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abstract: |
    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 CMB. We find $w<-0.8$ at the 68% confidence level. If we further consider that $w \geq -1$, 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 $z \sim 0$ from SDSS galaxies and obtain an independent estimate for the Hubble constant, $H_0 = 69 \pm
    12$ 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 $\Lambda$CDM universe with $w=-1$.
author:
- 'Raul Jimenez, Licia Verde, Tommaso Treu, and Daniel Stern'
title: |
    Constraints on the 