Radio ears: probing the universe's fate.

Will the universe continue expanding forever, or will it eventually stop expanding and begin to collapse? Powerful radio waves generated when pairs of high-energy jets of matter shoot out of the massive, compact centers of giant galaxies and crash into surrounding gas clouds may carry a remarkably direct answer to this question.

"This is a new tool for doing cosmology," says astrophysicist Ruth A. Daly of Princeton University, who has worked out a way to use the characteristics of these radio sources to detect changes in the universe's expansion rate.

Daly's method relies on the detection of a distinctive pattern of radio emissions from huge, distant galaxies with powerful, central energy sources thought to be black holes. in each case, the regions of radio emission appear as a pair of enormous, widely separated lobes that bracket and dwarf the galaxy itself.

The most energetic radio waves come from the outer edges of the lobes. There, the twin jets of particles traveling outward in opposite directions from the galactic center slam into the ambient gas and excite electrons, which whirl around in the gas cloud's magnetic field and emit intense radio waves.

Daly has found a way to estimate the distance between the two lobes of an extended radio source. Calculated on the basis of a link between the spectrum of radio frequencies emitted by a source and how long it took the lobes to form, this estimate doesn't depend on the distance of the radio source from Earth.

Daly can combine this information with observations from Earth of the source's angular width and its redshift the characteristic shift to longer wavelengths of radiation emitted by a receding source- to determine how much the rate of expansion of the universe has changed. "If the intrinsic sizes of the sources remain roughly constant with redshift, that means the universe is open," Daly says. In other words, expansion would continue forever.

When Daly applied her method to 10 radio sources several billion light-years from Earth, the results strongly favored an open universe. Now, she and a coworker are collecting and analyzing data on additional two-lobed radio sources to check these results. "I would like to know how representative those 10 sources are," she says.

Daly presented her findings at this week's American Astronomical Society meeting, held in Arlington, Va. A report describing the new technique and her preliminary results will appear in the May 1 ASTROPHYSICAL JOURNAL.