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 \pm 62  km sec ^ { -1 }  Mpc ^ { -1 } at z \simeq 0.5 and H ( z ) = 90 \pm 40  km sec ^ { -1 }  Mpc ^ { -1 } at z \simeq 0.8 .
We discuss the uncertainty in the expansion history determination that arises from uncertainties in the synthetic stellar-population models .
We then use these new measurements in concert with cosmic-microwave-background ( CMB ) measurements to constrain cosmological parameters , with a special emphasis on dark-energy parameters and constraints to the curvature .
In particular , we demonstrate the usefulness of direct H ( z ) measurements by constraining the dark-energy equation of state parameterized by w _ { 0 } and w _ { a } and allowing for arbitrary curvature .
Further , we also constrain , using only CMB and H ( z ) data , the number of relativistic degrees of freedom to be 4 \pm 0.5 and their total mass to be < 0.2 Â eV , both at 1 \sigma .