New light on dark energy

Dark energy is coming in from the cold.

Astronomers announced today that they have uncovered dark energy's influence on the evolution of galaxy clusters -- among the largest collapsed structures in the universe. In effect, dark energy -- an unexpected, repulsive force astronomers discovered in the late 1990s -- is putting a cap on how large these structures can get.

"This may well be called arrested development of the universe," says Alexey Vikhlinin, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who led the research team reporting the results. "We're observing the unambiguous signature of the effects of dark energy on the growth of structure" in the universe.

Dark energy is causing the very fabric of space-time to expand at an accelerating rate, researchers say. Over the very long haul, the Chandra results imply that the universe will not end in a "big rip," with everything violently torn apart.

Instead, Dr. Vikhlinin says, any objects too loosely bound by gravity to  overcome the repulsive force of dark energy will gradually vanish into the distance.

From the vantage point of the Milky Way -- or "Milkomeda" after its merger with the Andromeda Galaxy in about 3 billion years -- it will be lights out on the Vigro Cluster, a gathering of at least 1,300 galaxies in a gravitationally corralled herd some 60 million light-years from Earth.

The results, based on observations taken using NASA's Chandra X-Ray Observatory, are helping to shrink the list of possible answers to the question: What's responsible for dark energy?

Two candidates remain standing, explains David Spergel, a theorist in astrophysics at Princeton University in Princeton, N.J.

What's doing it?

One involves an ultralight among subatomic particles. Dubbed quintessence, the particle is associated with the kind of force field that interacts only very weakly with matter and is unstable. Some researchers suggest that this energy drove the rapid expansion that the universe underwent during its inflationary period, which is thought to have taken place during the first trillionth of a second after the big bang. Energy from this hypothesized quintessence field is one candidate for dark energy.

The other is something called vacuum energy -- a form of energy that stems from calculations in quantum physics. This energy would be present even if the universe held no matter. And it remains constant as the universe evolves, Dr. Spergel says.

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