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Massive neutrinos are currently the only dark matter candidates known to exist. Solar, atmospheric, reactor and accelerator neutrino experiments have each confirmed neutrino oscillations, implying that neutrinos have non-zero mass, but not pinning down the absolute values. This is very strong evidence for new physics beyond the Standard Model, and the basis for a remarkable synergy between High Energy Physics and Cosmology.
Various observational probes in cosmology permit us to study neutrino properties with an accuracy which, in many cases, is significantly better than that available to terrestrial experiments. Likewise, the knowledge about neutrinos gained in High Energy Physics experiments has helped to resolve astrophysical mysteries. But many fundamental questions remain. For example, we still do not know if the neutrino and its anti-particle are identical. It is also important to know the sum of neutrino masses as this is degenerate with the values of other cosmological parameters, e.g. the amplitude of fluctuations and the primordial spectral index.
Massive neutrinos have the effect of suppressing the matter power spectrum on small scales. Current results from galaxy redshift surveys and the Cosmic Microwave Background set an upper limit on the sum of the neutrino masses below 1 eV.
Very recently, using the MegaZ-LRG sample we derived a much tighter upper limit of 0.28 eV. This is one of the lowest and most robust values in the literature, using the largest-ever photometric redshift survey. It is already a significantly lower value than that which, e.g., the terrestrial KATRIN experiment will infer. We have also made a forecast for what DES+Planck will tell us, deducing an upper limit of 0.12 eV, or a detection with this uncertainty if the neutrino mass is significantly larger than 0.1 eV.