very weakly model dependent. Our constraint is slightly less stringent than that obtained by @SpergelWMAP03 from a joint likelihood analysis of [*WMAP*]{} with six external data sets ([*WMAP*]{} + CBI + ACBAR + 2dFGRS + Lyman$\alpha$ forest power spectrum + Type Ia supernovae + [*HST*]{} Key Project $H_0$ determination), but is tighter than the CMB-only ([*WMAP*]{}+CBI+ACBAR) determination and comparable to the [*WMAP*]{} + ACBAR + CBI + [*HST*]{} constraint [@SpergelWMAP03]. We speculate that the joint constraint from [*WMAP*]{} and all the external data sets considered in @SpergelWMAP03 could be further improved by adding stellar populations age determination as an additional external data set. We have restricted our analysis to values of $w > -1$ since theoretical models for $w < -1$ seem difficult to construct [@Carroll+03]. The use of external data sets (see e.g. @KLSW02 and references therein for other approaches to measure $w$) could help to remove this restriction and constrain $w$ for values $< -1$.

We have dated a large sample of galaxies and shown that a clear age–redshift envelope exists. By computing the relative ages of SDSS red galaxies at $z < 0.2$ we obtain an estimate of $H_0$ independent of and in good agreement with determinations 