The Cosmological Bulk Neutrino Catastrophe

Recent phenomenological models which posit extra spacelike dimensions in which (bulk) neutrinos are allowed to propagate are shown to have significant cosmological effects whenever the size of the largest extra dimension is R > 1 fm (1/R < 200 MeV). Specifically, limits from the cosmic microwave background (CMB) anisotropies as measured by BOOMERanG, big bang nucleosynthesis, deuterium and Li-6 photoproduction, diffuse photon backgrounds, and structure formation/age considerations are shown to translate into broad constraints on bulk neutrino schemes. These present challenges for many recent light neutrino mass models invoking large volume toroidal compactifications as well as those involving densely spaced Kaluza-Klein (KK) modes. We discuss how these models would have to be modified to escape constraint and under what finely-tuned circumstances bulk neutrino KK tower states could constitute a new kind of dark matter. Future CMB observations (e.g., MAP, Planck) may extend these constraints or discover signatures of bulk neutrino models with R ~ 0.1 fm