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 Å 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 ∼ 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 ∼ 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 ∼ 1.