NASA, ESA, and R. Massey / California Institute of Technology

Dark gatherings.
Dark matter is distributed in clumps and filaments. Regions farther from Earth are to the right side.

Mapping the Invisible

SEATTLE, WASHINGTON--Like a plate of spaghetti and meatballs thrown against a wall, a new 3-D map indicates that dark matter is scattered throughout the universe as a series of large clumps connected by elongated filaments. The findings, which are the result of one of the largest celestial observing campaigns ever undertaken, confirm theories about how the universe evolved and may shed light on the nature of the mysterious force known as dark energy.

Cosmologists believe the observed distribution of stars and galaxies in the universe was determined by dark matter, an invisible substance thought to make up about 85% of all matter in the universe (ScienceNOW, 5 January 2007). Dark matter clumped together first, and normal matter then flowed toward the densest regions, where it condensed into galaxies and stars. "Dark matter is like the scaffolding of the universe," says Richard Massey of the California Institute of Technology in Pasadena.

Massey's team confirmed this idea by spending many weeks of observing time on the biggest telescopes in space and on the ground, including the Hubble Space Telescope and the Japanese 8.3-meter Subaru telescope on Mauna Kea, Hawaii. Known as COSMOS (for Cosmological Evolution Survey), the project studied half a million galaxies in a region of the sky as large as 10 full moons, out to a distance of 8 billion light years.

Because gravity slightly bends light rays, the feeble images of distant galaxies are ever so slightly distorted by intervening clouds of dark matter--an effect known as weak lensing (ScienceNOW, 16 March 2000). By measuring how far away these "distorted" galaxies are from Earth, Massey and colleagues were able to plot the concentrations of intervening dark matter on their map, which was presented here yesterday at the 209th Meeting of the American Astronomical Society, as well as online yesterday in Nature.

The map shows that most galaxies are indeed found in regions where dark matter is concentrated. Elongated filaments of dark matter connect huge clusters as massive as a few quadrillion suns. Because looking far out into space also means looking far back in time, the map also provides information on the evolution of the large-scale structure of the universe, revealing that dark matter has become clumpier over time.

Discovering how that happened may eventually allow astronomers to learn more about the nature of dark energy, which battles gravity's attempts to keep the universe together, says team member Jason Rhodes of NASA's Jet Propulsion Laboratory in Pasadena.

Tony Tyson of the University of California, Davis, who was the first to realize the scientific potential of weak lensing back in 1983, says he's "excited" by the new map. The next step is to study weak lensing over the whole sky, which is necessary to get a handle on the nature of dark energy.

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