is less than the number of galaxies in the SDSS LRG sample). The solid line shows the theoretical position of the edge, which is remarkably close to observed edge. Note also that increasing the number of galaxies has improved the agreement between the observed and theoretical edge. Thus we conclude that small, recent bursts of star formation do not introduce a significant bias on the edge position; our primary uncertainty in determining $w(z)$ to high redshift is the sparse size of the sample of distant, early-types considered here. We can now attempt to recover $dz/dt$ from this simulation by looking at the shift in the edge position as a function of $z$ [see § 3.4 and @JL02] and thus deduce $w(z)$. We recover $w(z)=-1 \pm 0.2$ for the whole redshift range. This shows that, with a sample similar to the LRG sample extended to higher redshift, we should be able to measure $w(z)$ within a few 10%. This is similar to what was already concluded in @JL02.

Having established that the age–redshift relation for the old envelope is not significantly affected by the bias caused by later episodes of star formation, we now consider the observational limits that we can 