New Cosmological Upper Limit on Neutrino Mass

Neutrino news has been dramatic these past few years: neutrinos have been shown to oscillate from one type to another (See Update 375) and the solar neutrino problem has been resolved (See Update 586) after puzzling solar physicists for decades.

These results imply that at least one or more of the neutrino flavors (electron, mu, tau) have some mass and this, considering the number of nu's loose in the universe, means that even lightweight neutrinos will have had a palpable role in influencing the development of galaxies.

But how much nu mass is there and how big a role did nu's play? Particle physics experiments so far directly establish only values for the square of neutrino mass differences. From tritium decay experiments comes an upper limit of 2.2 eV for the electron neutrino. Upper limits for the mu or tau neutrinos are up in the MeV range.

The new mass limits come from looking at the distribution of galaxies across the canopy of the sky. The 2dF Galaxy Redshift Survey has scanned 250,000 galaxies (viewed 400 at a time with a telescope in Siding Spring Mountain, Australia). The galactic coordinates can be compared two at a time, providing a plot of the number of galaxies versus inter-galaxy distance.

Turned into a galactic "power spectrum," this correlation study can be used to estimate the likely density of the constituent species of matter in the universe: baryons (such as protons), cold dark matter (WIMPs), and hot dark matter (neutrinos are the leading candidate).

The 2dF work arrives at two big neutrino conclusions. (1) Neutrinos can account for no more than 13% of the matter in the universe and (2) the sum of all the nu masses (electron plus mu plus tau) is no more than 2.2 eV.

Group member Oystein Elgaroy (University of Cambridge, elgaroy@ast.cam.ac.uk, 44-1223-75 x17) says that this is the best upper limit for neutrino mass derived with relatively conservative assumptions on the total matter density in the universe. (Elgaroy et al., Physical Review Letters, 5 August 2002; also see Physical Review Focus, 12 July.)