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

We consider a scenario in which our observable universe is a 3-dimensional surface (3-brane) living in extra dimensions with a warped geometry. We explore the interesting possibility that 'lensed' density perturbations from other branes may serve as possible seeds for structure formation on our observable brane (without inflation), and, in addition, provide constraints on braneworld scenarios with warped bulk geometry. Because of the warped bulk metric, any perturbation generated on one brane (or in the bulk matter) appears to an observer on a second brane to have a significantly different amplitude. We analyze lensed perturbations in the Randall-Sundrum type scenarios and the 'shortcut metric' scenarios. For Lorentz violating metrics in the bulk, we find the attractive possibility that large density fluctuations that are causally produced elsewhere can lead to small density fluctuations on our brane on superhorizon (acausal) length scales, as required by structure formation. Our most interesting result is that the 'shortcut metrics' in which geodesics traverse the extra dimensions has the chance of providing an alternative to inflation with two important features: a possible solution to the horizon problem and a possible mechanism to generate large scale perturbations necessary for structure formation