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Published February 2010 | Submitted
Journal Article Open

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 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±0.5 and their total mass to be < 0.2 eV, both at 1σ.

Additional Information

© 2010 IOP Publishing Ltd and SISSA. Received July 19, 2009; Revised January 7, 2010; Accepted January 15, 2010; Published February 5, 2010. The work of DS was carried out at the Jet Propulsion Laboratory, operated by the California Institute of Technology under a contract with NASA. The work of RJ and LV is supported by funds from the Spanish Ministry for Science and Innovation AYA 2008-03531 and the European Union (FP7 PEOPLE-2002IRG4-4-IRG#202182). MK was supported by DoE DEFG03-92-ER40701 and the Gordon and Betty Moore Foundation. We thank the members of the SPICES and VVDS teams for making their spectroscopic data publicly available, and we acknowledge the use of the Legacy Archive for Microwave Background Data Analysis (LAMBDA). Support for LAMBDA is provided by the NASA Office of Space Science. Finally, the authors wish to recognize and acknowledge the very significant role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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