Gruber Cosmology Prize honors ‘dark matter’ astronomers

Marc Davis is one of four winners of the 2011 Gruber Cosmology Prize

Marc Davis is one of four winners of the 2011 Cosmology Prize of The Peter and Patricia Gruber Foundation.

University of California, Berkeley, astronomer Marc Davis will share with three other astronomers the 2011 Cosmology Prize of The Peter and Patricia Gruber Foundation, the foundation announced today (Wednesday, June 1).

The astronomers were honored for their computer simulations more than 20 years ago that convinced the world of the existence of “dark matter” and set off a so-far fruitless search to find out what it is.

Davis, a UC Berkeley professor of astronomy and physics who led the project, will share the $500,000 prize with George Efstathiou, the director of the Kavli Institute for Cosmology in Cambridge, England; Carlos Frenk, the director of the Institute for Computational Cosmology at Durham University in England; and Simon White, a director of the Max Planck Institute for Astrophysics in Garching, Germany.

The foundation cited the four scientists “for their pioneering use of numerical simulations to model and interpret the large-scale distribution of matter in the Universe.” The work by Davis, Efstathiou, Frenk and White, most of which was conducted at UC Berkeley, “galvanized support for ‘cold dark matter’ as the dominant form of matter in the Universe and has thus been instrumental in the crafting of our current cosmological paradigm,” the foundation said in a statement. The Gruber Prize recognizes both the discovery method that the team introduced as well as the collaboration’s subsequent discoveries.

Davis and his three colleagues will receive a gold medal at a ceremony this fall, where they each will deliver a lecture.

The team’s work was conducted in the 1980s, when astronomers recognized that some invisible matter permeated the Milky Way and other galaxies, but models of the Big Bang and the subsequent evolution of the universe were not sophisticated enough to distinguish between different theories.

The three reigning proposals were hot dark matter, composed of relativistic particles traveling at close to the speed of light; cold dark matter, comprising slow moving particles; and warm dark matter, midway between the two.

In 1981, while a young professor of astronomy at Harvard University in the Harvard-Smithsonian Center for Astrophysics (CfA), Davis surveyed 2,400 galaxies at various distances – an extraordinary census of how the heavens look on the largest scales – and made a discovery that astounded the astronomical community. The universe wasn’t just a uniform scattering of galaxies, but a “cosmic web” of galaxies grouped into filaments separated by vast voids.

“At the time, nobody had any idea what the large scale distribution of matter was, and mostly we didn’t think about it,” Davis said. “I soon saw that the best mathematical model of the Big Bang we had was wrong, there was a complete disconnect between our CfA observations and the theory.”

“At that point,” he said, “I became convinced that the only alternative was simulations,” that is, computer calculations of how thousands of galaxies move under the influence of gravity, with dark matter thrown into the mix.

Davis teamed up with White, who was a researcher at UC Berkeley’s Space Sciences Laboratory, recruited Frenk as a post-doctoral fellow, and collaborated with Efstathiou, who was then at Cambridge University’s Institute of Astronomy and had developed a clever way to simulate billions of years of galaxy evolution. Together, they were referred to as the DEFW collaboration, or the Gang of Four. Working mostly at UC Berkeley, the team in the late 1980s produced simulations that ruled out hot and warm dark matter, but showed that cold dark matter could produce the clumps, filaments and voids observed in space.

“That was the clincher,” Davis said. “We said that the universe looks like it is dominated by cold dark matter, and everybody was convinced after that.”

“The DEFW papers were instrumental in ushering in a new era where numerical simulations became a standard tool of cosmological studies,” according to Wendy Freedman, Crawford H. Greenewalt Chair and director of The Observatories of the Carnegie Institution of Washington, and chair of the 2011 Selection Advisory Board to the Gruber Cosmology Prize.

“Boy, that was just a fun thing to do, we really had a great time, we loved it,” Davis said. “And it really got a name for UC Berkeley, which became the center of all this work.”

Interestingly, the simulations also hinted at another “energy” in the universe aside from normal matter and cold dark matter. But like Einstein before them, the astronomers balked at including a cosmological constant in their calculations. A mere 10 years later, a mysterious “dark energy” was discovered to permeate the universe. Today, cosmologists estimate that ordinary matter – stars and stuff composed of stardust, including life – comprises only 4.6 percent of the universe. About 23.3 percent is dark matter, and the remainder, 72.1 percent, is dark energy.

Though simulations like those of the DEFW team can now precisely model the behavior of dark matter, the identity of this exotic matter is still unknown. Since their work’s culmination in 1989, many experiments looking for unusual elementary particles or dim substances between the stars have proved fruitless.

“Because of some wonderful experiments, we’re driving dark matter to real extremes, and we’re not sure what the hell to do now,” Davis said, referring to experiments like the Cryogenic Dark Matter Search that have set limits on what dark matter can be, but have yet to specify what it is. “The next best hope is the Large Hadron Collider (at CERN), but if that doesn’t tell us anything, we’re never going to know what dark matter is.”

Since his work on dark matter, Davis has been involved in several surveys, including an unprecedented survey of all galaxies at a redshift of one, performed with the Keck Telescope in Hawaii. The DEEP2 survey has proved the starting point for many subsequent observations and astronomy missions in space, he said.

“What got us engaged from the beginning was an interest in how galaxies are arranged in three dimensions and the question: Does it tell us anything about the nature of the universe?” Davis said. “The answer is, yes, it does, very clearly.”

The Gruber International Prize Program honors contemporary individuals in the fields of cosmology, genetics, neuroscience, justice and women’s rights, whose groundbreaking work provides new models that inspire and enable fundamental shifts in knowledge and culture. UC Berkeley and Lawrence Berkeley National Laboratory physicist Saul Perlmutter received the Gruber Cosmology Prize in 2007 for the discovery of dark energy.

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