Brian Dunbar Headquarters, Washington, D.C. January 15, 1994 (Phone: 202/358-0873) Embargoed until 9:30 a.m. Michael Finneran Goddard Space Flight Center, Greenbelt, Md. (Phone: 301/286-5565) RELEASE: 94-10 SATELLITE FINDS IMPRINT OF UNIVERSE ON GAMMA-RAY EXPLOSIONS Astronomers have uncovered new evidence that huge explosions, known as gamma-ray bursts, occurred in the far reaches of the universe and bear an imprint of the universe's expansion. Analysis of data from NASA's Compton Gamma Ray Observatory satellite by a team led by Dr. Jay Norris of Goddard Space Flight Center, Greenbelt, Md., now indicates that gamma-ray bursts show relative "time-dilation." This is an effect that would be created by many of the bursts occurring so far away in the universe that time is seen to be running noticeably slower there. Time dilation is a consequence of the General Theory of Relativity and the expansion of the universe. Thus, time intervals from very distant parts of the universe will be stretched as the gamma-ray bursts make their way across the expanse of space, which is itself expanding. This much sought-after result provides additional evidence that gamma-ray bursts are not limited to the area of the Milky Way galaxy as some researchers have suggested. "This is a great result, one of the most spectacular astrophysical discoveries of the decade," said Professor Bohdan Paczynski of Princeton University. Paczynski and Dr. Tsvi Piran of Harvard University and Hebrew University of Jerusalem had previously predicted the effect in gamma-ray bursts. Norris was cautious about the meaning of time-dilation. "Our result should not be taken as proof that the time-dilation is a result of cosmological expansion of the universe -- just that a difference in durations of bright and dim bursts does exist and must now be accounted for by any theory," Norris said. - more - - 2 - "If time-dilation is a result of cosmology," added Goddard-based Dr. Robert Nemiroff of George Mason University and a member of the Norris team, "then this is not only an important discovery about gamma-ray bursts, it is a discovery that gamma-ray bursts may be able to tell us about distant parts of our universe." The Norris team, which includes astrophysicists at Goddard and NASA's Marshall Space Flight Center, Huntsville, Ala., NASA's Ames Research Center, Mountain View, Calif., and the University of Pennsylvania, uncovered gamma-ray burst time-dilation in two ways. First, they showed that dim bursts typically have twice the duration of bright gamma-ray bursts. Next, they showed that dim bursts typically are "redder" in gamma-ray color than bright bursts, an effect that is a direct result of time-dilation on the gamma-ray burst spectrum. Norris stressed that while the time-dilation effect itself is well quantified, the spectral difference, though very significant, is yet to be calibrated. Bursts Outshine Entire Gamma-Ray "Sky" Gamma-ray bursts are huge explosions that have been detected only in the gamma-ray region of the spectrum. Some last only a fraction of a second, but others are as long as a few minutes. A gamma-ray burst explosion dramatically outshines the entire sky in the gamma-ray band. The origins of gamma-ray bursts have been an enigma since their discovery in the late 1960s by U.S. defense satellites. But the Compton Gamma Ray Observatory has allowed astronomers to study these bursts in more detail than ever. Until Compton's observations of hundreds of bursts, it was widely believed that the sources of these powerful phenomena were in the Milky Way galaxy. The celestial distribution of gamma-ray bursts is uniform unlike the appearance of the Milky Way galaxy, which looks like a band in the sky. The fact that gamma-ray bursts come from all directions is what originally suggested a cosmological, or extra-galactic, origin, and so a search for time-dilation began. If the time dilation measured by Norris's team is a good indication of gamma-ray burst distance, then these bursts are occurring far into the universe. In addition, for Compton to see them as bright as they are, the power of these explosions may be greater than anything ever seen before, as much as one quintillion suns. - more - - 3 - The astronomical community is cautious about accepting this result blindly. "I like to think I'm objective," said Dr. Thomas Cline, a long-time gamma-ray burst researcher at Goddard, "and although the outcome of this analysis is consistent with the hoped-for time-dilation effect, I'm still concerned that it might result from a real but misleading feature of the changing luminosity of gamma-ray bursts or a misleading but unreal feature of the data or the satellite. That, of course, only heightens the mystery." "We've found time-dilation using several statistical tests," said Norris, who then listed several detailed mathematical and statistical tests that were performed on the data. The spectral difference between bright and dim bursts, also seen in a statistical sense, was found by comparing the spectral "colors" of bursts across the durations of gamma-ray burst events. Dr. Virginia Trimble of the University of Maryland and California, Irvine, said: "For more than 50 years, astronomers have been looking for objects or phenomena whose observed properties are dominated by the large scale evolution and structure of the universe ('cosmological effects') rather than by observational selection or the detailed evolution of the individual objects. If the gamma bursts have indeed revealed such cosmological effects, then this is perhaps even more important as an astrophysical 'first' than as a contribution to our understanding of the bursters themselves." The Norris team analyzed data from Marshall's Burst and Transient Source Experiment (BATSE) on board Compton, whose principal investigator is Dr. Gerald Fishman. The Compton Gamma Ray Observatory, deployed from the Space Shuttle Atlantis on April 7, 1991, is managed by Goddard for the Office of Space Science at NASA Headquarters, Washington, D.C. - end - Editors Note: The subject of this release will be discussed at a press conference at the American Astronomical Society meeting on Saturday, Jan. 15, at 9:20 a.m. in the Alexandria Room of the Crystal Gateway Marriott, Arlington, Va.