The key to everything is in a crystal ball

Robert Matthews reports on the hunt for the universe's missing material now taking place in a North-East mine

IN the search for the key to the cosmos, a potash mine 3,600 feet beneath the North-East countryside would seem one of the least obvious places to look - and certainly one of the least congenial.

For the last five years, a team of researchers has been working in the stultifying, 90F (32C) at the bottom of the Boulby mine, hoping to answer an embarrassingly basic question: what is the universe actually made of?

The fact that no one really knows has been worrying scientists for more than 60 years. But now an experiment at the bottom of the Boulby mine may have come up with the answer, and cast important new light on the fate of the universe as well.

The discovery is the latest twist in a story that began in the Thirties when Fritz Zwicky, a Swiss-American astronomer, embarked on what should have been a routine project to measure the speed of galaxies moving through space. These huge collections of 100 billion stars typically move around the universe in clusters. Our own Milky Way galaxy belongs to a relatively small group of about three dozen galaxies, somewhat unimaginatively named the Local Group.

Zwicky was measuring the speeds of galaxies in the far larger and more distant Coma Cluster when he made a puzzling discovery. The hundreds of galaxies making up the cluster were zooming around each other at very high speed - so high, in fact, that the force of gravity between them should have been unable to prevent the galaxies from breaking away from each other. All the signs were, however, that the cluster was stable - and had been so for millions of years.

Various resolutions of the paradox suggested themselves. One obvious suggestion was that Zwicky's measurements were unreliable. A more radical alternative was that our understanding of the force of gravity is defective.

Zwicky steered a middle course, arguing that neither the measurements nor the law of gravity was at fault. Instead he suggested that the Coma cluster must also be harbouring vast amounts of invisible matter in addition to the billions of shining stars. This "dark matter" could then provide the extra mass and thus gravity needed to hold the cluster together.

Hardly a revolutionary notion, one might think: huge clouds of black dust and gas have long been known to lurk in our own galaxy. But over the years, the mystery of "dark matter" has only deepened.

More detailed studies of the behaviour of galaxies have shown that dark matter must actually make up the vast bulk of material in the universe. Current estimates suggest that when we look up at the night sky, the stars and galaxies we can see represent perhaps as little as one per cent of the total amount of matter actually out there in space.

Dark matter is now thought to be so plentiful that it plays a vital role in the ultimate fate of the universe. For if there is enough of it out there, its gravitational pull could halt and even reverse the expansion of the universe, which began 15 billion years ago in the Big Bang. If that were to happen, the result could be a catastrophic Big Crunch billions of years hence.

Most perplexing of all, however, is the recognition that most "dark matter" cannot be anything like ordinary gas and dust. If it were, it would have been cooked inside many stars by now and turned into chemical elements in concentrations quite different from those seen in today's universe.

So what exactly is dark matter? Enter the particle physicists, forever keen to propose some new form of matter - if only to give it a silly name, like quark, parton or gluon. They have certainly done themselves proud in naming candidates for dark matter.

Take the axion, a leading contender for dark matter. This sub-atomic particle was first dreamt up in the Eighties by theoreticians tackling an esoteric problem concerning protons and neutrons.

One of them, Frank Wilczek of the Institute of Advanced Study in Princeton, had always promised himself that if he had the chance to name a new particle, he would call it the axion - after a leading American brand of detergent.

But while the name has stuck, the axion itself has proved very elusive. If it exists at all, it must be incredibly light with a mass 1,000 billion times less even than the electron. On the other hand, there should be so many axions in the cosmos that together they could make up much of the dark matter. Despite more than a decade of searching, however, no one has so far detected a single axion - outside a supermarket that is.

Then there is the Macho, or Massive Compact Halo Object. This candidate for dark matter encompasses objects such as dwarf stars, pulsars and compact black holes, all of which are small, dark and perfectly-formed.

Over the last decade, evidence pointing to the existence of Machos in the "halos" surrounding galaxies has begun to accumulate. Even so there is little hope that they exist in the vast numbers needed to make up all the dark matter thought to exist out there.

From Machos it is but a short step to Wimps - Weakly-Interacting Massive Particles - and to recent events at the bottom of that mine near Redcar.

As with axions, Wimps were first dreamt up by theoreticians for esoteric reasons far removed from astronomy. Yet on paper at least, they make ideal candidates for dark matter. They are expected to be relatively hefty particles, weighing about 10 to 1,000 times as much as a proton, while interacting only weakly with matter - which presumably explains why we haven't seen any.

This is what the Boulby experiment was designed to put right. At the bottom of the mine is a large crystal of sodium iodide, a compound predicted to give off a flash of light whenever a Wimp passes through it.

The trouble is that the crystal will also flash if hit by cosmic rays - hence the decision to put the detector under thousands of feet of solid rock. Natural radiation can also cause problems, so the whole experiment is immersed in a huge vat of distilled water.

For almost five years, the team has patiently watched and waited, and seen nothing. But now their crystal has started emitting flashes - hundreds of them over recent months.

Whatever the flashes are, they do not appear to be false alarms caused by faulty equipment: a French-built Wimp detector lowered into the mine has also picked up identical signals. As to whether they are due to Wimps, however, the team will only give a definite maybe, as the signals are not quite what the team was expecting to see. More sensitive detectors are on their way, and should give an answer in a year or so.

So will there be enough Wimps to bring the universe to an end? Until astronomers have some estimate of the number of these particles out there, no one can say for certain.

Those hard-nosed scientists at the Boulby mine may yet uncover the fate of the universe by gazing into a crystal ball.