Posted: Wed, May 19, 2004, 8:47 AM ET (1247 GMT)
Images from the orbiting Chandra X-Ray Observatory have provided astronomers with new evidence that a mysterious "dark energy" is causing the expansion of the universe to accelerate. Astronomers used Chandra to study 25 distant galaxy clusters at x-ray wavelengths, relying on the property of such clusters to have similar ratios of hot gas — which can be detected by Chandra — to the overall mass of the cluster, to measure the distance to each cluster. Astronomers found that the expansion of the universe first decelerated after the Big Bang, but began to accelerate about six billion years ago. That acceleration is thought to be caused by dark energy, an unexplained phenomenon that acts against gravity. Astronomers studying the Chandra data concluded that 75% of the universe is composed of dark energy, a similar fraction to other studies that relied on observations of distant supernovae.

Related Links:

•  New Scientist article
•  SPACE.com article
•  Washington Post article
•  New York Times article
•  Chandra press release