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Pinning down dark matter's accelerating pull

By Robert C. Cowen / June 5, 2003



FOR ASTRONOMERS, the quest to understand the evolution of our universe has become a hunt for invisible "guiding hands."

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An international team of these hunters reported last week that it has found "the most direct evidence yet" that much of the matter in the universe is in a form that no eye or instrument can see.

The idea that the gravity of invisible "dark matter" may guide the motions of stars within galaxies and of galaxies in galaxy clusters has been around for some 70 years. Now lead investigator Francisco Prada of Germany's Max Planck Institute for Astronomy, together with colleagues from several other countries, have done what no dark-matter hunters could do before. They have studied such a large sample of galaxies that their statistics substantially reduce the uncertainties in their findings.

Their institutions are part of the Sloan Digital Sky Survey, which is mapping one-quarter of the entire sky in exquisite detail. This has given the hunters a data base of 250,000 galaxies within which they've found some 3,000 worth studying: large galaxies accompanied by small satellite galaxies.

The motions of the orbiting satellites reveal the gravitational influence of dark matter. While this is not a direct detection of that dark matter, it is what Anatoly Klypin of New Mexico State University in Las Cruces calls "a direct measurement of some of the properties predicted for dark matter." The galaxies all sit inside giant dark-matter concentrations that may be 50 times larger than they are.

While it's nice to know that dark matter really exists, astronomers have no idea what it's made of. Moreover, discovering its nature wouldn't end their hunt for "guiding hands." They estimate that dark matter constitutes about 27 percent of the mass of the universe. Add in the 3 percent of visible matter, and you have 30 percent of the cosmic mass. The other 70 percent may be more ghostly even than dark matter. Astronomers call it "dark energy." They can't see or measure it. They just think it should exist.

Over the past decade, astronomers have discovered that the well-known expansion of our universe appears to be accelerating. Something is overpowering the tendency for the mutual gravity of all the matter in the universe to slow the expansion. Dark energy, as now conceived, would exert the countervailing pressure.

Astronomers have reached that conclusion by tracking the universe's expansion. They found that an exploding star called a supernova type 1a has the same brightness wherever it lights up in the universe. This makes it a "standard" candle. You can tell how far away a galaxy is by how faint the candle appears. As light travels toward Earth, the expansion of space stretches its wavelength, making it increasingly red. Measuring how much light has stretched over particular periods maps the history of the expansion. These data indicate that expansion has begun to accelerate.

The supernovae research itself is also speeding up. Saul Perlmutter at the Lawrence Berkeley National Laboratory in California noted in the April issue of Physics Today that even Scrooge-like telescope scheduling committees have "dramatically increased" time allotments for such study. He suggests that "we live in an unusual time, perhaps the first golden age of empirical cosmology."

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