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Measuring deceleration using supernovae
•Plot of mv  vs. z   is really a plot of distance vs. velocity
•If faint (Þdistant Þearlier) 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
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•Surprise results from 1998 indeed do suggest accelerating expansion
•May be due to “cosmological constant” proposed by Einstein
–AKA “Dark energy” or “Quintessence”
From our text:  Horizons, by Seeds