2003ApJ...593..622J

We place constraints on the redshift-averaged, effective value of the equation of state of dark energy, w, using only the absolute ages of Galactic stars and the observed position of the first peak in the angular power spectrum of the cosmic microwave background (CMB). We find w←0.8 at the 68% confidence level. If we further consider that w≥-1, this finding suggests that within our uncertainties, dark energy is indistinguishable from a classical vacuum energy term. We detect a correlation between the ages of the oldest galaxies and their redshift. This opens up the possibility of measuring w(z) by computing the relative ages of the oldest galaxies in the universe as a function of redshift, dz/dt. We show that this is a realistic possibility by computing dz/dt at z∼0 from Sloan Digital Sky Survey (SDSS) galaxies and obtain an independent estimate for the Hubble constant, H₀=69±12 km.s^–1.Mpc^–1. The small number of galaxies considered at z>0.2 does not yield, currently, a precise determination of w(z), but shows that the age-redshift relation is consistent with a standard ΛCDM universe with w=-1.