New Doubts Are Cast on Einstein's Cosmological Constant

By DENNIS OVERBYE

Published: January 12, 2006

Einstein was wrong.

Einstein was right.

He was wrong about being wrong.

An astronomer from Louisiana State University said yesterday that a new analysis of cosmic history cast doubts on Einstein's cosmological constant, the leading explanation for the mysterious force that appears to be pushing apart the universe.

But other astronomers said that conclusion itself was in doubt.

The astronomer, Bradley E. Schaefer, said his analysis showed that the force, known as dark energy, was not constant, as Einstein would have predicted, but was growing more violent as cosmic time went on.

"The cosmological constant does not look good," said Dr. Schaefer, who used the violent flashes called gamma ray bursts as cosmic mileage markers to describe the history of the expansion of the universe.

In an interview Dr. Schaefer cautioned that it was just a preliminary result with years before a final answer came in. He presented his report at a meeting of the American Astronomical Society in Washington.

Several astronomers said it was Dr. Schaefer, not Einstein, who was wrong. His conclusion, they said, was undermined by mathematical and statistical flaws. Moreover, some astronomers questioned whether the properties of gamma ray bursts were known precisely enough to serve as cosmological beacons.

"I flat out don't believe this result," said Adam Riess, an astronomer at the Space Telescope Science Institute in Baltimore who was a discoverer of dark energy eight years ago.

Einstein first proposed the constant in 1917, as a way of explaining how the universe could be static despite the force of gravity. It was a sort of universal antigravity embedded in space. He abandoned that theory as a blunder when the universe proved to be expanding.

But in 1998, the cosmological constant gained new life. It was then that competing teams of astronomers, using exploding stars known as Type 1a supernovas as cosmic mileage markers, discovered that the expansion of the universe appeared to be accelerating, as if a dark antigravitational force were indeed at work.

As Steven Weinberg of the University of Texas said, Einstein's biggest blunder was believing the cosmological constant was a blunder.

Since the 1998 discovery, astronomers have been racing to chart the history of the expansion of the universe more precisely to pin down the properties of dark energy. Observations by Dr. Riess and his colleagues with the Hubble Space Telescope two years ago have determined that the universe hit the gas pedal about five billion years ago.

Dr. Schaefer used as his mileage markers 52 gamma ray bursts, which are 100 to 1,000 times as powerful as Type 1a supernovas, and can thus be seen much farther away, or back in time. The bursts, which can be seen only from space, have been studied by satellites like the High Energy Transient Explorer, or HETE.

The most distant burst, at 12.8 billion light-years, occurred when the universe was 6 percent of its present age, Dr. Schaefer said.

His measurements put the dark energy in a controversial category named phantom energy, which if it continued unabated would rip apart the cosmos in a few billion years.

But Donald Lamb, an astronomer at the University of Chicago, and others who were interviewed before Dr. Schaefer's talk but were made familiar with its conclusions, disagreed.

They said Dr. Schaefer had been forced to his finding about the Einstein constant by his use of a mathematical parameter called w-prime, a measure of how fast the violence of the dark energy appears to be changing with distance in the universe.

At large distances, Dr. Lamb said, the parameter becomes mathematically meaningless, and theorists have dropped it.

Moreover, he said, if Dr. Schaefer's analysis is valid, his results agree with Einstein's constant, within the measurements' uncertainties.

"It's not a meaningful discrepancy," Dr. Lamb said, adding that a statement like Dr. Schaefer's required stronger evidence. "The bottom line is the result doesn't show Einstein was right. And it doesn't show he was wrong."

(Page 2 of 2)

That was echoed by Robert R, Caldwell, a dark-energy theorist at Dartmouth, who added that if dark energy evolved as swiftly as Dr. Schaefer said, it would have interfered with the formation of galaxies.

Another criticism is that most of Dr. Schaefer's data points are from the early universe, more than five billion years ago, before dark energy became a dominant force in the universe. The action, they say, is all at later times, when dark energy's effects are more easily seen, but his data is sparse there.

In his presentation, Dr. Schaefer said it was too early to make claims about the usefulness and ultimate meaning of his proposal.

"The first results are pointing to the cosmological constant not being constant, a point I don't want to push too much," he said. "This is the first demonstration of cosmologically useful results from a new method. We can't be too confident in the results right now."

The dispute shines an awkward light on what some astronomers regard as a promising and powerful tool for cosmology, gamma ray bursts. Dr. Schaefer called his work a "proof of purchase" for the use of the bursts, which can emit as much light in a second as our Sun does in a billion years, to investigate dark energy and other puzzles of the long night.

George Ricker, a gamma ray astronomer at M.I.T., said of Dr. Schaefer's work: "It's great that he's doing this. He's drawing attention to fact that this is possible."

Other astronomers said the use of gamma ray bursts for cosmology was in its infancy at best.

Until recently, astronomers did not even know what they were. Lately, the bursts have been traced to the titanic implosions of very massive stars into black holes, and astronomers have begun to learn how to calibrate the flashes.

Lorenzo Amati of the Institute of Space Astrophysics and Cosmic Physics in Bologna, Italy, and his colleagues discovered a correlation between the total luminosity of a gamma ray burst and the wavelength at which it appears brightest.

Using such techniques, astronomers can estimate the intrinsic luminosity of a gamma ray burst within 25 percent, Dr. Lamb said. That is only two or three times the uncertainty associated with the Type 1a supernova explosions, considered the gold standard for cosmological work.

Dr. Lamb was the lead author of a report last summer outlining how a gamma ray burst mission could be used to investigate dark energy.

Such a mission is unlikely soon, however, because NASA's science budget suffers overruns from the James Webb Space Telescope, and scientists are trying to find money to maintain the HETE satellite, which is scheduled to be turned off.

Optical astronomers point out that it will take many more gamma ray bursts than supernovas to reach the same precision. Lacking a sound theoretical reason to believe that gamma ray bursts should be standard candles, they are reluctant to give them much credence.

Dr. Riess said, "The news is that we are really stumped about this dark energy puzzle and are being forced to be very creative to probe it."