Dark Matter Mapped in 3-D, Scientists Report

John Roach
for National Geographic News
January 8, 2007
A new three-dimensional map of the universe's dark matter provides compelling evidence that the mysterious substance is the scaffolding upon which stars and galaxies are assembled.

Scientists are still not sure exactly what dark matter—an invisible form of matter that does not give off or reflect light yet accounts for the vast majority of mass in the universe—is made of. (Related: "Dark Matter Proof Found, Scientists Say" [August 22, 2006].)

But "without dark matter as we are seeing here, the universe wouldn't exist as it is today," Richard Massey, an astronomer at the California Institute of Technology (Caltech) in Pasadena, said at press briefing yesterday in Seattle, Washington.

Massey likened the dark matter to scaffolding that both holds the universe together as it expands after the Big Bang and provides a framework for the assembly of normal matter.

The map was published online yesterday by the science journal Nature. Massey and his colleagues presented details of the study at the 209th meeting of the American Astronomical Society in Seattle, Washington.

"For the first time we have been able to map out the large-scale distribution of this invisible, mysterious dark matter," Massey said.

Searching the COSMOS

The map was derived from nearly a thousand hours of observations of 575 overlapping patches of sky with the Hubble Space Telescope's high-resolution cameras (see an interactive Hubble diagram). The imaging was part of a project called COSMOS—the Cosmic Evolution Survey.

(Related: "Hubble Repair Mission Approved by NASA" [October 31, 2006].)

"It's the largest project that's ever been done by the space telescope," noted Caltech team member Nick Scoville, principal investigator for COSMOS.

Additional space and ground telescopes in Hawaii and Chile helped give the map its depth and color.

Richard Ellis, also a Caltech team member and a co-principal investigator for COSMOS, said the research collectively shows "what an important role dark matter plays in shaping the universe."

Gravitational Lens

The three-dimensional mass map was developed by measuring the shapes of about a half million faraway galaxies.

To reach Earth, the light from the galaxies must pass through dark matter. The matter bends and warps the path the light takes.

While the researchers cannot see galactic light undistorted, statistical analyses of hundreds of galaxies allow the scientists to predict how the galaxies should look in the absence of dark matter, Massey explained.

The researchers then inferred dark matter's presence and distribution by the way the galactic light bent, a technique known as gravitational lensing.

The map, which stretches back halfway to the beginning of the universe, shows dark matter strung out in a network of filaments that grow over time.

The filaments intersect in massive structures at the same places where galaxy clusters are located, supporting theoretical models that normal matter—such as galaxies—accumulates along the densest concentrations of dark matter.

Since the map stretches back in time, the researchers are also able to show how dark matter has grown increasingly clumpy as it has collapsed under gravity.

Astronomers likened the challenge of mapping the universe's mass to mapping a city with only the aid of aerial photos taken at night. Looking at the new maps is equivalent to seeing a city, its suburbs, and country roads in daylight.

Darkness to Light?

As astronomers continue to study the growth of clustering in the dark matter, they may also gain insight to the nature of dark energy, another mysterious force believed to permeate the universe.

Dark energy is a repulsive force opposing gravity. It is thought to influence how dark matter clumps together.

"This growth of these [dark matter] structures is a competition between the attractive force of gravity and the repulsive force, or almost an antigravity, of the dark energy," said Jason Rhodes, a study collaborator from the Jet Propulsion Laboratory in Pasadena.

Follow-up missions, Rhodes added, will use the same gravitational lensing technique as the COSMOS mission to determine the nature of dark energy.

And what about the nature of dark matter?

"We haven't got an answer of what is dark matter yet," Massey said. "We got the first step though—that is, where is the dark matter."

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