Astrophysicists Detect Cosmic Shear, Evidence Of Dark Matter

"Pieces of the puzzle are falling into place," said Morris Aizenman of NSF's Directorate for Mathematical and Physical Sciences. "Within the first few months of this millennium, we have solved the riddle of the geometry of the universe and are now on the threshold of exploring its structure."

Since it cannot be seen by telescopes, the nature and distribution of dark matter have puzzled astronomers for decades. Using the NSF Cerro Tololo Inter-American Observatory in Chile and a method known as weak gravitational lensing, the scientists were the first to map the distribution of dark matter over large swaths of the sky.

The research team included David Wittman, Anthony Tyson and David Kirkman of Bell Labs, the research and development arm of Lucent Technologies; Ian Dell'Antonio of NSF's Kitt Peak National Observatory and Brown University; and Gary Bernstein of the University of Michigan at Ann Arbor. Their results will be published in the May 11 issue of Nature.

Gravitational lensing relies on Einstein's prediction in his general theory of relativity that gravity bends light. The team analyzed the light from 145,000 very distant galaxies for evidence of distortions produced by the gravitational pull of dark matter that lay in their paths. To observers, the light from distant spherical objects is distorted by this gravitational pull into elliptical shapes, an effect known as cosmic shear. By analyzing the cosmic shear produced in thousands of galaxies, the researchers were able to determine the distribution of dark matter over large regions of the sky.

"The cosmic shear measures the structure of dark matter in the universe in a way that no other observational measurement can," Tyson said. "We now have a powerful tool to test the foundations of cosmology."

The measurements allowed the astrophysicists to test current predictions of the ultimate fate of the universe. According to models favored by cosmologists, the amount of dark matter helps determine the geometry of the universe. The researchers were able to rule out a scenario known as the standard cold dark matter model, in which there is enough ordinary matter and dark matter in the universe to eventually stop its expansion through gravitational force. Measurements of cosmic shear will enable a comparison of the dark matter distribution in the late universe with that of the early universe measured by observations of the cosmic microwave background.

The team used the Big Throughput Camera, designed and built expressly to measure cosmic shear, to take images of 145,000 distant galaxies along three different lines of sight. The camera was installed in the upgraded 4-meter Blanco telescope at the Cerro Tololo observatory.