A dark force in the universe: scientists try to determine what's revving up the cosmos.

Three years ago, observations of distant, exploding stars blew to smithereens some of astronomers' most cherished ideas about the universe. To piece together an updated theory, they're now thinking dark thoughts about what sort of mystery force may be contorting the cosmos.

According to the standard view of cosmology, the once infinitesimal universe has ballooned in volume ever since its fiery birth in the Big Bang, but the mutual gravitational tug of all the matter in the cosmos has gradually slowed that expansion.

In 1998, however, scientists reported that a group of distant supernovas were dimmer, and therefore farther from Earth, than the standard theory indicated. It was as if, in the billion or so years it took for the light from these exploded stars to arrive at Earth, the space between the stars and our planet had stretched out more than expected. That would mean that cosmic expansion has somehow sped up, not slowed down. Recent evidence has only firmed up that bizarre result (SN: 3/31/01, p. 196).

In 1929, Edwin P. Hubble discovered that distant galaxies are fleeing from one another as if the entire universe is swelling in size. Ever since, astronomers have been hoping to answer a key question: Will the expansion of the universe, slowed by gravity, go on forever, or will the cosmos eventually collapse into a Big Crunch?

Despite decades of effort and countless studies devoted to the ballooning of the universe, the recent findings stunned astronomers. Few suspected that all along they were asking the wrong question.

"For 70 years, we've been trying to measure the rate at which the universe slows down. We finally do it, and we find out it's speeding up," says Michael S. Turner of the University of Chicago.

An accelerated expansion would seem to contradict all common sense, says Andreas J. Albrecht of the University of California, Davis. Throw a ball into the sky, and after it reaches a certain height, it will come back down, he notes. Now imagine throwing another ball upward and finding that instead of it falling back down, it somehow keeps moving up faster and faster. For that to happen, there would have to be some force pushing upward on the ball strongly…