Gravitational double layers
  I analyze the properties of thin shells through which the scalar curvature R is discontinuous in gravity theories with R + R^2 Lagrangian on the bulk. These shells/domain walls are of a new kind because they possess, in addition to the standard energy-momentum tensor, an external energy flux vector, an external scalar pressure/tension and, most exotic of all, another energy-momentum tensor contribution resembling classical dipole distributions on a shell: a double layer. I prove that all these contributions are necessary to make the entire energy-momentum tensor divergence-free. This is the first known occurrence of such a type of double layer allowed in a gravity theory. I present explicit examples in constant-curvature 5-dimensional bulks, with a brief study of their properties: new physical behaviors arise. 
Jos\'e M. M. Senovilla
gr-qc hep-th math-ph math.MP