Cosmological bounds on the masses of stable, right-handed neutrinos

Previous authors have used the present mass density of the universe to constrain the masses of stable, left-handed neutrinos. We generalize these arguments to constrain the masses of stable, right-handed neutrinos which interact with strength $G\le G_F$. Light neutrinos (≤1 MeV) are constrained to be less massive than ∼100 eV-2 keV, depending upon $G$ and whether they are of the Dirac or Majorana type. Heavy neutrinos must be more massive than $\simeq \left(\frac{G_F}{G}\right)$ GeV, and less massive than about 10 TeV.