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Cosmos at full throttle

A baffling force called dark energy is causing the universe to expand at a faster rate than previously thought.

By Peter N. SpottsStaff writer of The Christian Science Monitor / May 27, 2004



Will the universe eventually collapse in the "big crunch," expand forever in the "big loneliness," or be torn to bits in the "big rip"?

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The key to answering those questions appears to lie in a mysterious form of energy that has been cracking the cosmic whip on the universe for the past 6 billion to 7 billion years. This "dark energy" appears to be causing the universe to grow at an accelerated rate rather than a rate scientists previously thought would slow forever.

Six years after astronomers first stunned the scientific world with this discovery, researchers say dark energy still baffles them. Yet several studies reported over the past year have strengthened the evidence for dark energy's role as cosmic gas pedal, researchers say.

The results "point to the promise of improving our understanding of dark energy," says Michael Turner, head of the astronomy and astrophysics department at the University of Chicago. That understanding, he says, is critical to answering fundamental questions about the origin and future of the universe and the nature of matter and space-time.

Last week, researchers from Britain, Germany, and the United States announced results from studies of hot gas surrounding vast clusters of galaxies. The effort, using NASA's Chandra X-ray Observatory, was designed to determine the amount of dark energy the universe holds, compared with other forms of matter and energy. In addition, the team took a tentative stab at trying to see if the amount of dark energy changes with time - key to determining its nature.

Anyone reading their results might be excused for feeling a bit special. The team found that 4 percent of the universe is made of ordinary matter. Another 21 percent consists of so-called dark matter, inferred from its gravitational effects on matter. The remaining 75 percent consists of dark energy, which exerts a form of pressure that makes it act like gravity thrown into reverse. These figures are consistent with results reported last year from satellite measurements of the big bang's afterglow - the cosmic microwave background.

If the quantity of dark energy is constant, astronomers say, the universe will continue to expand at an increasing rate. In about 20 billion years or so, only about 100 galaxies might be visible from Earth. Think of it as the "big lonely." If dark energy were to change with time, it could relax to let gravity once again dominate, prompting the universe to collapse in the "big crunch." Or if the pace speeds up, it could lead to the "big rip," in which the fabric of space-time stretches so rapidly that even atoms get torn apart.

Based on the team's observations, dark energy is holding steady and "behaves much like the cosmological constant in Einstein's theories" about the evolution of the universe, says Steve Allen, an astronomer at Cambridge University in England and the team leader.

Essentially, this means that the amount of energy per volume of space remains constant. If this observation holds up under more rigorous programs, it would substantially narrow the range of explanations for what dark energy really is.

That's not bad for a "constant" that Albert Einstein dubbed his greatest blunder. In 1917, when he pondered the implications of his general relativity theory for the universe, most astronomers believed that the size of the universe didn't change. Other galaxies appeared on astronomers' photographic plates, but many thought the fuzzy images were nebulae or clusters of stars in the Milky Way.

When Einstein applied his equations to the observed universe, his numbers led him to an unsettling conclusion. Given the way his equations showed gravity distorting the shape of space-time, and given the amount of matter and energy in the universe to exert gravity, the universe could not remain static. It would have to collapse through gravitational attraction.

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