f556c24b__04_22_2004____Background_Info__2004
[1990] Help astronomers determine the fate of the universe by precisely measuring the expansion rate and by looking for evidence of deceleration. Expansion rate values were uncertain by a factor of 2. [Future] By observing more supernovae, astronomers expect to improve the constraints on the nature of dark energy. [2004] 
Provided proof of the existence of dark energy, which is accelerating the universe's expansion. Hubble data also placed constraints on the nature of dark energy.
HubbleSite - NewsCenter - The Lure of the Rings (04/22/2004) - Background Info
2004