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Today an astronomer announced that he may have found an important clue in one of the most profound mysteries in the universe. That is: Why the stars in the sky are moving away from us at an ever-increasing pace. Some scientists have suggested that the vacuum of space is filled with some strange form of energy, but as NPR's Richard Harris reports, new results call that idea into question.

RICHARD HARRIS reporting:

When astronomers look out into the sky, they see that all the stars and galaxies are moving away from us. Part of their explanation is that the universe started with a big bang, a cosmic explosion. In 1999, astronomers set out to measure the speed of expansion by looking at distant supernovas, exploding stars, to see how fast they were moving away. To everyone's surprise, they found that the material isn't just coasting away from us in space as you'd expect in the aftermath of an explosion, it's actually picking up speed as it goes. Bradley Schaefer is an astronomer at Louisiana State University.

Mr. BRADLEY SCHAEFER (Louisiana State University): Something is out there pushing the universe, making it expand, making it speed up, and while we don't know what this is really, it's been given a name, dark energy, and you can say that, well, this dark energy is, well, pushing the universe, pushing the acceleration.

HARRIS: A leading notion is that this dark energy is something that pervades what we had thought of as empty space. It's sometimes called quantum vacuum energy, or it's called the cosmological constant. The idea is that it's a constant through all space and through all time. Schaefer decided to test this idea by probing deeper back in time to see if the constant was the same way back then. He did this by studying objects called gamma ray bursters.

Mr. SCHAEFER: They're an extreme version of supernova, we think, and as such they're roughly a hundred times more luminous than supernovae, and so you can see them much farther out in the universe than you can see supernovae.

HARRIS: Today at a meeting of the American Astronomical Society, Schaefer announced his early results from this study of gamma ray bursters. He says their movement suggests that dark energy was different way out in space, and therefore way back in time.

Mr. SCHAEFER: And so the first results of this new method happen to be pointing towards the direction of the cosmological constant not being constant, which would be to say that it appears that the dark energy is changing with time.

HARRIS: Schaefer, however, stopped short of making a definitive claim about the nature of the universe from these results.

Mr. SCHAEFER: Now for an important claim like this, extraordinary claims require extraordinary evidence.

HARRIS: And his evidence is not strong enough at the moment to be absolutely convincing. Even so, it is attracting a lot of attention at the astronomy meeting, including from heavyweights such as Michael Turner from the University of Chicago and the National Science Foundation.

Mr. MICHAEL TURNER (University of Chicago; National Science Foundation): It's very intriguing. I don't think it rises to the level of me being able to issue Brad a ticket to Sweden.

HARRIS: The Nobel Prize will have to wait for more convincing results, Turner says, but he adds that a breakthrough in this field would clearly be gold medal material.

Mr. TURNER: Cosmic acceleration, I believe, is the biggest mystery in all of science.

HARRIS: Turner says it's hard enough for scientists to understand how the supposedly empty vacuum of space can be suffused with some form of cosmic energy. If Schaefer's data hold up and the idea of a constant and pervading energy is thrown out, then what?

Mr. TURNER: The alternatives really are mind stretching.

HARRIS: Maybe the universe contains hidden dimensions, for example. Turner says the uncertainty about Schaefer's results won't last long. Other astronomers have also been trying to measure the expansion of the universe way back in time, so he says in a few years we should know whether this is just a statistical fluke or a real clue about the fabric of our universe. Richard Harris, NPR News.

NORRIS: And you can get a look at those gamma ray bursters Richard Harris mentioned at our Web site, npr.org.

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