Elusive Neutrinos Thought to Be Key to Future of Universe

Two of the most esoteric fields of science are joining in a marriage that could help determine whether the universe will go on expanding forever or ultimately collapse on itself in a final big bang.

Physicists who are building a new generation of multimillion-dollar instruments to detect gravity waves are joining with astronomers who are trying to measure what may be immeasurable--a subatomic particle so small it defies imagination, but so numerous that if it has even a little mass it may well determine the future of the universe.

The marriage of these two very different fields could produce quality data that “we can really believe in,” said UCLA professor David Cline, the prime mover behind the program.

Most of the universe is believed to be made up of mysterious “hidden matter” that cannot be seen. Cosmologists speculate that if there is enough hidden matter, the force of gravity will be so great that the universe eventually will stop expanding and finally collapse.

But what could be massive enough to cause the universe to ultimately contract, and yet still remain invisible?

Some theorists believe the answer might lie in neutrinos, which are manufactured continuously when stars like the sun burn their fuel. Neutrinos are also created in great bursts when stars explode or collide. It was, in fact, just such an explosion that brought several dozen experts to Los Angeles last week for a symposium hosted by UCLA.

Discover Supernova

Two years ago, an exploding star, called a supernova, was discovered in the southern sky, and it was the brightest supernova seen in 400 years, according to historical records. The discovery sent astronomers around the world scrambling for their instruments to witness one of nature’s most spectacular shows, and it has spawned several major new areas of research because a supernova is one of nature’s rarest laboratories, offering a view deep into the heart of the physics of the universe.

Scientists who have studied the supernova have found that most of their earlier theories have stood the test well. Just as the theories suggested, the supernova has demonstrated that exploding stars produce the heavy elements that make up so much of the universe.

More recently, scientists have discovered what they had expected to find at the center of the supernova, a spinning neutron star called a pulsar that sends off energy like a spinning lighthouse. But no one expected the pulsar to be spinning as rapidly as it apparently is: 2,000 times a second.

That rapid spin rate is one of the bigger surprises in Supernova 1987A, which is in a nearby galaxy called the Large Magellanic Cloud, about 170,000 light years away.

Equally important, however, was the discovery that the supernova was sending out neutrinos. In all, 20 neutrinos were detected in two large detectors, one in Japan and the other in Ohio. That alone was considered a significant achievement, since neutrinos do not have an electrical charge and thus are extremely difficult to capture. Trillions of them are constantly passing through the Earth without ever touching anything, but occasionally one collides with an atom in a neutrino detector, and the debris from that collision tells scientists something--but not much--about neutrinos.

Not Enough Data

So while the detection of neutrinos from Supernova 1987A was somewhat of a triumph, the data gained was not adequate to determine whether neutrinos have enough mass to be significant in the cosmological question concerning the fate of the universe.

And that set UCLA’s Cline to thinking. Cline, who teaches both physics and astronomy, decided that if he could get the gravity wave physicists working with the neutrino astronomers, they might collectively solve one of the great problems of modern astronomy.

The way to determine whether a neutrino has any mass is to figure out the speed at which it travels. If it has no mass, it would be a packet of pure energy, traveling at the speed of light. Even a slight mass would slow it a little.

For Supernova 1987A, scientists tried to determine the speed of the neutrinos on the basis of the time of the sighting of the explosion. Since they arrived at Earth at virtually the same time as the visible light, the 20 detected neutrinos did not have enough mass to be significant.

But all 20 of those were of one type, called electron neutrinos. There are two other types, called the muon and tau neutrinos, which are stronger candidates for having significant mass, and which were not detected from Supernova 1987A.

New Detectors Planned

More-sensitive detectors are planned for the years ahead, but even if muon and tau neutrinos can be detected, there remains the problem of determining how long it took them to get here.

And that brought the gravity physicists and the neutrino astronomers together last week for the UCLA conference.

A number of major gravity wave detectors are being planned around the world, including a $100-million joint project by Caltech and MIT that the National Science Foundation is expected to finance. The new detectors should be sensitive enough to detect gravity waves created by a supernova.

In his General Theory of Relativity, Albert Einstein postulated that some events, such as colliding or exploding stars, would send out bursts of radiation in the form of gravity waves. That concept has become generally accepted among physicists, although no one has been able to prove it. The hope is that the next generation of gravity detectors, especially the one planned by Caltech and MIT, will be sensitive enough to detect them.

Since gravity waves travel at the speed of light, the difference between the time that gravity waves are detected and the time that neutrinos are detected would offer a precise way of determining the mass of all three types of neutrinos.

Network of Detectors

Cline and the other scientists who were at the UCLA symposium are trying to establish a framework for gravity wave detectors scattered around the world to work in concert with the equally scattered neutrino detectors.

“We need to do it now,” said Stanford University physics professor William Fairbank. “This could become a very exciting program.”

The neutrino detectors could be put on alert as soon as the gravity wave detectors report their first hits. That would ensure that all the data from that period would be preserved for future study.

If the neutrinos arrive simultaneously with the gravity waves, as many scientists expect, that will rule them out as candidates for the “missing mass,” and scientists can start looking for something else. If they arrive sometime later--and that could even be a year later if the supernova is far enough away--it will mean that these tiny, illusive elements will have the last word on the future of the universe.