Cosmological Lower Bound on Heavy-Neutrino Masses

The present cosmic mass density of possible stable neutral heavy leptons is calculated in a standard cosmological model. In order for this density not to exceed the upper limit of 2×${10}^{-29\mathrm{}}$ g/${\mathrm{cm}}^3$, the lepton mass would have to be greater than a lower bound of the order of 2 GeV.