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”