| Summary: | 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 H0, 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 Ωk and ΩDE 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 Nrel = 3.45±0.33 using WMAP 7-years data in combination with South Pole Telescope data and our H(z) determinations (Nrel = 3.71±0.45 using Atacama Cosmology Telescope data instead of South Pole Telescope). We exclude Nrel > 4 at 95% CL (74% CL) using the same datasets combinations. We also put competitive limits on the sum of neutrino masses, Σmν < 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.
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