Upper Bound on Neutrino Masses from Galaxy Surveys

Cosmology not only probes the absolute mass scale of the neutrino but is a completely independent method to test against. In any case, it is imperative to include an accurate prescription for the neutrino in cosmology, as any failure to do so can bias the other cosmological parameters. A cosmological constraint on the sum of the neutrino masses is primarily a constraint on the relic big bang neutrino density Ω_ν. One can relate this density to the sum of the mass eigenstates ∑m_ν as given by Ω_ν= ∑m_ν/(93.14 h² eV). The direct effects of the neutrinos depend on whether they are relativistic or nonrelativistic and the scale under consideration. Neutrinos have a large thermal velocity as a result of their low mass and subsequently erase their own perturbations on scales smaller than the free streaming length. This subsequently contributes to a suppression of the statistical clustering of galaxies over small scales and can be observed in a galaxy survey. The abundance of neutrinos in the Universe can also have a direct effect on the primary CMB anisotropies if nonrelativistic before the time of decoupling (i.e., when sufficiently massive). However, one of the most clear effects at this epoch is a displacement in the time of matter-radiation equality. All these cosmological effects can be used to impose bounds on the neutrino mass. Previous studies have capitalized on these signatures and have started to place sub eV constraints on the absolute mass scale . We utilize the new Sloan Digital Sky Survey MegaZ luminous red galaxy (LRG) DR7 galaxy clustering data  to provide the first photometric galaxy clustering constraint on the neutrino and, combining with the CMB, examine the complementarity of these early- and late-time probes. With an almost comprehensive combination of probes this renders one of the tightest constraints on the neutrinos in cosmology and therefore physics.

Cosmological observations provide independent constraints on the neutrino mass scale provided that a few assumptions (a flat universe with Gaussian and adiabatic primordial fluctuations and a constant spectral index for example) can be made. Compared to the prospects of current-to-next generation particle neutrino experiments (like KATRIN) it may be that astronomical surveys of the cosmic microwave background anisotropies or optical surveys of the large scale structure of the Universe will place the tightest constraints on neutrino masses for some time. Continue reading from the excerpt above written by Shaun Thomas, Filipe Abdalla, and Ofer Lahav on their invited viewpoint article in Physical Review Letters (freely available):Upper Bound of 0.28 eV on Neutrino Masses from the Largest Photometric Redshift Survey.