Balloon Sounds Out the Early Universe.

Intrepid explorers braved the unknown to find that the world is round. Now, a detector suspended from a balloon circling the frigid Antarctic has measured the curvature of the universe and revealed that it's perfectly flat.

The balloon-borne experiment detected tiny fluctuations in the temperature of the cosmic microwave background, the whisper of radiation left over from the Big Bang. This energetic radiation cooled to microwave energies as it traveled through space for some 13 billion years.

The new data, from the Italian-U.S. experiment BOOMERANG (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics), represent the most detailed images ever taken of the infant universe, says Andrew E. Lange of the California Institute of Technology in Pasadena.

Several fundamental parameters lie hidden in the maps of the microwave background that Lange and his colleagues have constructed, researchers say.

The new findings, several cosmologists note, strongly support a theory called inflation, in which a burst of expansion enlarged the cosmos from subatomic size to cosmic proportions--all within a minuscule fraction of a second. In the process, tiny fluctuations in density would have been amplified, giving rise to today's superclusters of galaxies.

Lange and his colleagues describe their findings in the April 27 NATURE.

"This is what we've been hoping for the last few years, that we would enter into an era of precision cosmology, where we're able to study the properties of the early universe," comments Wayne Hu of the Institute for Advanced Study in Princeton, N.J. In 1991, the Cosmic Background Explorer first revealed fluctuations in the microwave background but averaged temperature variations over huge patches of sky. Such large-scale measurements can't trace in detail the conditions in the infant cosmos, Hu notes.

BOOMERANG measures the microwave background on much smaller scales. Its data provide the clearest evidence for primordial sound waves, which theorists have suggested are part and parcel of the microwave background. These sound waves "probe the conditions of the early universe as a kind of cosmic ultrasound," Hu writes in a commentary in the same issue of NATURE.

In the very hot, very young universe, he explains, matter and photons--particles of light--were tightly coupled. Photons bounced between electrons and couldn't travel freely. Whenever gravity compressed the matter, the pressure exerted by the photons offered resistance, reversing the motion and setting up acoustic oscillations--alternations of higher and lower pressure with the same physical form as sound waves. The compression raised the temperature of the microwave background ever so slightly, while expansion lowered it--creating the hot and cold spots seen by BOOMERANG and similar experiments.

After about 300,000 years, the cosmos cooled enough for electrons and protons to combine to form hydrogen atoms. Because photons aren't bounced back and forth by atoms, they could suddenly stream freely into space. This radiation, detected by BOOMERANG 13 billion years later, reveals the pattern of the sound waves, which bears the imprint of the shape and other characteristics of the early universe.

Cosmologists have long tried to measure cosmic curvature, the extent to which matter and energy curve space according to the principles of Einstein's theory of general relativity, notes Michael S. Turner of the University of Chicago and the Fermi National Accelerator Laboratory in Batavia, Ill. A host of other experiments, he says, including a test flight of BOOMERANG, had already suggested that the universe isn't curved. This means that the cosmos has just the right density of matter and energy to expand forever instead of collapsing in a Big Crunch. The new data, from BOOMERANG's 11-day flight in late 1998, show "that the pattern of hot and cold spots on the microwave sky is undeniably that of a flat universe," Turner adds. "Our equations really seem to mean something."

Combined with other measurements, the new work confirms a gap in the cosmic ledger book. Because the mass that's been measured isn't enough to make the universe flat, there must be some additional "dark energy," Turner says. This energy could cause the universe to rev up its rate of expansion, a bizarre notion that recent observations support (SN: 2/12/00, p. 106).

BOOMERANG reveals that the temperature fluctuations in the microwave background are greatest when measured over patches of sky of a certain size. This size, theorists say, corresponds to the longest sound wave that existed when the universe was 300,000 years old. If the inflation theory is correct, the microwave background must exhibit a series of such peaks corresponding to shorter wavelengths, just as a musical instrument plays several overtones. With only 5 percent of the 1998 experiment's data analyzed, BOOMERANG neither reveals nor excludes a second peak, Lange says.

It's too soon to worry, says Turner. Other tests now under way and the expected launch next year of the Microwave Anisotropy Probe may yet reveal the missing peaks, he says.

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