A Small Cosmological Constant, Grand Unification and Warped Geometry
  We explore a mechanism to obtain the observational small value for the 4-dimensional vacuum energy through an exponential warp-factor suppression. Intriguingly the required suppression scale relates directly to the GUT scale. We demonstrate the mechanism explicitly in a 5-dimensional brane-world setup with warped geometry. Upon lifting the setup to 10-dimensional IIB string-theory, the relevance of the GUT scale becomes clear as the IIB string-theory description, which is based on D3-brane stacks, gives rise to a spontaneously broken SU(5) supersymmetric GUT theory with low-energy MSSM spectrum and Higgs doublet-triplet splitting. 
Axel Krause (Humboldt U., Berlin)