Astronomers have taken another significant step toward proving the existence of dark energy, the mysterious force that seems to be stretching the cosmos at an accelerating rate. A team from the University of Hawaii, Honolulu, has observed how dark energy directly affects the largest structures in the universe. If confirmed, the findings could bring astronomers closer to understanding a phenomenon that has bedeviled them for more than a decade.
In the mid-1990s, an international team of researchers attempted to measure the rate by which the expansion of the universe--begun 13.7 billion years ago in the big bang--was decelerating. They planned to do so by carefully calibrating the brightness of a well-studied type of supernova in very distant galaxies. Because the brightness of each of these supernovae should be exactly the same, the researchers could calculate the distance and velocity of their host galaxies. But by 1998, the team had reached a stunningly different conclusion: The expansion of the universe was not slowing down--it was accelerating, driven by some unknown effect that came to be called dark energy. Ever since, researchers have struggled to detect the strange force, which also seems to constitute 76% of the mass-energy balance of the universe.
To conduct the new study, the Hawaiian team, led by astronomer Istvan Szapudi, combined two large-scale observations of the cosmos that already had been completed: the cosmic microwave background (CMB), which represents the last, dying embers of the big bang, and the Sloan Digital Sky Survey, which comprises images of millions of galaxies. Current theory predicts that dark energy would slightly heat the CMB radiation passing through dense regions of galaxies called superclusters, each about a half-billion light-years across. In contrast, dark energy would slightly cool the microwaves passing through equally vast areas of mostly empty space called supervoids. As they will report in an upcoming issue of Astrophysical Journal Letters, based on data collected from observations of 50 superclusters and 50 supervoids, the CMB was either heated or cooled almost precisely as expected.
The study is "the best of its type undertaken to date," says astrophysicist Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland. Riess, whose team published the first paper on dark energy, adds, however, that although the method used by the Hawaiian team provides "fairly direct evidence for dark energy," the force itself still hasn't been detected. Nor do the results help scientists determine which of their theories best describes what dark energy is--in particular, whether it is "vacuum energy" inherent in space itself or some sort of "quintessence" force that might change over time. "We've still got a cosmic mystery on our hands," Riess says.
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