Cosmological Constraints on Bulk Neutrinos

Recent models invoking extra spacelike dimensions inhabited by (bulk) neutrinos are shown to have significant cosmological effects if the size of the largest extra dimension is $R\gtrsim 1\mathrm{f}\mathrm{m}$. We consider effects on cosmic microwave background anisotropies, big bang nucleosynthesis, deuterium and ${}^{\mathrm{6}}\mathrm{L}\mathrm{i}$ photoproduction, diffuse photon backgrounds, and structure formation. The resulting constraints can be stronger than either bulk graviton overproduction constraints or laboratory constraints.