Solar , atmospheric , and reactor neutrino experiments have confirmed neutrino oscillations , implying that neutrinos have non-zero mass , but without pinning down their absolute masses .
While it is established that the effect of neutrinos on the evolution of cosmic structure is small , the upper limits derived from large-scale structure could help significantly to constrain the absolute scale of the neutrino masses .
In a recent paper the 2dF Galaxy Redshift Survey ( 2dFGRS ) team provided an upper limit m _ { \nu, tot } < 2.2 { eV } , i.e .
approximately 0.7 eV for each of the three neutrino flavours , or phrased in terms of their contribution to the matter density , \Omega _ { \nu } / \Omega _ { m } < 0.16 .
Here we discuss this analysis in greater detail , considering issues of assumed ‘ priors ’ like the matter density \Omega _ { m } and the bias of the galaxy distribution with respect to the dark matter distribution .
As the suppression of the power spectrum depends on the ratio \Omega _ { \nu } / \Omega _ { m } , we find that the out-of-fashion Mixed Dark Matter model , with \Omega _ { \nu } = 0.2 , \Omega _ { m } = 1 and no cosmological constant , fits both the 2dFGRS power spectrum and the CMB data reasonably well , but only for a Hubble constant H _ { 0 } < 50 { km } { s } ^ { -1 } { Mpc } ^ { -1 } .
As a consequence , excluding low values of the Hubble constant , e.g .
with the HST Key Project , is important in order to get a strong upper limit on the neutrino masses .
We also comment on the improved limit obtained by the WMAP team , and point out that the main neutrino signature comes from the 2dFGRS and the Lyman \alpha forest .