Cosmic chronometers: constraining the equation of state of dark energy. I: H(z) measurements

We present new determinations of the cosmic expansion history from red-envelope galaxies. We have obtained for this purpose high-quality spectra with the Keck-LRIS spectrograph of red-envelope galaxies in 24 galaxy clusters in the redshift range 0.2 < z < 1.0. We complement these Keck spectra with high-quality, publicly available archival spectra from the SPICES and VVDS surveys. We improve over our previous expansion history measurements in Simon et al. (2005) by providing two new determinations of the expansion history: H ( z ) = 97±62 km sec −1 Mpc −1 at z ≃ 0.5 and H ( z ) = 90±40 km sec −1 Mpc −1 at z ≃ 0.9. We discuss the uncertainty in the expansion history determination that arises from…