The cosmological constant problem from a brane-world perspective

We point out several subtleties arising in brane-world scenarios of cosmolo gical constant cancellation. We show that solutions with curvature singular ities are inconsistent, unless the contribution to the effective four-dimen tional cosmological constant of the physics that resolves the singularities is fine-tuned. This holds for both at and curved branes. Irrespective of t his problem, we then study an isolated class of at solutions in models wher e a bulk scalar field with a vanishing potential couples to a 3-brane. We g ive an example where the introduction of a bulk scalar potential results in a non-zero cosmological constant. Finally we comment on the stability of c lassical solutions of the brane system with respect to quantum corrections.