Cosmological constant, near brane behavior and singularities

We show that the classical cosmological constant in type II flux compactifications can be written as a sum of terms from the action of localized sources plus a specific contribution from non-trivial background fluxes. Exploiting two global scaling symmetries of the classical supergravity action, we find that the flux contribution can in many interesting cases be set to zero such that the cosmological constant is fully determined by the boundary conditions of the fields in the near-source region. This generalizes and makes more explicit previous arguments in the literature. We then discuss the problem of putting D3-branes at the tip of the Klebanov-Strassler throat glued to a compact space in type IIB string theory so as to engineer a de Sitter solution. Our result for the cosmological constant and a simple global argument indicate that inserting a fully localized and backreacting D3-brane into such a background yields a singular energy density for the NSNS and RR 3-form field strengths at the D3-brane. This argument does not rely on partial smearing of the D3-brane or a linearization of field equations, but on a few general assumptions that we also discuss carefully.