Sterile neutrino production in models with low reheating temperatures

By numerically solving the appropriate Boltzmann equations, we study the production of sterile neutrinos in models with low reheating temperatures. We take into account the production in oscillations as well as in direct decays and compute the sterile neutrino primordial spectrum, the effective number of neutrino species, and the sterile neutrino contribution to the mass density of the Universe as a function of the mixing and the reheating parameters. It is shown that sterile neutrinos with non-negligible mixing angles do not necessarily lead to $N_\nu\sim 4$ and that sterile neutrinos may have the right relic density to explain the dark matter of the Universe. If dark matter consists of sterile neutrinos produced in oscillations, X-rays measurements set a strong limit on the reheating temperature, $\tr\gtrsim 7\mev$. We also point out that the direct decay opens up a new production mechanism for sterile neutrino dark matter where cosmological constraints can be satisfied.