Cosmic Chronometers: Constraining the Equation of State of Dark Energy. I: H(z) Measurements

TL;DR

This paper presents new measurements of the universe's expansion rate using galaxy spectra, and combines these with CMB data to better constrain dark energy properties and cosmological parameters.

Contribution

It provides two new H(z) measurements at redshifts 0.5 and 0.8, improving previous expansion history constraints and demonstrating their utility in dark energy studies.

Findings

New H(z) measurements at z=0.5 and z=0.8 with uncertainties

Constraints on dark energy parameters w0 and wa using combined data

Limits on relativistic degrees of freedom and total neutrino mass

Abstract

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/Mpc at z = 0.5 and H(z) = 90 +- 40 km/sec/Mpc at z = 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 w0 and wa 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 +- 0.5 and their total mass to be < 0.2 eV, both at 1-sigma.