Boston Globe, May 10, 1981


By Robert Cooke, Globe Staff

Sometime far in the distant future our universe will either collapse inward, recycling itself in a cataclysmic Big Bang, or go on expanding forever to become a cold, lifeless collection of burnt-out cinders adrift in space.

And which way it goes, it seems, depends on whether a tiny, elusive critter called the neutrino has any mass at all.

Most of us, of course, needn't worry about the fate of the universe one way or the other, since whatever happens will be billions of years in the future.

But for physicists the question is important scientifically, and they've been debating, rather warmly, which of these two end-of-the- universe scenarios seems most likely to be true.

This weekend, over in Cambridge at the Harvard/Smithsonian Center for Astrophysics, the debate is continuing to rage at a Neighborhood Meeting of astrophysicists who think yes, there is enough stuff out there to recycle the universe, and those who say no, too bad, sorry about that.

One can understand why the argument over the mass, or lack of mass, of the neutrino has reached high pitch: If the neutrino is massive, it creates as many theoretical problems as it solves. It will mean years worth of work must be rethought and uncountable calculations and equations must be refigured.

The tiny neutrino itself has long been thought of as a ghostly particle that has no mass and no electric charge. This means it would be traveling at the speed of light and, since it couldn't interact with anything, would travel through the whole Earth unimpeded, without bumping into anything.

This problem of the fate of the universe isn't simple, certainly, but it boils down essentially to finding out how much matter there really is in the universe. And there the little neutrino may make all the difference.

Basically, too, it's all a matter of gravity. If enough stuff exists out there in space, for example, then there's also enough gravity to halt eventually the expansion of the universe, turn it around and cause it to collapse inward on itself billions of years hence. That, theorists say, could lead to another Big Bang and creation of another expanding universe.

If, on the other hand, there's not enough matter in the heavens, then there's also not enough gravity to bring everything back together again.

With this question in mind, then, scientists, and especially astronomers and astrophysicists, have been studying the skies, trying to determine how much material really exists. But after adding up all the stars, galaxies, gas and everything else that can be measured, they've come up rather short, not

finding enough to "close" the universe and cause it to recycle itself.

At the same time, however, the things they do see, especially the stars, galaxies and clusters of galaxies, behave in ways that suggest there's much more mass out there than has yet been measured. Thus the gravitational effects seem to say there's a lot more mass out there, but it's invisible.

"In some work I did here with colleagues last year, on the biggest scale measured so far, on superclusters, we found that 95 percent of the mass (needed to explain the gravitational effects) is unaccounted for," said Dr. Marc Davis, at the Harvard/Smithsonian Center for Astrophysics in Cambridge.

"We got an enormous gravitational effect, but nothing else. And it seems that's how it's going to be in the future -- being completely undetectable except for gravity.

"What sort of dark matter could do this?" Davis asked. "It must be something that doesn't emit (signals) in any part of the spectrum, meaning it's dark. So what is it?

"If the universe was filled with Earths, or with Jupiters or even with bricks (all of which are dark bodies), we wouldn't see it. It could also be black holes, or it could be neutrinos."

This, then, has become the puzzle of the "missing mass." Where is it? Or does it not exist?

This debate over the missing mass re-awakened recently when a California scientist, Dr. Frederick Reines, and his co-workers reported -- on the basis of experiments with sub-atomic particles - that the neutrino may, after all, have a little bit of mass.

Despite these results, very few physicists seem willing to bet much that Reines has really seen evidence for neutrinos with mass. What they're waiting for is more and better data from other experiments.

But on the other hand, physics theorists have been suggesting there's no reason the neutrino can't have mass, even though there's no proof that it does.

But if this is true, if the neutrino has a small amount of mass, it would change the whole equation dramatically.

"Neutrinos with mass," according to the American Physical Society, "would have profound consequences for astrophysics and cosmology. Neutrinos with mass (if they really do have mass) may constitute most of the mass of the

universe, and at the very least could make up the mass which is apparently missing' in clusters of galaxies."

Traditionally, however, neutrinos have been considered to be utterly massless and to carry no electric charge. In this condition, they are thought to move at the speed of light and be able to pass unobstructed clear through the Earth.

In their calculations about what might, or must, have happened perhaps 15 billion years ago when the universe was born, physicists have calculated that billions of neutrinos should have been created along with each proton, neutron and electron. Indeed, enough neutrinos should have been created to match the number of photons (particles of light) that were created.

In addition, so many neutrinos are thought to be released in the nuclear reactions occurring inside the stars that even if the neutrino has only a very, very small amount of mass, these neutrinos might easily double the mass density of the whole universe.

If all that is true, then the universe should now harbor some 450 neutrinos per cubic centimeter, a huge amount of mass.

The idea that a huge number of neutrinos may provide the gravitational glue that holds the clusters of galaxies together, however, is meeting with solid resistance. Some American theorists think, indeed, that the presence of massive neutrinos might make it difficult, or even impossible, for galaxies and clusters of galaxies to form at all. Russian theorists, to the contrary, think the presence of massive neutrinos might make it easier for galaxies to form.

Of course many of these ideas depend, too, on how massive the neutrinos are, if they have mass at all. The best estimates now are that they would fall into the range of just a few electron volts, between 3 and 20 electron volts. This amounts to only .002 of the mass of the tiny electron.

If the neutrino does have even a small amount of mass, then it means it won't travel at the speed of light, and should be capable of coming to rest someplace. If this is true, then theorists suggest there may be neutrinos going slow enough so they can become trapped in galaxies, and can perhaps participate in the evolution of galaxies.

All of which is a nice puzzle, a riddle that should keep whole squads of physicists employed for some time to come.

Copyright 1981, 2001 Globe Newspaper Company