Lensed density perturbations in braneworlds: Towards an alternative to perturbations from inflation

We consider a scenario in which our observable universe is a 3-dimensionalsurface (3-brane) living in extra dimensions with a warped geometry. We showthat ``lensed'' density perturbations from other branes serve as possible seedsfor structure formation on our observable brane (without inflation), and, inaddition, provide constraints on braneworld scenarios with warped bulkgeometry. Due to the warped bulk metric, any perturbation generated on onebrane (or in the bulk matter) appears to an observer on a second brane to havea significantly different amplitude. We analyze lensed perturbations in theRandall-Sundrum type scenarios and the ``shortcut metric'' scenarios. ForLorentz violating metrics in the bulk, we find the attractive possibility thatlarge density fluctuations that are causally produced elsewhere can lead tosmall density fluctuations on our brane on superhorizon (acausal) lengthscales, as required by structure formation. Our most interesting result is thatthe ``shortcut metrics'' in which geodesics traverse the extra dimensionsprovide an alternative to inflation with two important features: a possiblesolution to the horizon problem and a mechanism to generate perturbationsnecessary for structure formation.Comment: 27 page