A team of astronomers has used a "galactic balance" to weigh the neutrino, placing the tightest upper bound yet--2.2 eV (about a quarter of a millionth of the mass of the electron)--on the tiny particle¿s mass.

Neutrino properties are notoriously difficult to measure because the particles are electrically neutral and interact only weakly with matter. But because the universe contains so many neutrinos, about 350 per cubic centimeter at the present time, even a very small shift in estimates for neutrino mass can have profound consequences for cosmology. "The clustering of matter in the universe depends directly on the total mass of neutrinos," explains team member Oystein Elagory of the University of Cambridge. "If we can measure the clustering we can therefore get a direct constraint on the neutrino mass."

Elagory and his colleagues used data from the 2dF Galaxy Redshift Survey to look at how 160,000 galaxies are distributed in our local universe. Their findings show that neutrinos could account for at most one-eighth of dark matter needed by currently favored cosmological models. That means that the major source of the invisible mass in the universe remains unknown.

Meanwhile, results from the Sudbury Neutrino Observatory have set a lower mass limit of 50meV for one type of neutrino. Now, with Elgaroy¿s result, scientists are coming closer than ever to pinning the elusive particle down. Elgaroy¿s work appears in a forthcoming issue of Physical Review Letters.