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Dark energy search gets murkier

Supernova measurements muddle scientists’ efforts to explain universe’s accelerating expansion

LOOKING DARKER  Using supernova measurements from the Pan-STARRS PS1 telescope on Maui, researchers calculated a cosmological parameter related to dark energy. If verified, the findings could force cosmologists to develop a new explanation for this energy, which pushes the universe apart.

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New measurements of light from distant supernovas could complicate cosmologists’ already-frustrating attempts to explain the mysterious dark energy that is pushing apart the universe.

In the new analysis, scientists combined data from 146 recently discovered supernovas with previously published results and calculated an important cosmological parameter. Their result is inconsistent with the simplest explanation for the universe’s accelerating expansion, which suggests that the strength of dark energy has remained constant over the life of the universe.

If confirmed, the finding could imply that matter in the universe will eventually be torn apart, a scenario known as the Big Rip. But before reaching that conclusion, the researchers say they must ferret out potential sources of error and uncertainty in their measurements. “It’s very possible, and I think a lot of people would say likely, that one of the big measurements is off,” says study coleader Daniel Scolnic, an astrophysicist at Johns Hopkins University.

Dark energy first made headlines in 1998, when researchers found that light from faraway supernovas was dimmer than expected, suggesting that the universe is expanding at a faster and faster pace. To explain this acceleration, scientists proposed the existence of dark energy, which imbues the cosmos with a negative pressure that pushes space outward. Most physicists suspect that dark energy is a form of vacuum energy known as the “cosmological constant” because its strength never varies.  If so, a number called w, which relates the pressure pushing space apart to the density of dark energy, must equal –1.

But the new analysis, posted online October 14 at arXiv.org, arrives at a different value. Combining data from the Hawaii-based Panoramic Survey Telescope & Rapid Response System, or Pan-STARRS, with previous astronomical surveys, the researchers calculate w to be –1.186. If correct, this value of w would force cosmologists to pursue more complicated theories about the universe’s expansion, in which the strength of dark energy increases over time.

That’s not happening yet. Even the study’s authors stress that they are not advocating throwing out the cosmological constant. “My gut feeling is that w is probably –1,” says study coleader Armin Rest, an astronomer at the Space Telescope Science Institute in Baltimore.

Rest thinks the finding most likely results from some kind of systematic error. In a companion paper also posted online October 14 at arXiv.org, the team determines that the largest source of error involves differences in how the Pan-STARRS telescope and other groups’ telescopes captured the supernovas’ light. Errors can also arise from interference from dust in the Milky Way, an incomplete understanding of supernova physics and other factors.

Glenn Starkman, an astrophysicist at Case Western Reserve University in Cleveland, is not convinced the new paper advances scientists’ understanding of dark energy. He says the results mostly confirm what cosmologists already know: that different cosmological surveys disagree about parameters such as w that are crucial to dark energy models. “It’s evidence that there’s discord in the model but not yet evidence that w is less than –1,” he says.

But George Efstathiou, director of the Kavli Institute for Cosmology at the University of Cambridge, believes that whether due to systematic error or as-yet-undiscovered physics, the results do provide useful information. “These are very difficult measurements to make,” he says. “The more independent data there is, the better it is for the field.”

Editor's Note: This story was updated on November 11, 2013, to correct the description of dark energy and its connection to the constant known as w.

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