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). We
found that for these galaxies the 4000\AA break is a spectral feature that
correlates with the relative ages of ETGs. In this paper we describe the
method, explore its robustness using theoretical synthetic stellar population
models, and apply it using a SDSS sample of $\sim$14 000 ETGs. Our motivation
to look for a new technique has been to minimise 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 $H_0 = 72.6 \pm 2.8$ (stat)
$\pm2.3$ (syst) (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 $w = -1 \pm 0.2$ (stat) $\pm0.3$
(syst). Given the fact that the SDSS ETG sample only reaches $z \sim 0.3$, 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 $z \sim 1$.
