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KnightRidder News Service

It sounds more like "Star Wars" than real science, but astronomers are taking seriously new evidence that a mysterious "dark energy" pervades empty space. You can't see it or feel it, but this strange force is so powerful that it counteracts gravity and makes stars and galaxies fly apart faster than scientists previously thought. Without it, the universe would have caved in ages ago. "There is now tantalizing evidence for an extra repulsion force that overwhelms gravity on cosmic scales," Martin Rees, Britain's astronomer royal, told a symposium at the Library of Congress last week.

Understanding this force will be "one of the grand challenges for the millennium to come," said Neta Bahcall, a leading cosmologist at Princeton University. In scientific papers and talks, the dark force goes by a variety of exotic names: "cosmic dark energy ... negative gravity ... vacuum energy ... zero-point energy ... X-matter." To describe it, cosmologists have even revived the ancient term "quintessence" - the name medieval scholars gave to an invisible substance in which heavenly bodies supposedly floated.

Some scientists equate the force with the "cosmological constant" - a notion proposed more than 80 years ago by Albert Einstein to explain why gravity doesn't cause the universe to collapse of its own weight. The "constant" was a number Einstein inserted in his equations to make them come out even - a gimmick your high school math teacher probably would call cheating. The great physicist later repudiated his own idea, calling it his "biggest blunder," but it has popped up again in respectable scientific circles. A repulsive force is now considered the best way to explain why the universe appears to be epanding ever faster.

In the 70 years since astronomer Edwin Hubble discovered the universe is getting bigger, scientists have debated whether it will continue to grow or slow down, halt and reverse course under the pull of gravity. Early this year, two international teams of astronomers announced that recent observations of supernova - massive exploding stars at extremely great distances - provided convincing evidence that the expansion is not only continuing but also gaining speed. These observations "suggest that the expansion of the universe is accelerating, indicating the existence of a cosmological constant or dark energy," Princeton's Bahcall wrote in the May 28 issue of Science magazine.

The supernova evidence is forcing a major shake-up in astrophysics, the science of space-time, Bahcall said. "The most popular explanation is that space itself could have additional properties, a kind of springiness, an energy, a negative pressure, that tends to make space expand all by itself," Robert Kirshner, associate director of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., said at a NASA science briefing. Though this exotic form of energy is difficult to measure, astronomers believe it dominates the universe.

The best available data indicate that matter - atoms, chairs, trees, people, planets and stars - makes up barely one-third of the universe, according to Michael Turner, chairman of the department of astronomy and astrophysics at the University of Chicago. The remaining two thirds consists of gravity-defying negative energy, such as Einstein's cosmological constant. "We need the cosmological constant to balance the books, since matter makes up only 35 percent of the density of the universe," Turner said.

The balancing act was described by Sean Carroll, a theoretical physicist at the University of California, San Diego, as a tug of war. "In a universe with both matter and vacuum energy, there is a competition between the tendency of [vacuum energy] to cause acceleration and the tendency of matter to cause deceleration," Carroll said. "The ultimate fate of the universe depends on the precise amounts of each component." As they try to figure out the nature of the dark force, astronomers who usually deal with outer space are drawing on the latest theories of inner space - the weird world of subatomic physics.

Modern physics teaches that a vacuum, such as space, is not really empty, but rather is filled with infinitesimally small particles that constantly flicker in and out of existence. "Space is a simmering sea of particles living on borrowed time and borrowed energy," Turner said. "Otherwise empty space is seething with these ghostly entities," astrophysicists Fred Adams of the University of Michigan and Greg Laughlin of the University of California, Berkeley, declared in their new book, The Five Ages of the Universe. "The energy required to make these particles is borrowed from the vacuum and then quickly repaid when the particles annihilate each other and subsequently disappear back into nothingness," Adams and Laughlin explained.

It is these particles, they believe, that create the "negative pressure" that drives the universe to expand at an accelerating rate. Engineers at NASA's Marshall Space Flight Center in Huntsville, Ala., and at Lockheed Martin, the giant aerospace company headquartered in Bethesda, Md., are tinkering with possible ways to extract useful sources of power from the vacuum - ultimately perhaps driving spaceships to the stars. Although dreams of interstellar voyages fueled by anti-gravity are probably at least a century away from being fulfilled, the idea of some such dark force, once considered outlandish, is now in the scientific mainstream. "We have to take it seriously," said Harvard's Kirshner.