Neutrino masses and the number of neutrino species fromWMAP and 2dFGRS

Abstract

We have performed a thorough analysis of the constraints which canbe put on neutrino parameters from cosmological observations, mostnotably those from the WMAP satellite and the 2dF galaxy survey.For these data we find an upper limit on the sum of active neutrinomass eigenstates of ∑mν ⩽ 1.0 eV (95% conf.), but thislimit is dependent on priors.We find that the WMAP and 2dF data alone cannot rule out the evidence from neutrinoless double beta decay reported bythe Heidelberg–Moscow experiment.In terms of the relativistic energy density in neutrinos or otherweakly interacting species we find, in units of the equivalentnumber of neutrino species, Nν, that Nν = 4.0+3.0−2.1(95% conf.).When BBN constraints are added, the bound on Nν is 2.6+0.4−0.3(95% conf.), suggesting that Nν could possibly be lower thanthe standard model value of 3. This can, for instance, be the casein models with very low reheating temperature and incompleteneutrino thermalization.Conversely, if Nν is fixed to 3 then the data from WMAP and2dFGRS predict that 0.2458 ⩽ YP ⩽ 0.2471 (95% conf.), which is significantlyhigher than the observationally measured value.The limit on relativistic energy density changes when a smallνe chemical potential is present during BBN. In this casethe upper bound on Nν from WMAP, 2dFGRS and BBN isNν ⩽ 6.5. Finally, we find that a non-zero ∑mν can be compensatedby an increase in Nν. One result of this is that the LSND resultis not yet ruled out by cosmological observations.