'Dark energy' pushed universe's growth, astronomer says
NEW YORK -- The Hubble Space Telescope has shown that a mysterious form of energy first conceived by Albert Einstein, then rejected by the famous physicist as his "greatest blunder," appears to have been fueling the expansion of the universe for most of its history.
This so-called "dark energy" has been pushing the universe outward for at least 9 billion years, astronomers said Thursday.
"This is the first time we have significant, discrete data from back then," said Adam Riess, a professor of astronomy at Johns Hopkins University and researcher at NASA's Space Telescope Science Institute.
He and several colleagues used the Hubble to observe 23 supernovae -- exploding white dwarf stars -- so distant that their light took more than half the history of the universe to reach the orbiting telescope. That means the supernovae existed when the universe was less than half its current age of approximately 13.7 billion years.
Because the physics of supernova explosions is extremely well-known, it is possible for the astronomers to gauge not just their distance, but how fast the universe was expanding at the time they went off.
"This finding continues to validate the use of these supernovae as cosmic probes," Riess said.
He and his colleagues describe their research in a paper that is scheduled for publication in the February 10 issue of Astrophysical Journal.
The idea of dark energy was first proposed by Einstein as a means of explaining how the universe could resist collapsing under the pull of gravity. But then Edwin Hubble -- the astronomer for whom the NASA telescope is named -- demonstrated in 1929 that the universe is expanding, not a constant size. That led to the big-bang theory, and Einstein tossed his notion on science's scrap heap.
There it languished until 1998, when astronomers who were using supernova explosions to gauge the expansion of the universe made a shocking observation. It appeared that older supernovae, whose light had traveled a greater distance across space to reach the Hubble telescope, were receding from Earth more slowly than simple big-bang theory would predict. Nearby supernovae were receding more quickly than expected. That could only be true if some mysterious force were causing the expansion of the universe to accelerate over time.
Cosmologists dubbed the force "dark energy," and ever since they've been trying to figure out what it is.
"Dark energy makes us nervous," said Sean Carroll, a theoretical physicist at the California Institute of Technology who was not involved in the supernova study. "It fits the data, but it's not what we really expected."
Answers may come once NASA upgrades the Hubble Space Telescope in a space shuttle mission scheduled for 2008. NASA and the Department of Energy are also planning to launch an orbiting observatory specifically designed to address the mystery in 2011.
Dark energy could be some property of space itself, which is what Einstein was thinking of when he proposed it. Or it could be something akin to an electromagnetic field pushing on the universe. And then there's the possibility that the whole thing is caused by some hitherto undiscovered wrinkle in the laws of gravity.
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