Effects of sterile neutrinos and an extra dimension on big bang nucleosynthesis

By assuming the existence of extra-dimensional sterile neutrinos in the big bang nucleosynthesis (BBN) epoch, we investigate the sterile neutrino (nu(s)) effects on the BBN and constrain some parameters associated with the nu(s) properties. First, for the cosmic expansion rate, we take into account effects of a five-dimensional bulk and intrinsic tension of the brane embedded in the bulk and constrain a key parameter of the extra dimension by using the observational element abundances. Second, effects of the nu(s) traveling on or off the brane are considered. In this model, the effective mixing angle between a nu(s) and an active neutrino depends on energy, which may give rise to a resonance effect on the mixing angle. Consequently, the reaction rate of the nu(s) can be drastically changed during the cosmic evolution. We estimated abundances and temperature of the nu(s) by solving the rate equation as a function of temperature until the sterile neutrino decoupling. We then find that the relic abundance of the nu(s) is drastically enhanced by the extra dimension and maximized for a characteristic resonance energy E-res greater than or similar to 0.01 GeV. Finally, some constraints related to the nu(s), i. e., mixing angle and mass difference, are discussed in detail with the comparison of our BBN calculations corrected by the extra-dimensional nu(s) to observational data on light element abundances.