The Einstein equations on the 3-brane world - art. no. 024012

We carefully investigate the gravitational equations of the brane world, in which all the matter forces except gravity are confined on the 3-brane in a 5-dimensional spacetime with Z(2) symmetry. We derive the effective gravi tational equations on the brane, which reduce to the conventional Einstein equations in the low energy limit. From our general argument we conclude th at the first Randall-Sundrum-type theory predicts that the brane with a neg ative tension is an antigravity world and hence should be excluded from the physical point of view. Their second-type theory where the brane has a pos itive tension provides the correct signature of gravity. In this latter cas e, if the bulk spacetime is exactly anti-de Sitter spacetime, generically t he matter on the brane is required to be spatially homogeneous because of t he Bianchi identities. By allowing deviations from anti-de Sitter spacetime in the bulk, the situation will be relaxed and the Bianchi identities give just the relation between the Weyl tensor and the energy momentum tensor. In the present brane world scenario, the effective Einstein equations cease to be valid during an era when the cosmological constant on the brane is n ot well defined, such as in the case of the matter dominated by the potenti al energy of the scalar field.