Domain walls without cosmological constant in higher order gravity
  We consider a class of higher order corrections with arbitrary power $n$ of the curvature tensor to the standard gravity action in arbitrary space-time dimension $D$. The corrections are in the form of Euler densities and are unique at each $n$ and $D$. We present a generating functional and an explicit form of the corresponding conserved energy-momentum tensors. The case of conformally flat metrics is discussed in detail. We show that this class of corrections allows for domain wall solutions since, despite the presence of higher powers of the curvature tensor, the singularity structure at the wall is of the same type as in the standard gravity. However, models with higher order corrections have larger set of domain wall solutions and the existence of these solutions no longer depends on the presence of cosmological constants. We find for example that the Randall-Sundrum scenario can be realized without any need for bulk and/or brane cosmological constant. 
Krzysztof A. Meissner and Marek Olechowski