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Astronomers aim to shine light on universe's 'dark energy'

It's almost three-quarters of what the universe is made up of – but scientists are still trying to figure out what it is.

By Peter N. SpottsStaff writer of The Christian Science Monitor / September 13, 2007

In nearly a decade since it was discovered, a mysterious cosmic feature dubbed "dark energy" has lain like a downed redwood across the path of scientists trying to reach the holy grail of physics – a fundamental theory of matter and its basic forces.

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Unlike gravity, which binds galaxies together and gathers them into large clusters, dark energy would drive them apart. For several billion years, gravity acted as a brake, slowing a universe ballooning since it formed some 13.7 billion years ago in what cosmologists call the "big bang." These days, however, dark energy appears to be taking over and speeding that expansion.

So far, no one has devised a widely ac­­cepted reason why dark energy exists. Nor has anyone figured out why it acts as a repellent. Yet cosmologists now calculate that dark energy is 74 percent of the universe's inventory of matter and energy.

"It's almost unfair that the universe is teasing us in this way. It gives us this dramatic clue, then shuts up," says Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena. "We want to understand this dramatic fact much better. But in order to do that, we need to get more information about it."

Late last week, a National Research Council (NRC) panel became the latest group to answer that call. It recommended that the National Aeronautics and Space Administration and the US Department of Energy underwrite a mission to take dark energy's measure. After examining a range of possible NASA missions grouped under the heading "Beyond Einstein," the panel concluded that efforts to probe dark energy had the right mix of critical science questions, available technologies, and reasonable cost.

If NASA proceeds as the NRC recommends, it would join attempts by other researchers using ground-based telescopes to find the missing pieces that would help solve the dark-energy puzzle.

Dark energy was discovered in 1998 by two groups working independently. They found that the universe was expanding faster than it should be, given the density of matter and energy the universe was estimated to contain.

Researchers spent the first six months or so after the discovery trying to answer the question, "Is this right?" says Adam Riess, an astronomer at Johns Hopkins University in Baltimore and a member of one of two teams. Among other things, their observations led them to estimate that the vast majority of the universe's inventory of matter and energy was this befuddling dark energy.

Since then, Dr. Riess continues, re­­searchers have used various approaches to confirm the discovery and to pin down the point in the universe's history where, over very large distances, gravity began to yield to dark energy.

Four years ago, for instance, scientists using ground-based telescopes and a satellite to study the microwave "hiss" left over from the big bang gave the discovery a major boost. The satellite – the Wilkinson Microwave Anisotropy Probe – measured hot spots and cold spots in this background radiation, which represents the universe when it was 300,000

years old. The hot spots/cold spots correspond to different densities of matter across the sky. That pattern matched with the large-scale structure of the universe astronomers see today. It also confirmed that dark energy accounted for 74 percent of the universe's total "matter-energy density."