Constraining the expansion rate of the Universe using low-redshift ellipticals as cosmic chronometers

We present a new methodology to determine the expansion history of the Universe analyzing the spectral properties of early-type galaxies (ETG), based on the study of the redshift dependence of the 4000 angstrom break. In this paper we describe the method, explore its robustness using theoretical synthetic stellar population models, and apply it using a SDSS sample of similar to 14 000 ETGs. Our motivation to look for a new technique has been to minimize the dependence of the cosmic chronometer method on systematic errors. In particular, as a test of our method, we derive the value of the Hubble constant H-0 = 72.6 +/- 2.9(stat) +/- 2.3(syst) km Mpc(-1)s(-1) (68% confidence), which is not only fully compatible with the value derived from the Hubble key project, but also with a comparable error budget. Using the SDSS, we also derive, assuming w = constant, a value for the dark energy equation of state parameter w = -1 +/- 0.2(stat) +/- 0.3(syst). Given the fact that the SDSS ETG sample only reaches z similar to 0.3, this result shows the potential of the method. In future papers we will present results using the high-redshift universe, to yield a determination of H(z) up to z similar to 1.