2002ApJ...573...37J

We propose to use relative galaxy ages as a means of constraining cosmological parameters. By measuring the age difference between two ensembles of passively evolving galaxies at somewhat different redshifts, one could determine the derivative of redshift with respect to cosmic time, dz/dt. At high redshifts, z∼1-2, this measurement would constrain the equation of state of the dark energy, while at low redshifts, z≲0.2, it would determine the Hubble constant, H₀. Furthermore, we show that d²z/dt² directly tracks the equation of state of the dark energy; thus, measurements of this quantity at low redshifts can be used to constrain it. The selected galaxies need to be passively evolving on a time much longer than their age difference.