Brane world cosmology: Gauge-invariant formalism for perturbation

In the present paper the gauge-invariant formalism is developed for perturbations of the brane world model in which our universe is realized as a boundary of a higher dimensional spacetime. For the background model in which the bulk spacetime is (n+m) dimensional and has the spatial symmetry corresponding to the isometry group of an n-dimensional maximally symmetric space, gauge-invariant equations are derived for perturbations of the bulk space-time. Further, for the case corresponding to the brane world model in which m=2 and the brane is a boundary invariant under the spatial symmetry in the unperturbed background, relations between the gauge-invariant variables describing the bulk perturbations and those for brane perturbations are derived from Israel’s junction condition under the assumption of Z₂ symmetry. In particular, for the case in which the bulk spacetime is a constant-curvature spacetime, it is shown that the bulk perturbation equations reduce to a single hyperbolic master equation for a master variable, and that the physical condition on the gauge-invariant variable describing the intrinsic stress perturbation of the brane yields a boundary condition for the master equation through the junction condition. On the basis of this formalism, it is pointed out that it seems to be difficult to suppress brane perturbations corresponding to massive excitations for a brane motion giving a realistic expanding universe model.