Galaxy survey sheds light on dark matter

By looking at some of the brightest objects in the universe, astronomers are learning new details about its darkest stuff.

Simulation shows that galaxies (dots) trace the dark matter (gray). Red box indicates a dense cluster of galaxies; green box, a more typical, lower-density region. A. J. Benson et al./Monthly Notices Roy. Astro. Soc.

Known as dark matter because it emits no light, this substance accounts for at least 90 percent of the mass of the cosmos. Astronomers hold that dark matter keeps each galaxy intact, holds galaxy clusters together, and originally prompted gas and dust to coalesce into galaxies.

Using 127,000 galaxies whose distances were measured as part of the largest galaxy survey so far, researchers have determined that on scales of millions of light-years, dark matter is distributed in exactly the same way as the galaxies are. Gravity pulls galaxies into a configuration of sheets and filaments separated by huge voids. This pattern accurately traces the invisible dark matter, says Alan F. Heavens of the University of Edinburgh.

A team led by Heavens and Licia Verde of both Rutgers University in Piscataway, N.J., and Princeton University recently posted its findings on the Internet (

Analyzing galaxies from the same survey in a different way, another group came to the same conclusion about dark matter. Ofer Lahav of the University of Cambridge in England, Sarah L. Bridle of the University of Edinburgh, and their colleagues have posted their results at

The new data, which end a 20-year debate about whether galaxies are a good tracer of dark matter, “will place strong constraints on theories of where and how galaxies form,” Heavens says.

In agreement with previous studies, his team finds that dark matter comes up short. It provides only about one-third of the density of matter and energy required to keep the universe flat.

Yet a flat universe, in which parallel lines never meet, is just the type of cosmos indicated by measurements of the cosmic microwave background, the radiation left over from the Big Bang (SN: 4/28/01, p. 261: Sounds of the universe confirm Big Bang). Since dark matter can’t keep the universe flat, some additional source of matter or energy must be at play.

Studies of distant supernovas suggest that there is such a source but that it has an odd property: It causes the universe to rev up the rate at which it is expanding (SN: 3/31/01, p. 196: Starry Data Support Revved-Up Cosmos). This mystery source, dubbed dark energy, is an entity entirely separate from dark matter.

The combination of the results of the two galaxy studies and their correlation with the supernova and microwave background observations “is a clear indication that we are on the right track to understanding the origin and evolution of our universe,” says Verde.

“The studies represent a new and important measurement of the density of matter in the universe,” notes David N. Spergel of Princeton. “They appear to be consistent with the emerging consensus . . . of a universe with roughly 30 percent dark matter, 3 percent ordinary matter, and 67 percent dark energy.”

The teams relied on data from the 2dF (2-degree field) galaxy redshift survey, which has mapped the locations of 210,000 galaxies and will measure another 40,000. A more detailed survey, the Sloan Digital Sky Survey, will ultimately map the positions of a million galaxies.

A second result from Lahav’s team may help reveal the state of the infant cosmos. The researchers compared the fluctuations in the density of galaxies in the 2dF survey with larger scale fluctuations in the cosmic microwave background. Those fluctuations are believed to be the seeds of the structure in the cosmos today.

The analysis shows that today’s fluctuations are 20 percent smaller than indicated by the simplest extrapolation of those in the microwave background. The mismatch could be a clue about the nature of the brief early era of hyperexpansion that may have begun the Big Bang. The new results suggest that this inflation slowed down as it petered out, or ended when the universe was still very hot, or both, says Joel R. Primack of the University of California, Santa Cruz.

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