Data from galaxy clusters suggest dark energy is constant over time: density value may resemble Einstein's cosmological constant.

VANCOUVER, Canada -- Chalk up another victory for the dark side.

Comparing X-ray observations of distant and nearby clusters of galaxies, astronomers say they have found new, independent evidence for the existence of dark energy, the mysterious entity that is accelerating cosmic expansion. By combining the new data with that from other studies, the team finds that dark energy appears to have maintained the same value over time, just as Einstein's proposed cosmological constant does.

Some theories of dark energy suggest that the repulsive force associated with this mystery substance may grow stronger with time, causing the universe to end in a Big Rip, with every planet and person ultimately ripped apart. While the new findings suggest that dark energy has maintained a constant strength throughout cosmic history and therefore won't tear the universe entirely apart, they still allow some wiggle room.

The new X-ray study by itself allows dark energy to vary by 50 percent from its current density over time, says Alexey Vikhlinin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. When combined with other studies, the new data suggest the density could vary by only 10 percent.

Vikhlinin and his colleagues used NASA's Chandra X-ray Observatory to record X-ray emissions from 86 massive clusters of galaxies, each heavier than 100 trillion suns. The team found two sets of clusters. The first set of 37 dates from between 6.4 billion and 9.8 billion years after the birth of the universe and the second set of 49 dates from between 11.8 billion and 13 billion years after. Vikhlinin reported his team's findings in Vancouver on December 11 at the Texas Symposium on Relativistic Astrophysics, and the findings also appeared online December 15 at arxiv. org/abs/0812.2720.

Because the present-day densities of clusters are precisely known and fixed, researchers seek to find the fingerprints of dark energy by measuring the density of clusters back in time. At earlier times, because the universe was more compact, gravity's inward pull was stronger relative to dark energy's outward push. That means in the most well-accepted model of the universe, the presence of dark energy would lead to a larger number of massive clusters at early times than in a universe with no dark energy. This is precisely what the team found.

"Clusters of galaxies are the most massive objects in the universe that can be used as tracers of the growth of structure," says Daisuke Nagai of Yale University, a member of Vikhlinin's team. "Dark energy, if present, tends to slow down the evolution of cluster abundance due to its repulsive force. The rate of the evolution, in turn, depends sensitively on the nature or form of dark energy."

Adam Riess of the Space Telescope Science Institute in Baltimore, a member of one of the teams that discovered dark energy a decade ago, says the new work sounds promising. "We're going to need all hands on deck, all methods working pretty well if we're going to figure out what dark energy is."

Doing a study like Vikhlinin's, but using visible light, Brian Gerke of the Stanford Linear Accelerator Center in Menlo Park, Calif., and his colleagues found similar results among groups of galaxies. Gerke also reported the findings at the Texas Symposium on December 11.