spacetoday.net: Astronomers pinpoint when expansion of universe accelerated

Posted: Sun, Oct 12, 2003, 2:08 PM ET (1808 GMT)
Astronomers announced Friday that the expansion of the universe started to accelerate about five billion years ago because of the influence of a mysterious dark energy. A group led by Adam Reiss of the Space Telescope Science Institute observed a number of distant supernovae in order to determine at what rates the universe has been expanding. They determined that the combined gravity of the matter in the universe had been causing the expansion had been slowing down since the Big Bang until about five billion years ago, when dark energy and its repulsive force caused the expansion to accelerate. The accelerating expansion of the universe had been discovered by astronomers several years ago, but prior to this recent research it was unclear how long the acceleration had been taking place. The nature of the dark energy causing the acceleration remains a mystery.