Raising the bar: new constraints on the Hubble parameter with cosmic chronometers at z ∼ 2

One of the most compelling tasks of modern cosmology is to constrain the expansion history of the Universe, since this measurement can give insights on the nature of dark energy and help to estimate cosmological parameters. In this letter are presented two new measurements of the Hubble parameter H(z) obtained with the cosmic chronometer method up to $z\sim2$. Taking advantage of near-infrared spectroscopy of the few very massive and passive galaxies observed at $z>1.4$ available in literature, the differential evolution of this population is estimated and calibrated with different stellar population synthesis models to constrain H(z), including in the final error budget all possible sources of systematic uncertainties (star formation history, stellar metallicity, model dependencies). This analysis is able to extend significantly the redshift range coverage with respect to present-day constraints, crossing for the first time the limit at $z\sim1.75$. The new H(z) data are used to estimate the gain in accuracy on cosmological parameters with respect to previous measurements in two cosmological models, finding a small but detectable improvement ($\sim$5 %) in particular on $\Omega_{M}$ and $w_{0}$. Finally, a simulation of a Euclid-like survey has been performed to forecast the expected improvement with future data. The provided constraints have been obtained just with the cosmic chronometers approach, without any additional data, and the results show the high potentiality of this method to constrain the expansion history of the Universe at these redshifts.