Big bang nucleosynthesis constraints on bulk neutrinos

We examine the constraints imposed by the requirement of successful nucleosynthesis on models with one large extra hidden space dimension and a single bulk neutrino residing in this dimension. We solve the Boltzmann kinetic equation for the thermal distribution of the Kaluza-Klein modes and evaluate their contribution to the energy density at the big bang nucleosynthesis epoch to constrain the size of the extra dimension $R^{-1}\equiv \mu$ and the parameter ${\sin }^{2}2\theta$ which characterizes the mixing between the active and bulk neutrinos.