Constraining the time evolution of dark energy, curvature and neutrino properties with cosmic chronometers

We use the latest compilation of observational Hubble parameter measurements estimated with the differential evolution of cosmic chronometers, in the redshift range 0<z<2, to place constraints on cosmological parameters. We used a Markov-Chain Monte-Carlo approach to sample the parameter space for the cosmic chronometers dataset alone and in combination with other state-of-the art cosmological measurements: CMB data from the latest Planck 2015 release, the most recent estimate of the Hubble constant H0, a compilation of recent baryon acoustic oscillation data, and the latest type Ia cosmological supernovae sample. From late-Universe probes alone (z<2) we find that w0 = -0.9 ± 0.18 and wa = -0.5 ± 1.7, and when combining also Planck 2015 data we obtain w0=-0.98± 0.11 and wa=-0.30±0.4. These new constraints imply that nearly all quintessence models are disfavoured by the data; only phantom models or a pure cosmological constant are favoured. This is a remarkable finding as it imposes severe constraints on the nature of dark energy. For the curvature our constraints are Ωk = 0.003 ± 0.003, considering also CMB data. We also find that H(z) data from cosmic chronometers are important to constrain parameters that do no affect directly the expansion history, by breaking or reducing degeneracies with other parameters. We find that Neff = 3.17 ± 0.15, thus excluding the possibility of an extra (sterile) neutrino at more than 5 σ, and put competitive limits on the sum of neutrino masses, Σ mν< 0.27 eV at 95% confidence level. Finally, we constrain the redshift evolution of dark energy by exploring separately the early and late-Universe, and find a dark energy equation of state evolution w(z) consistent with that in the ΛCDM model at the ± 0.4 level over the entire redshift range 0 < z < 2.