Brane cosmological perturbations

We address the question of cosmological perturbations in the context of brane cosmology, where our Universe is a three-brane where matter is confined, whereas gravity lives in higher-dimensional spacetime. The equations governing the bulk perturbations are computed in the case of a general warped universe. The results are then specialized to the case of a five-dimensional spacetime, a scenario which has recently attracted a lot of attention. In this context, we decompose the perturbations into "scalar," "vector," and "tensor" modes, which are familiar in the standard theory of cosmological perturbations. The junction conditions, which relate the metric perturbations to the matter perturbations in the brane, are then computed.