PHILADELPHIA, JAN. 17 -- Data collected by Astro, the observatory complex carried into orbit on the shuttle Columbia last month, have shot down a promising theory about what makes up the "missing mass" in the cosmos, scientists said today.
The Hopkins Ultraviolet Telescope (HUT), one of four Astro instruments that flew in the shuttle's cargo bay, failed to detect the presence of an invisible nuclear particle called the tau neutrino, thought by some theorists to pervade galaxies and galactic clusters.
"Our findings," said HUT scientist Arthur Davidsen of Johns Hopkins University, "exclude the predictions of this theory at a 99.7 percent confidence level."
Astronomers have known for years that there has to be much more mass in the universe than has been detected -- perhaps as much as 10 times more. Without the gravitational forces that this "dark matter" creates, spiral galaxies would have flown apart like shattered pinwheels and other vast conglomerations would have broken up into wandering clots of gas and star debris.
The puzzle has inspired various theories. One that generated considerable excitement was advanced by Oxford University cosmologist Dennis Sciama: The dark matter might consist of heavy tau neutrinos created in the first moments after the Big Bang, the primeval fireball believed to have produced the universe. If these particles -- which so far have never been seen by scientists -- actually exist, and if they have a measurable mass, then their presence might solve one of the most confounding enigmas of modern science.
One of HUT's experiments was designed to detect the presence of these particles in a cluster of galaxies called Abell 665. By calculating how much mass the entire cluster contains, the HUT scientists determined the number of neutrinos that should be there. The neutrinos themselves are invisible; but when they decay, they give off a telltale emission of photons (light particles) in the ultraviolet range. So measuring these emissions should reveal the neutrino population just as one could determine the number of frogs in a pond by listening to the volume of the croaking.
The HUT results, however, showed that the specific kind of neutrinos predicted by Sciama's theory were not there.
"Of course, neutrinos could still have mass, and could account for the dark matter," Davidsen said. "But they do not decay at the rate predicted by this theory."
Other Astro results announced yesterday were more encouraging to theorists. Data from the mission's Wisconsin Ultraviolet Polarimeter Experiment, for example, provided evidence in what project scientist Kenneth Nordsieck called "unprecedented detail" for a 25-year-old theory about the composition of the interstellar medium, the dust and gas between stars.
WUPPE's findings showed that there are at least three distinct components to the dust: particles of graphite (a form of carbon); elongated silicate particles that contain impurities such as iron and thus tend to align themselves along a galaxy's magnetic field and polarize some of the radiation passing through the dust; and small nonaligned particles.
Astro's Broad Band X-Ray Telescope group from NASA's Goddard Space Flight Center in Greenbelt displayed a number of findings, including new evidence that iron is much more widely distributed in galaxies than previously thought; and Goddard's Ultraviolet Imaging Telescope team released photographs that will aid scientists in understanding the structure and forces at play in such objects as globular star clusters, spiral galaxies and supernova fragments -- the dismembered remnants of exploded stars.
Because of a series of technical glitches, the Astro team was able to hit only 30 percent of its planned astronomical targets before its nine-day mission ended.