Is an enigmatic force called dark energy speeding the universe's expansion? An international team of more than 50 astronomers has brought us one step closer to an answer by computing the velocity of thousands of galaxies. Scientists hail the technique as a powerful new tool that can gauge how dark energy opposes gravity.
The idea of dark energy was born 10 years ago, when a team of astrophysicists led by Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland, submitted a paper to the Astrophysical Journal. The scientists suggested that the expansion of the universe was accelerating, rather than slowing down or beginning to reverse, as many researchers had predicted. The implications were astounding: Not only was there a cosmic force that seemed to be stronger than gravity, albeit only at gigantic scales, but also this force--later dubbed dark energy--seemed to constitute more than three-quarters of everything in the universe. Researchers continue to puzzle over the most basic questions about dark energy--including whether it exists. The problem is that other explanations for the universe's expansion are possible. For instance, gravity might behave differently at the biggest scales. So scientists have been applying ever more powerful and sensitive instruments to get to the bottom of the mystery.
The new attempt, described in tomorrow's issue of Nature, involves determining the speeds of about 13,000 loosely associated galaxies in the same general neighborhood of the universe and at the same approximate distance from Earth: 7 billion light-years. Astronomers know that, over time, galaxies tend to clump together in clusters and, eventually, superclusters. Therefore, examining this bunch of galaxies' motions while they're just beginning to link up could help reveal whether dark energy interferes with gravity.
The results: So far, so good. The data compiled from the survey show that something indeed is distorting the speed of the galaxies beyond the mere effects of gravity. The only problem is that scientists regard the survey as too limited to produce an irrefutable conclusion.
"We are catching galaxies well before they end up in clusters," and therefore the galaxies are producing "the subtle effect we look for," says lead author Luigi Guzzo of the Osservatorio Astronomico di Brera in Milan, Italy. It's the first time the technique has been used for this purpose, says co-author Enzo Branchini of the University of Rome III in Italy. The next step, he says, will be to go for confirmation by surveying many more galaxies--perhaps as many as 100 million--at different distances, meaning at different ages of the universe, to see if the effects produced by this first round remain consistent. It could eventually take that many samples, Branchini says, to choose one possible cause over another.
"This is great news," Riess says. What constitutes dark energy "remains one of the most pressing questions in cosmology and physics." And cosmologist Robert Caldwell of Dartmouth College says although the results are preliminary, "it is going to be useful to have measurements of the rate of galaxy clustering" to add to the mix of calculations about why the universe's expansion is accelerating.
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