Conformally Invariant Brane-universe and the Cosmological Constant
  A relativistic 3-brane can be given a conformally invariant, gauge-type, formulation provided the embedding space is six-dimensional. The implementation of conformal invariance requires the use of a modified measure, independent of the metric in the action. A brane-world scenario without the need of a cosmological constant in 6D can be constructed. Thus, no ``old'' cosmological constant problem appears at this level. 
E.I.Guendelman, E. Spallucci