Groping for invisible matter

What we can see constitutes only about 10 per cent of the universe's mass. Where is the rest hidden?

Published: Wednesday 15 March 1995

-- THE universe is bloated almost beyond measure but about 90 per cent of its bulk is invisible. Intrigued cosmologists have postulated several theories about the missing folds of mass, but so far not one has as much as poked its nose out.

Now, scientists at the Los Alamos National Laboratory in New Mexico, USA, have pinned down a particle called the neutrino -- hitherto presumed massless -- which could account for at least 10-20 per cent of the invisible matter. If correct, the new findings could indicate whether the cosmos will expand forever or whether it would eventually start clamming up.

The exact mass of this elusive particle is yet to be determined. But the scientists estimate that it would be between 1 and 5 electron volts (the mass of an electron, a constituent of atoms and hitherto the lightest particle known, is more than 500,000 electron volts). An electron volt is a measure of energy, which when converted into mass using Einstein's E=mc2 equation equals one-billionth of the mass of a proton, a fundamental particle of visible matter.

The observations regarding the neutrino by the Los Alamos National Laboratory have been repeated twice over a period of 5 months and analysed to detect any misleading signals. The scientists are so sure about their interpretation that they have gone public even before the results appear in the reputed journal, Physical Review Letters.

Astrophysicist D Hywel White, leader of the Los Alamos team, says that it will take at least 2 more years of experimenting before they can measure the neutrino's mass with any precision. In this effort, the Los Alamos group will be assisted by 2 other laboratories -- the Fermi National Accelerator Laboratory in Illinois, US, and CERN, the European particle physics laboratory near Geneva.

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