the Hubble constant H 0 , to derive new constraints on several cosmological parameters. The new Hubble parameter data are very useful to break some of the parameter degeneracies present in CMB-only analysis, and to constrain possible deviations from the standard (minimal) flat ΛCDM model. The H ( z ) data are especially valuable in constraining Ω k and Ω DE in models that allow a variation of those parameters, yielding constraints that are competitive with those obtained using Supernovae and/or baryon acoustic oscillations. We also find that our H ( z ) data are important to constrain parameters that do no affect directly the expansion history, by breaking or reducing degeneracies with other parameters. We find that N rel = 3.45±0.33 using WMAP 7-years data in combination with South Pole Telescope data and our H ( z ) determinations ( N rel = 3.71±0.45 using Atacama Cosmology Telescope data instead of South Pole Telescope). We exclude N rel > 4 at 95% CL (74% CL) using the same datasets combinations. We also put competitive limits on the sum of neutrino masses, Σ m ν < 0.24 eV at 68% confidence level. These results have been proven to be extremely robust to many possible systematic effects, such as the initial choice of stellar population synthesis model adopted to estimate H ( z ) and the progenitor-bias.