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CMU astrophysicist on team probing 'dark energy'

Using a telescope at the South Pole, scientists studied radiation from the early universe

Friday, December 13, 2002

By Byron Spice, Post-Gazette Science Editor

Most of the universe consists of matter that can't be seen and a mysterious energy that nobody understands, and today, astrophysicists are releasing the latest exacting measurements of what they do not know.

Using a telescope at the South Pole, a scientific team has studied the remnants of radiation released from the early universe and calculated that 30 percent of the universe's energy is in the form of "dark matter," a strange form of matter that doesn't interact with light, and that 65 percent of the energy is an even stranger "dark energy" that is accelerating the expansion of the universe.

That means that the light and matter that we can see and feel around us, as well as that comprising all of the stars we see in the sky, represents just 5 percent of all of the universe's energy.

"All of this makes cosmology sound fishy," admitted Jeff Peterson, a Carnegie Mellon University astrophysicist and one of the investigators. Nobody had even anticipated something like dark energy when Peterson and his Carnegie Mellon team were building the Viper telescope, which is 6 feet in diameter and was installed five years ago at the South Pole.

But in a strange way, the results also are consistent with other observations, all of which have to do with what's called the cosmic microwave background. This is radiation that was released about 300,000 years after the Big Bang, when the universe had cooled enough so that protons, neutrons and electrons could form atoms. Before that, the universe was an opaque swarm of subatomic particles.

The new findings, which have been submitted to the Astrophysical Journal for publication, come from National Science Foundation-sponsored observations using a sensitive new receiver, called the Arcminute Cosmology Bolometer Array Receiver, or ACBAR, mated to the Viper telescope. The investigators looked at subtle temperature differences in the microwave background, which equate with variations in the density of energy. From a physics point of view, energy and matter are equivalent.

Microwave radiation is absorbed by water vapor, so observations of the cosmic background are best made in areas with dry, thin air, such as Antarctica, or space.

The results from ACBAR are similar to those reported in the summer by the Cosmic Background Imager, a set of radio antennas located high in Chile's Atacama Desert.

The ACBAR investigators, principally from the University of California, Berkeley, Case Western Reserve University in Cleveland and Carnegie Mellon, anticipate that similar findings will be reported next month from NASA's Microwave Anisotropy Probe, or MAP, a space-based observatory.

Berkeley's William Holzapfel, a Peters native, said the MAP satellite is surveying the entire sky, while those using the South Pole telescope studied just a small chunk of sky in much greater detail. The two results should be complementary, much as a map of the United States would complement a street map of Pittsburgh.

The results also are consistent with theoretical predictions, added Holzapfel, a co-principal investigator.

For years, cosmologists have talked about dark matter, arguing that the universe behaved in ways suggesting that it contained more mass than could be found in visible stars.

Though the ACBAR findings indeed suggest that dark matter is a major component of the universe, "I think the story is all about dark energy," Peterson said.

No one suspected such a thing existed until almost six years ago, when observations of distant exploding stars first provided evidence that the expansion of the universe was not being slowed by gravity, but was actually speeding up for reasons unknown.

"Dark energy" isn't an entirely satisfactory way to describe this phenomenon, Peterson acknowledged, "but things are changing so fast we've run out of names to call stuff."

Byron Spice can be reached at bspice@post-gazette.com or 412-263-1578.

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