A Challenge to Evolution of Universe
Thursday, January 12, 2006
In the quest to decipher the evolution of the cosmos, no topic generates greater interest among scientists than "dark energy," the mysterious force that appears to be causing the universe to expand at an ever-accelerating rate.
Yesterday, Louisiana State University astronomer Bradley E. Schaefer tossed a grenade into this debate, presenting new research to suggest that the force dark energy exerts may have varied over time. That casts new doubt on the validity of Albert Einstein's "cosmological constant" only a few years after astronomers rescued the concept from scientific oblivion.
"I'm not pushing this as a proof," Schaefer said in an interview at this week's meeting of the American Astronomical Society in the District, where he presented his research. "It's pointing against the cosmological constant, but it's a first result describing how dark energy changes with time. We need more people to test the results and get more information."
Schaefer based his findings on analysis of ultra-bright cosmic explosions called gamma-ray bursts, detected as far as 12.8 billion light-years away. He found that the most distant explosions appeared brighter than they should have been if the universe were accelerating at a constant rate.
"As you go back in time, the universe is pushing [outward] less and less," he said. "At some point, the pressure of dark energy is zero and is exerting no force on the universe. There is no explanation for it."
Schaefer's findings, the first attempt to use gamma-ray bursts to study dark energy, produced a result that disagreed with accumulating evidence gleaned from observing a different kind of blast -- the exploding stars called supernovae. That work suggested that the expansion of the universe is accelerating in accordance with Einstein's cosmological constant.
"The idea of using a gamma-ray burst as a distance indicator is a very exciting one," said California Institute of Technology astronomer Richard Ellis, a supernova cosmologist. "The trouble is there are no ways to check the techniques. I'm not saying it's no good, but I can't believe it's as precise as supernovae."
The concept of dark energy emerged in 1999 as a way to explain the fact that the expansion of the universe, once thought to be slowing ever since the big bang about 13.7 billion years ago, was accelerating. That resurrected the idea of a cosmological constant, introduced by Einstein more than 80 years ago as a "fudge factor" to explain why the universe then appeared to be in equilibrium, rather than being pulled together by gravity.
A few years later, however, astronomer Edwin Hubble discovered that the universe was not in stasis, after all, but was expanding. There was no "constant." Einstein condemned his own idea as "my greatest blunder."
But in the 1990s, astronomers found ways to use supernovae as cosmic "standard candles" whose luminosity could be analyzed to track the history of the universe's expansion as far back as 9.8 billion light-years.
That led to the 1999 discovery that the expansion of the universe was accelerating rather than slowing. There had to be some "repulsive force" overcoming the gravity that should have been causing the universe to come together.
Astronomers called the force dark energy, and "it mimics the cosmological constant," said Michigan Technological University astronomer Robert J. Nemiroff. Einstein may have been right after all.
Astronomers estimate that dark energy makes up 70 percent of the universe, but they do not know what it is. Solving the mystery is as all-consuming as any passion in physics. "It's so spooky," said Astronomical Society President Robert B. Kirshner, a cosmology expert at the Harvard-Smithsonian Center for Astrophysics. "Everybody is looking for ways to get at it."
Schaefer said he had been interested for several years in using gamma-ray bursts -- the brightest explosions in a violent universe -- to look far deeper into the past than astronomers could reach with supernovae.
The key were the launches of the Massachusetts Institute of Technology's HETE-2 satellite in 2000, and NASA's Swift satellite in 2004. Both were designed to locate and observe gamma-ray bursts, which last milliseconds to several minutes.
Schaefer analyzed 52 of the bursts and found that the most distant of them were brighter than the cosmological constant would have predicted -- indicating that the universe's expansion was accelerating at a slower rate than it is today.
Schaefer "gets full marks for coming up with a new technique," Ellis said in a telephone interview, but "we still really don't have a clue" what dark energy is. "It's mayhem at the moment" in cosmology, he added. "The theorists are having a field day, because there's no data."