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Astronomy: Exploring the Expanding Universe

The Hunt for Dark Matter

Even with all of the galaxies that Bothun and others expect to find, researchers still say much of the matter in the universe is unaccounted for.

According to the Big Bang theory, the nuclei of simple atoms such as hydrogen and helium would have started forming when the universe was about one second old. These processes yielded certain well-specified abundances of the elements deuterium (hydrogen with an extra neutron), helium, and lithium. Extensive observations and experiments appear to confirm the theory's predictions within specified uncertainties, provided one of two assumptions is made: (1) the total density of the universe is insufficient to keep it from expanding forever, or (2) the dominant mass component of the universe is not ordinary matter. Theorists who favor the second assumption need to find more mass in the universe, so they must infer a mass component that is not ordinary matter.

Computer simulation of dark matter - click for details Part of the evidence for the second theory was compiled by Vera Rubin, an astronomer at the Carnegie Institution of Washington who received NSF funding to study orbital speeds of gas around the centers of galaxies. After clocking orbital speeds, Rubin used these measurements to examine the galaxies' rotational or orbital speeds and found that the speeds do not diminish near the edges. This was a profound discovery, because scientists previously imagined that objects in a galaxy would orbit the center in the same way the planets in our galaxy orbit the Sun. In our galaxy, planets nearer the Sun orbit much faster than do those further away (Pluto's orbital speed is about one-tenth that of Mercury). But stars in the outer arms of the Milky Way spiral do not orbit slowly, as expected; they move as fast as the ones near the center.

What compels the material in the Milky Way's outer reaches to move so fast? It is the gravitational attraction of matter that we cannot see, at any wavelength. Whatever this matter is, there is much of it. In order to have such a strong gravitational pull, the invisible substance must be five to ten times more massive than the matter we can see. Astronomers now estimate that 90 to 99 percent of the total mass of the universe is this dark matter-it's out there, and we can see its gravitational effects, but no one knows what it is.

At one of NSF's Science and Technology Centers, the Center for Particle Astrophysics at the University of California, Berkeley, investigators are exploring a theory that dark matter consists of subatomic particles dubbed WIMPs, or "weakly interacting massive particles." These heavy particles generally pass undetected through ordinary matter. Center researchers Bernard Sadoulet and Walter Stockwell have devised an experiment in which a large crystal is cooled to almost absolute zero. This cooling restricts the movements of crystal atoms, permitting any heat generated by an interaction between a WIMP and the atoms to be recorded by monitoring instruments. A similar WIMP-detection project is under way in Antarctica, where the NSF-supported Antarctic Muon and Neutrino Detector Array (AMANDA) project uses the Antarctic ice sheet as the detector.

In the spring of 2000, NSF-supported astrophysicists made the first observations of an effect predicted by Einstein that may prove crucial in the measurement of dark matter. Einstein argued that gravity bends light. The researchers studied light from 145,000 very distant galaxies for evidence of distortion produced by the gravitational pull of dark matter, an effect called cosmic shear. By analyzing the cosmic shear in thousands of galaxies, the researchers were able to determine the distribution of dark matter over large regions of the sky.

Cosmic shear "measures the structure of dark matter in the universe in a way that no other observational measurement can," says Anthony Tyson of Bell Labs, one of the report's authors. "We now have a powerful tool to test the foundations of cosmology."

PDF Version
Voyage to the Center of the Sun
New Tools, New Discoveries
At the Center of the Milky Way
The Origins of the Universe
The Hunt for Dark Matter
Shedding Light on Cosmic Voids
Visualizing the Big Picture
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