Why is it important that the supernovae being observed, which led to the idea of dark energy, are high z (remember z is another name for redshift) supernovae? high z supernovae are always paired with cepheid star systems, so we can calculate their distances. high z supernovae are brighter than regular supernovae. high z means that the supernovae are far enough away for us to detect changes in the expansion rate. high z supernovae have light redshifted enough to see in the visible spectrum with our telescopes.