A blown-up portion of the sky, observed through a telescope in Chile, that the researchers used in their analysis. Photo: Bell Labs

Astrophysicists at Bell Labs have detected the long-sought large-scale distribution of an invisible form of matter, called "dark matter," that pervades the universe.

This invisible matter makes its presence felt through its gravity and constitutes more than 90 percent of the mass in the universe. Since it cannot be seen by telescopes, its nature and distribution have puzzled astronomers for decades, making the quest to understand it one of the biggest challenges of science. This is the first time its distribution has been mapped over significant regions of the sky that were thought to be devoid of clumps of matter.

To obtain the dark matter distribution, David Wittman, Anthony Tyson and David Kirkman of Bell Labs, along with two collaborators, Ian Dell'Antonio of both the National Optical Astronomy Observatory and Brown University, and Gary Bernstein of the University of Michigan at Ann Arbor, used a method known as weak gravitational lensing, in which they analyzed the light from 145,000 very distant galaxies for evidence of distortions produced by dark matter that lay in its path.

Einstein's general theory of relativity predicts that gravity bends light; in weak gravitational lensing, dark matter in the foreground produces a distortion or "shear" in the shapes of background galaxies. By analyzing the cosmic shear produced in these thousands of galaxies, the researchers were able to obtain the distribution of dark matter over large regions of the sky. They are reporting their results in the May 11 issue of the journal Nature.

Testing the foundations of cosmology
"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 cosmic shear measurement allowed the astrophysicists to test current predictions of the ultimate fate of the universe. According to models favored by cosmologists, the amount of cosmic dark matter will determine whether the universe will continue to expand forever, slow down to a halt, or one day collapse on itself.

Based on their analysis, Wittman and his colleagues were able to rule out a well-known cosmological 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. Instead, their observations support an alternative universe, which contains a certain amount of vacuum energy that causes it to expand more rapidly over time.

The figure shows a schematic view of weak gravitational lensing by large-scale mass structure: distant galaxy orientation is correlated on scales characteristic of the lensing dark matter structures. Light bundles from two distant galaxies which are projected closely together on the sky follow similar paths and undergo similar gravitational deflections by intervening dark matter concentrations. Apparent orientations of distant galaxies are thus correlated on angular scales of less than a few degrees. The larger the mass in the gravitational deflectors, the larger the faint galaxy ellipticity correlations on a given angular scale. These ellipticity correlations of distant galaxies reveal the statistics of the large-scale dark matter distribution in the intervening universe -- a central diagnostic of the underlying cosmology. Photo: Bell Labs, Lucent Technologies

A special camera measured cosmic shear
The astrophysicists used a camera they designed and built expressly to measure cosmic shear to take images of 145,000 distant galaxies using the 4-meter Blanco telescope at the National Science Foundation's Cerro Tololo Interamerican Observatory in Chile. They figured out a way to control imaging errors -- a problem introduced when the light from distant sources passes through the Earth's atmosphere as well as a telescope's optics -- by using thousands of foreground stars to correct these errors.

Cosmic shear introduces certain similarities in the images of background galaxies that appear close together on the sky. Light from such galaxies passes through similar intervening volumes of dark matter and gets bent by the dark matter's gravity. Encoded in the distorted light is information about the dark matter distribution.

The researchers were able to set up an automated data processing pipeline to analyze the thousands of galaxy images and decode the dark matter distribution. Since galaxies are typically football-shaped, detecting any additional stretching of background galaxies is difficult and many thousands of galaxies had to be averaged to conclusively detect cosmic shear.

The future is bright for dark matter
"This technique is maturing rapidly," Tyson said. "In the future, similar studies will use tens of millions of distant galaxies to see how the distribution of mass in the universe evolved with time."

Further measurements of cosmic shear planned by Tyson and his colleagues, when combined with observations of the cosmic microwave background -- a diffuse radiation that is a remnant of the Big Bang explosion that created the universe and is present everywhere -- will enable them to compare the dark matter distribution in the universe today with what it was when the universe was a lot younger. When they succeed, they will cast light on one of cosmology's most intractable problems.

"The future is bright for dark matter," Tyson joked.