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MICHELE NORRIS, Host:

Today, scientists are reporting new details about the mysterious force known as dark energy. Astronomers stumbled across dark energy a decade ago. Since then, they've been trying to learn more about what it is and where it's hiding. As NPR's Joe Palca reports, dark energy seems to exist where scientists once said there was nothing.

JOE PALCA: Scientists have a way of hijacking words and giving them new meanings. In the 1960s, physicists made off with the words color and charm to describe subatomic particles known as quarks. Now, they're trying to take over the word nothing.

D: Fifty years ago, nothing was considered very boring. Today, nothing is the focus of much forefront research in physics and in astrophysics.

PALCA: Michael Turner is an astrophysicist at the University of Chicago. According to Turner, scientists now think the nothing of space is actually filled with a poorly understood force called dark energy. After the big bang, the universe went through a period of expansion, but the laws of gravity decreed that eventually that expansion would slow down, certainly by now some 13 billion years after the big bang. But when they did the measurements a decade ago, to their astonishment they discovered not only that the expansion wasn't slowing down, it was speeding up. Something had to be driving it, but what? Turner says astrophysicists are still trying to figure that out.

D: Where we stand right now is we have words that we tell one another, which is that the universe is filled with the weirdest substance that we've ever run across, called dark energy. And that unlike ordinary stuff whose gravity is attractive, the gravity of dark energy is repulsive. And that repulsive gravity is pushing the universe apart causing the expansion to speed up and not slow down.

PALCA: And Turner says astrophysicists believe all that dark energy is actually hiding out in what used to be called nothing. In other words, at least to astrophysicists, nothing is really something. But exactly what dark energy is, is still a mystery. Scientists have been looking for other signs of how it's shaping the universe. Now, they have one. Alexey Vikhlinin and his colleagues at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, used the Chandra X-ray observatory to study the growth of galaxy clusters. Dark energy should have a measurable effect on that growth. In fact, the rate of growth should change at the same time the expansion of the universe starts speeding up. Vikhlinin says that's just what they saw.

NORRIS: This change in the rate of growth has occurred exactly at the point where we believe the universe entered that acceleration phase, so this is an unmistakable signature of dark energy effects.

PALCA: Vikhlinin's results will appear in the February 10th issue of the Astrophysical Journal. Astrophysicist Michael Turner says, of course, there could be another explanation for the behavior of the galaxy clusters and the accelerating expansion.

D: It could be that all this talk about dark energy is just much ado about nothing.

PALCA: In the Shakespearean sense of the word.

D: And that there is no dark energy out there. It's just that we don't understand gravity.

PALCA: But that would mean Einstein got it wrong when he made predictions about gravity in his theory of general relativity. Vikhlinin says that kind of radical rewriting of the laws of physics is not necessary. His results, although strange, do fit in Einstein's theories.

NORRIS: And what we find is all the effects can in fact be still described by Einstein's gravity.

PALCA: Or, to put it in another way, for now at least it appears Einstein didn't get nothing wrong. Joe Palca, NPR News, Washington.

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