Dark energy was first proposed six years ago when observations of distant supernova explosions hinted that the expansion of the universe is accelerating, rather than slowing down as expected. Data from the Wilkinson Microwave Anisotropy Probe (WMAP) on the cosmic microwave background radiation also supported the existence of this unseen force. In the new work, an international team of astronomers used the Chandra X-ray Observatory to study 26 galaxy clusters located between one billion and eight billion light-years away. The researchers measured the distance to the galaxy clusters and determined the amount of hot gas in each one. Plotting the results over cosmic time, the scientists determined that the universe¿s expansion started speeding up about six billion years ago, driven by dark energy. "Our Chandra method has nothing to do with other techniques," remarks study co-author Robert Schmidt of the University of Potsdam in Germany, "so they¿re definitely not comparing notes, so to speak."
The new results, which will be published in an upcoming issue of the Monthly Notices of the Royal Astronomy Society, agree with previous estimates that 75 percent of the universe is dark energy. The scientists hope that future experiments--using Chandra, WMAP and the upcoming Constellation-X mission--will allow them to calculate more precisely just how much dark energy exists and whether that amount might be changing over time. Notes Michael Turner of the University of Chicago: "Until we better understand cosmic acceleration and the nature of dark energy, we cannot hope to understand the destiny of the universe."