The End

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Aside from that detail, the Einstein connection made the idea of dark energy, or antigravity, seem somewhat less nutty when Schmidt and Perlmutter weighed in. Of course, some astrophysicists had lingering doubts. Maybe the observers didn't really have the supernovas' brightness right; perhaps the light from faraway stellar explosions was dimmed by some sort of dust. The unique properties of a cosmological constant, moreover, would make the universe slow down early on, then accelerate. That's because dark energy grows as a function of space. There wasn't much space in the young, small universe, so back then the braking force of gravity would have reigned supreme. More recently, the force of gravity fell off as the distance between galaxies grew and that same increase made for more dark energy. Nobody had probed deeply enough to find out what was really going on in the distant past.

Or rather, nobody had got enough data. Back in 1997, astronomers Mark Phillips of the Space Telescope Science Institute and Ron Gilliland of the Carnegie Institute of Washington had used the Hubble Space Telescope to spot a distant supernova designated SN 1997ff and, with the help of Peter Nugent, a Lawrence Berkeley astronomer on Perlmutter's team, had determined its speed of recession from Earth. Nugent couldn't figure out the distance, though: determining the brightness of a Type Ia calls for not just one but several measurements, spread over time.

On the rival team, Riess knew of the discovery, but he learned soon afterward that other Hubble photos had also caught the supernova, completely by chance. So one day last summer, he recalls, "I called Peter and began fishing around for information. I guess I wasn't especially cagey. He said almost right away, 'Are you asking about 1997ff?'"

Rather than try to scoop each other, the friendly rivals decided to cooperate--and soon realized they had stumbled onto something truly astonishing. The new supernova, some 50% closer to the beginning of the universe than any supernova known before, was far brighter than had been predicted. That neatly eliminated the idea of dust, since a more distant star should have been even more dust-dimmed than nearer ones. But the level of brightness also signaled that this supernova was shining when the expansion of the cosmos was still slowing down. "Usually," says Riess, "we see weird things and try to make our models of the universe fit. This time we put up a hoop for the observations to jump through in advance, and they did--which makes it a lot more convincing."

PROBING THE COSMIC FIREBALL

What makes it still more convincing is that an entirely different kind of observation--the long-standing search for lumpiness in the cosmic background radiation--now suggests independently that dark energy is real. The lumps themselves were first detected about a decade ago, thanks to the Cosmic Background Explorer satellite. At the time, astrophysicist and COBE spokesman George Smoot declared that "if you're religious, it's like seeing God."