Massive Neutrino Alters Conception of Universe

The best things in the universe come in small packages. Since the revolution in physics early in the century, scientists have thought the neutrino--a sub-atomic particle distinct from protons and neutrons--to be no more than a massless bundle of energy. Recent research suggests, however, that some of these "little neutrons" of the sub-atomic family might actually be very dense particles of matter, capable of exerting a gravitational pull.

David N. Schramm, a University of Chicago astrophysicist, speculates that relic neutrons--those left over from the Big Bang, the gaseous explosion believed to have formed the universe more than 9 billion years ago--outnumber the protons, neutrons, and electrons that comprise ordinary matter by about 10 billion to 1. The average cubic centimeter in the universe contains about 450 of these relic neutinos. Schram contends that if these particles have even a tiny mass, unlike the current description of conventional physics, scientists can construct a radically different view of the universe and explain several cosmological riddles.

Theories of massive neutrinos have been circulating for more than a decade, but only in the last two years have two independent experiments, one performed by a Soviet research team and another by scientists at the University of California at Irvine, provided evidence that even the lightest neutrinos have masses close to ten electron volts. Ten electron volts comprise about 0.002 per cent of the mass of an electron.

Evidence of massive neutrinos is significant for constructing a new picture of the universe, because of the different effect the laws of relativity would have in describing the motion of these particles in motion.

Particles with zero mass, such as photons, are constrained by relativity to travel at no other speed than that of light, for only at that speed can they carry energy without mass. At the other end of the scale, particles with mass can never reach the speed of light, according to the laws of relativity.

But if neutrinos have even a tiny mass, they would have the capability to slow down and come to rest like protons and neutrons. The tiny mass of the neutrinos would not only increase their gravitational pull, but those particles moving slowly enough could gravitationally bind themselves together and form galaxies and clusters of galaxies.

The evolution of galaxies through this type of neutrino clustering might explain one of cosmology's most perplexing paradoxes--the "missing mass" problem. This paradox arises because the amount of visible matter in the universe--both stars and luminous gas--is dwarfed by the total amount of matter estimated from studies of gravitational dynamics, velocity measurements and spectroscopic data.

Scientists addressing this riddle have postulated the existence of a sort of invisible "dark material" at the outer ranges of galaxy clusters. Researchers conclude that normal matter, comprised of protons and neutrons, can account for this dark component of individual galaxied and small groups of galaxies.

But they can not explain the incredible masses of the largest clusters. Recent research indicates, however, that the clusters might have captured the slow-moving relic neutrinos from the Big Bang and bound them in their gravitational fields. These neutrinos would then form the massive invisible halo surrounding the cluster.

Schramm says the mass of the neutrinos might make them the predominant form of matter in the universe, showing how one small particle can mean an awful lot in the world of astrophysics.