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The new supernova's remoteness was even more important for another reason. "If dark energy is really the explanation for what we see," says Riess, a member of the rival team, "then its effect should have been weaker in the early universe." That's because while the force of gravity between galaxies falls as they move farther apart, dark energy is a property of space and gets stronger as the universe expands. Shortly after the Big Bang, when the universe took up relatively little space, there wasn't much dark energy. Now much bigger, the modern universe has more space and thus more energy to shove galaxies apart. Sure enough, this distant supernova shows that the expansion was slower long ago.
While the new observations go a long way toward confirming that dark energy is real, astronomers would love to see a few more distant supernovas, just to be sure. Unfortunately, that won't be happening soon. The Hubble pictures that Riess and Nugent analyzed were all taken purely by chance, while the telescope was looking for other things. Aiming at distant galaxies in hopes a supernova will go off is an inefficient use of the telescope's valuable time. The best bet would be a satellite devoted to such a project--and indeed, Perlmutter and others are working on that idea, although it will take years to get off the ground.
If space really does seethe with dark energy, the fate of the universe, a matter of longstanding debate, will be clear. With more dark energy today than yesterday, and more of the stuff tomorrow than today, the cosmos should fly apart faster and faster as time goes by. There will be no Big Crunch, as some have predicted, with billions of galaxies falling in on one another in a fiery apocalypse. Tens of billions of years from now, our Milky Way galaxy will find itself alone in empty space, with its nearest neighbors too far away to see. In the end, the stars will simply wink out--and the universe will end not with a bang but with the meekest of whimpers.