New cosmological upper limit

New cosmological upper limit on neutrino mass. Recent neutrino results imply that one or more of the three neutrino flavors (ν_e, ν _μ , ν _τ ) have some mass (see Physics Today, July 2002, page 13 ). Considering the number of neutrinos loose in the universe, even a small mass means they will have significantly influenced the development of galaxies. Various physics experiments have established an upper limit of 3 eV for the ν_e and whopping upper limits in the MeV range for the ν _μ and ν _τ . Now, a worldwide collaboration of astronomers has looked at the distribution of 250 000 galaxies in the 2 Degree Field Galaxy Redshift Survey and measured large-scale structure statistics in the form of a power spectrum. They compared the data with calculated power spectra using a model that included baryons, cold dark matter, massive neutrinos (hot dark matter), and a cosmological constant. The model had a few reasonable assumptions—for example, that all three types of neutrinos drop out of thermal equilibrium at the same temperature and that the spectrum of primordial fluctuations from which galaxies evolved is scale-independent—and an appropriate treatment of previously measured cosmological parameters. The group then arrived at two big conclusions: Neutrinos can account for no more than 13% of the matter in the universe, and the sum of all three neutrino masses is no more than 2.2 eV Group members Øystein Elgarøy and Ofer Lahav say that this is the best upper limit for neutrino mass derived with relatively conservative assumptions on cosmological parameters. (Ø. Elgarøy et al., Phys. Rev. Lett. 89 , 061301, 2002 http://dx.doi.org/10.1103/PhysRevLett.89.061301 .)