of the Higgs doublet ), so is called a sterile neutrino. Or, mass can be generated by the Majorana mechanism , which would require the neutrino and antineutrino to be the same particle. The strongest upper limit on the masses of neutrinos comes from cosmology : the Big Bang model predicts that there is a fixed ratio between the number of neutrinos and the number of photons in the cosmic microwave background . If the total energy of all three types of neutrinos exceeded an average of 50 eV per neutrino, there would be so much mass in the universe that it would collapse. [ citation needed ] This limit can be circumvented by assuming that the neutrino is unstable; however, there are limits within the Standard Model that make this difficult. A much more stringent constraint comes from a careful analysis of cosmological data, such as the cosmic microwave background radiation, galaxy surveys , and the Lyman-alpha forest . These indicate that the summed masses of the three neutrinos must be less than 0.3 eV . [ 37 ] In 1998, research results at the Super-Kamiokande neutrino detector determined that neutrinos can oscillate from one flavor to another, which requires that they must have a nonzero mass. [ 38 ] While this shows that neutrinos have mass, the absolute neutrino mass scale is still not known. This is because neutrino oscillations are sensitive only to the difference in the squares of the masses. [ 39 ] The best estimate