Collapse and bounce of null fluids
  Exact solutions describing the spherical collapse of null fluids can contain regions which violate the energy conditions. Physically the violations occur when the infalling matter continues to move inwards even when non-gravitational repulsive forces become stronger than gravity. In 1991 Ori proposed a resolution for these violations: spacetime surgery should be used to replace the energy condition violating region with an outgoing solution. The matter bounces. We revisit and implement this proposal for the more general Husain null fluids including a careful study of potential discontinuities and associated matter shells between the regions. Along the way we highlight an error in the standard classification of energy condition violations for Type II stress-energy tensors. 
Bradley Creelman and Ivan Booth
gr-qc