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Deriving the expansion history of the Universe is a major goal of moderncosmology. To date, the most accurate measurements have been obtained with TypeIa Supernovae and Baryon Acoustic Oscillations, providing evidence for theexistence of a transition epoch at which the expansion rate changes fromdecelerated to accelerated. However, these results have been obtained withinthe framework of specific cosmological models that must be implicitly orexplicitly assumed in the measurement. It is therefore crucial to obtainmeasurements of the accelerated expansion of the Universe independently ofassumptions on cosmological models. Here we exploit the unprecedentedstatistics provided by the Baryon Oscillation Spectroscopic Survey (BOSS) DataRelease 9 to provide new constraints on the Hubble parameter $H(z)$ using theem cosmic chronometers approach. We extract a sample of more than 130000 of themost massive and passively evolving galaxies, obtaining five newcosmology-independent $H(z)$ measurements in the redshift range $0.3<z<0.5$,with an accuracy of $\sim$11-16\% incorporating both statistical and systematicerrors. Once combined, these measurements yield a 6\% accuracy constraint of$H(z=0.4293)=91.8\pm5.3$ km/s/Mpc. The new data are crucial to provide thefirst cosmology-independent determination of the transition redshift at highstatistical significance, measuring $z_{t}=0.4\pm0.1$, and to significantlydisfavor the null hypothesis of no transition between decelerated andaccelerated expansion at 99.9\% confidence level. This analysis highlights thewide potential of the cosmic chronometers approach: it permits to deriveconstraints on the expansion history of the Universe with results competitivewith standard probes, and most importantly, being the estimates independent ofthe cosmological model, it can constrain cosmologies beyond -and including- the$\Lambda$CDM model.

A 6% measurement of the Hubble parameter at $z\sim0.45$: direct evidence of the epoch of cosmic re-acceleration