When an exploding star sent subatomic particles hurtling through detectors deep beneath the earth in February, scientists were elated. Not only did their sighting of so-called electron neutrinos give them their first direct glimpse at the inner workings of a supernova. They also hoped that data from the sightings would help settle the question of whether these particles alone have enough mass to cause the gravitational collapse of the universe.
Far from the unambiguous answer some had hoped for, recent calculations by several researchers using supernova data indicate: maybe, maybe not. Some even suggest that the issue of neutrino mass may not be as relevant to the universe's future as once thought.
``It's not a simple problem,'' says physicist Fredrick Reines of the University of California at Irvine, because the answer one gets depends on the assumptions a researcher makes. Initial calculations of the mass do not ``close the book'' on the issue, he says.
Neutrinos are a class of subatomic particles that are electrically neutral, have virtually no mass, and travel at the speed of light. Because they react only weakly with other forms of matter, they are very hard to spot. In fact, scientists refer to what they see in their detectors as ``neutrino events,'' because they don't see the neutrino directly. Instead they see its effect on other matter, such as pure water. This characteristic has led astrophysicists to include neutrinos in a category called dark matter, unseen matter whose existence in the universe is inferred by its gravitational effect on visible matter.
It's this dark matter that appears to hold the key to the future of the universe. The theory currently in vogue holds that the universe contains just the density of mass needed to keep it from collapsing. As a result, this ``flat'' universe could be expected to expand forever, though at an ever slower rate. But, says physicist Alan Guth of the Massachusetts Institute of Technology, when astronomers add up the mass of what they can see plus the mass of the dark matter thought to surround those objects, they come up with only about 20 to 40 percent of the mass density that the theory predicts. The ability to detect and pin down the remaining mass would either help confirm a flat universe, or presumably tip the balance either toward a greater expansion or eventual collapse.
Neutrinos became an early candidate for ``dark matter'' because researchers knew they existed and calculations suggested that they were abundant, according to Lawrence M. Krauss, a physicist at Yale University and one of the researchers working with the neutrino data from the supernova.
An initial estimate of the mass of the electron neutrinos from Supernova 1987A has come from Sheldon Glashow of Harvard University and John Bachall of the Institute for Advanced Study in Princeton, N.J. They put an upper limit of 11 electron volts on the electron neutrino's mass. (For comparison, the energy represented by 100 trillion electron volts would light a 100-watt bulb.) At that level, electron neutrinos could be fairly well ruled out as having enough mass to close the universe. That would still leave two other types of neutrinos that are thought to be much more massive than electron neutrinos and just as numerous.
But Dr. Krauss, speaking to a group of researchers at the Harvard-Smithsonian Center for Astrophysics earlier this week, put an upper limit of from 20 to 30 electron volts on the electron neutrino's mass. There are indications that others are coming to similar conclusions. If it turns out that the electron neutrino's mass falls within that range, these particles could in principle close.
All this assumes that neutrinos make up the dominant form of matter in the universe. But recent attempts to run computer simulations of the evolution of the universe based on that assumption aren't squaring with the structure of the universe that astronomers see.
While not eliminating neutrinos from consideration, these discrepancies are leading researchers to look for other forms of ``dark matter.''