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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.

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Observers saw no evidence for change, so he reasoned that there must be some form of "negative energy" offsetting gravity's tug. By tweaking his numbers, he could get a static universe.

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In the 1920s, Edwin Hubble burst Einstein's bubble. Using the most powerful telescopes of the day, Hubble showed that the fuzzy patches were galaxies, and that the galaxies appeared to be speeding away. The universe was expanding.

For nearly 60 more years, astronomers would trot out Einstein's blunder to explain one new phenomenon or another, only to find later that more conventional explanations were correct.

Meanwhile, particle physicists in the 1960s were working on ideas in quantum mechanics in which a vacuum could exhibit a form of energy. And when they applied Einstein's theory of general relativity to this vacuum energy, thought to permeate the cosmos, it produced the gravitational repulsion that mimicked Einstein's cosmological constant. The work gave the feature an underpinning in physics it had lacked.

The only task left was to observe it in nature. That came in 1998, when two teams working independently reported observations that showed space expanding at a more rapid rate than it should be.

This time, invoking a gravitationally repulsive dark energy appears to be the correct answer. But how it relates to physicists' discoveries about the forces of nature and the subatomic particles associated with them remains a mystery. Dark energy could indeed be Einstein's cosmological constant. It could be a quantum field dubbed "quintessence." Or it could be a new aspect of gravity itself.

"Keep in mind that we call this dark energy, but that gives a false impression that we understand what it is. We really don't," says Adam Riess, astronomer with the Space Telescope Science Institute in Baltimore, Md. Early this year, Dr. Riess and colleagues added what many astronomers call a significant advance in observing dark energy.

In February, the team published results of Hubble Space Telescope observations that spanned a range of distances and periods in the universe's history. They found the period, some 6 billion years ago, when the shift occurred from a slowdown in the rate of expansion to an acceleration - a turning point that has become known as the "big jerk." The team used the light from a powerful "standard candle" - a type of exploding star, or supernova - to gauge distance. Then they used spectrographic data on these objects to determine the speed at which the galaxies containing the supernovae were receding.

A third major contribution came last year from a set of studies involving the cosmic microwave background measurements from a NASA satellite and observations from the Sloan Digital Sky Survey. Both pointed to dark energy as the dominant ingredient in the universe's recipe. And by combining data from the two, four teams working independently found evidence for the action of dark energy on the scale of galaxy clusters, which cover a huge expanse of space and embrace from 50 to 1,000 galaxies.

Taken together, the X-ray, supernova, and microwave-survey studies represent "extraordinary evidence" for dark energy, the University of Chicago's Turner said at a briefing last week.

The next step is to learn whether dark energy varies with time. Already, Riess and his colleagues have been awarded a generous amount of observing time with the Hubble Space Telescope next year to get a more detailed look at dark energy's workings. Ground- and space-based optical telescopes are being planned to study it. Ground-based radio telescopes are getting into the act. And Chandra will be surveying more clusters at a wider range of distances.

The discovery of dark energy has handed researchers "the most profound problem in all of science," Turner says. Solving it "will require a full-court press."

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