Measuring deceleration using supernovae
Plot of mv  vs. z   is really a plot of distance vs. velocity
If faint (distant earlier) objects show slightly higher z
than expected from extrapolation based on nearby (present day) objects,
then expansion rate was faster in the past and has been decelerating
Surprise results from 1998 indeed do suggest accelerating expansion
May be due to “cosmological constant” proposed by Einstein
AKA “Dark energy” or “Quintessence”