April 10, 2003: New Hubble Space Telescope observations of a pair of very distant exploding stars, called Type Ia supernovae, provide new clues about the accelerating universe and its mysterious "dark energy." Astronomers used the telescope's Advanced Camera for Surveys to help pinpoint the supernovae, which are approximately 5 billion and 8 billion light-years from Earth. The farther one exploded so long ago the universe may still have been decelerating under its own gravity.See the rest:
The camera's powerful vision picked out the faint glow of the distant supernovae. Then it dissected the light from the exploding stars to measure the distances and to study how they faded over time. After careful analysis, astronomers confirmed that these supernovae are a special kind of exploding star, Type Ia supernovae. Astronomers call them reliable distance indicators because when they explode, they glow at a predictable peak brightness. Knowing how bright they actually are and how bright they appear to be through a telescope, astronomers are able to calculate their distance from Earth.
Examining the light from distant exploding stars can shed light on the early universe. Information from studies of Type Ia supernovae about five years ago revealed the stunning news that galaxies appeared to be moving away from each other at an ever-increasing speed. They have attributed this accelerating expansion to a mysterious factor, a dark energy believed to permeate the universe.
But astronomers also found evidence that dark energy wasn't always in control. In 2001 studies of a far-flung supernova yielded the first evidence that the expanding universe was once decelerating, that gravity was slowing the universe's expansion. They have very little data, though, on the period of transition between these two phases, when the repulsion produced by dark energy began to surpass the tug of gravity.
Credit: NASA and J. Blakeslee (JHU)