Strategy to shed gamma light on the mystery of dark energy


Astronomers are a big-picture bunch. They want to know how galaxies form, how planets behave and what the light from distant stars tells us about the dawn of time.

But the group that gathered in Washington last week waded into an issue that's really big, even by cosmological standards. They're trying to measure dark energy -- the unseen but very real force that's causing the universe to expand.

"I believe this is the biggest mystery in all of science," said Michael Turner, an astronomer at the University of Chicago who joined hundreds of colleagues at the annual meeting of the American Astronomical Society.

Until now, most astronomers have tried to measure dark energy by looking at the light from supernovae -- created by the explosions of stars.

But Bradley E. Schaefer, an astronomer at Louisiana State University, wants to change that. And he's willing to challenge Albert Einstein and most of the world's cosmologists to do it.

Schaefer says his analysis of 52 gamma ray bursts -- the most powerful explosions in space -- shows that they are a good (and maybe better) yardstick for measuring how quickly the universe is expanding.

He also says that when Einstein was calculating the rate of expansion of the universe, he got it wrong.

There's a lot at stake besides bragging rights.

Schaefer and others expect to see about $1 billion in federal and private research money spent over the next few years to explore dark energy.

Their work is designed to shed light on the forces that formed the universe, the forces that might one day destroy it and the physics behind both.

"The age and fate of the universe is a question every culture has always asked about," Schaefer says. "It's all about what the universe really is, what it did to get the way it is and how it's changing."

To find out, scientists have to deal with incredibly powerful forces.

Supernovae, for example, are brilliant explosions of stars about the size of our sun that can last for several weeks. Most gamma ray bursts are believed to be quicker, more intense explosions of even larger stars -- the most powerful blasts in the universe.

New theories

In their meetings, Schaefer and his colleagues are debating a lot of new theories that few outside the astrophysics fraternity comprehend. In fact, dark energy wasn't even named until the late 1990s.

Because Schaefer is challenging both the astrophysics establishment and the godfather of modern-era physics, he knows he's entering a minefield.

"I anticipate a lot of criticism," he said shortly after announcing his findings last week in Washington. "There are whole groups in this field who will want to shoot this thing down."

He's right about that. Many scientists argue that too little is known about gamma ray bursts to consider using them as a dark-energy yardstick.

"The claims being made at this point are overreaching," said Michael Wood-Vasey, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Massachusetts.

Using gamma ray bursts to judge dark energy is a bit like trying to judge the distance to a person on the horizon without knowing how tall he is, says Adam Riess, a Johns Hopkins University professor and astronomer at the Space Telescope Science Institute in Baltimore. But he and others say that the bursts could be used as a tool as scientists learn more about them.

What's the fight all about?

Astronomers once believed the universe was largely a static place. But in the 1920s, Edwin Hubble, the astronomer for whom the space telescope is named, discovered that light from distant galaxies was moving away from us. That meant the universe was -- and is -- expanding.

Einstein, whose fundamental theories predated Hubble's work, had figured out the same thing. But because no one at the time could prove the expansion theory by observation, he came up with an equation known as the cosmological constant to account for the phenomenon he couldn't explain.

Eventually, though, scientists began to chart the universe's expansion the same way that drivers will judge the distance to a passing car -- by watching its taillights recede.

By the early 1990s, scientists began using a specific type of supernova, known as Type Ia, as their cosmic taillight because they understood its properties. Formed by the collapse and explosions of white dwarf stars -- which are about the size of our sun -- type Ia supernovae create an intense burst of spewing gases that radiate for weeks with a uniform brightness.

"They're as bright as whole galaxies," said Ray Carlberg, a cosmologist at the University of Toronto and part of an international team studying dark energy.

Berkeley Lab

In 1997, scientists at the University of California's Berkeley National Lab, working with astronomers at the world's largest telescopes, began taking images of patches of sky that allowed them to track the movements of dozens of Type Ia supernovae.

They would take images, wait three weeks and snap more images of the same skies, calculating how far the supernovae traveled during the three-week intervals by measuring their changes in brightness, along with the degree of "red shift." (As starlight travels through space, the light shifts from the blue to the red end of the spectrum.)

On Jan. 1, 1998, the scientists announced in the journal Nature that 42 of their recently discovered supernovae were dimmer than expected -- which meant the universe is not only expanding, as Hubble found, but expanding faster than anyone thought.

So some other force was at work, accelerating the expansion -- and they called it dark energy.

"At the time I didn't think it was a good name, but it caught on fairly quickly," said Saul Perlmutter, a senior scientist at the Berkeley Lab and a co-discoverer of the acceleration phenomenon.

Teams of astronomers around the world have tracked type Ia supernovae ever since.

"It's still the most direct way to see an acceleration," Perlmutter said.

Last fall, Carlberg and others studied images of supernovae over 20 nights with a 340-million-pixel camera and concluded that Einstein's cosmological constant was essentially correct. Future observations are expected to fine-tune those results, Carlberg says.

Not so fast, says Schaefer. He has been analyzing gamma ray bursts that go off like camera flashes. That speed had made them difficult to detect. But that has changed with the deployment of more recent satellites, including NASA's Swift and HETE probes, which are designed to track the bursts, Schaefer said.

Using data from 52 gamma ray bursts collected by NASA satellites, Schaefer found that the most distant bursts were brighter than Einstein's cosmological constant said they should be.

Although Schaefer's initial findings will have to be tested by observation of more gamma ray bursts, he's pretty sure Einstein was wrong. "The cosmological constant apparently isn't constant," he said.

Although he concedes that he could be mistaken, he argues that the gamma ray bursts he used are a thousand times brighter than supernovae and make a better tool for probing deep space.

The most distant Type Ia supernova is about 9.8 billion light years away. But with images from Swift and HETE, astronomers have tracked gamma ray bursts up to 12.8 billion light-years away.

Filling in the gap

"There's a gap where you don't have any information, and that's a gap that gamma ray bursts can go about filling," Schaefer said.

Many scientists say they're anxious to see Schaefer publish his findings in a peer-reviewed journal, as he promises to do in the coming months.

"What he has may be a new ruler for measuring something, and that's great," Carlberg said. "But whether that ruler is made out of rubber and won't really work or whether it gives an accurate measurement remains to be seen."

An article about dark energy in Friday's Health & Science section incorrectly described the size of white dwarf stars. They have a radius only 1 percent as large as the sun's but are far more dense, with up to 40 percent more mass. The article credited the Berkeley National Lab with discovering dark energy in 1998. The discovery was announced that year by both the Berkeley lab and an international group known as the High-z Supernova Search Team.The Sun regrets the errors.
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