Supersymmetry and Brane Cosmology

We consider a five-dimensional brane world scenario where the fifth dimension is compactified on $S^1/Z_2$. We show that the familiar four-dimensional cosmology on our brane is easily recovered during a primordial stage of inflation if supersymmetry is exploited. Even if some vacuum energy density appears localized on our three brane, heavy supersymmetric bulk fields adjust themselves and acquire a nontrivial configuration along the extra-dimension. This phenomenon redistributes uniformly the energy density across the bulk and the resulting energy-momentum tensor does not display any singularity associated to the initial localized energy density on our three-brane. No jumps across the brane are present for the derivatives of the metric and Einsteins equations are solved by constant solutions along the fifth dimension. Our findings make it clear that cosmological phenomena in the supersymmetric brane world scenario must be studied taking properly into account bulk supersymmetric states. This comment is particularly relevant when applied to (super)gravity since in supersymmetric brane world scenarios, even though chiral matter and gauge fields may be restricted to live on boundaries, gravity multiplets always propagate in the bulk.