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Cosmic Push: Finding pieces of a dark puzzle

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12:51pm, January 18, 2006
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Scientists call it the most elemental riddle in all of physics and astronomy: What's tearing apart the universe by turning gravity's familiar tug into a cosmic push?

Astronomers discovered the handiwork of this mysterious push, dubbed dark energy, 8 years ago, when studies revealed that cosmic expansion isn't slowing down, as had been predicted, but is speeding up (SN: 5/22/04, p. 330: Dark Doings). One of the leading theories is that dark energy is distributed uniformly in space and time—akin to the case for what Albert Einstein called the cosmological constant (SN: 12/17/05, p. 390: Cosmic Expansion: Supernovas shed light on dark energy). Understanding dark energy would unify the force of gravity with the subatomic realm, providing deep insights into the origin and evolution of the universe, says cosmologist Sean Carroll of the University of Chicago.

So, when a report last week indicated that dark energy behaves even more strangely than researchers had suspected, it garnered extraordinary publicity—as well as vehement reactions among researchers. The study, described by astronomer Bradley Schaefer of Louisiana State University in Baton Rouge, indicates that dark energy varies over time. The work suggests that dark energy put the brakes on cosmic expansion in the past but is now accelerating it, Schaefer reported at the January meeting of the American Astronomical Society in Washington, D.C.

Such a universe would have some mind-bending properties, including a previously unsuspected force in nature generated by an unknown elementary particle. Furthermore, Einstein's well-supported theory of gravitation would require major modifications, adds Carroll.

The implications of Schaefer's study are so provocative, says Carroll, that for now, "no one believes it." He and others also question the data.

In fact, the most important aspect of the new study, Schaefer and Carroll agree, is not the result but the method used to get it. This is the first time that gamma-ray bursts, the most powerful explosions in the universe, have been used to analyze dark energy.

Schaefer acknowledges that his findings aren't as precise as those from more-established methods for studying dark energy.

Astronomers originally found evidence for dark energy by using another, dimmer type of explosion known as a supernova type 1a. Because gamma-ray bursts can be seen at much greater distances than supernovas can, they could, in theory, be used to probe cosmic expansion at earlier times. Gamma-ray bursts vary in brightness much more widely than supernovas do, making it more of a challenge to use them. To compensate, Schaefer took into account five properties of 52 gamma-ray bursts to gauge their intrinsic brightness.

Gamma-ray bursts "hold great promise" for studying dark energy, says cosmologist Don Lamb of the University of Chicago, but he adds that he disagrees with Schaefer's analysis of dark energy at large distances. Lamb's team is doing its own study of gamma-ray bursts and to date has found that dark energy does indeed resemble the cosmological constant, Lamb told Science News.

With satellites expected to find a wealth of gamma-ray bursts over the next few years, scientists will have an opportunity to determine what these explosions can reveal about dark energy, says Schaefer.

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