Optical scalars in spherical spacetimes
  Consider a spherically symmetric spacelike slice through a spherically symmetric spacetime. One can derive a universal bound for the optical scalars on any such slice. The only requirement is that the matter sources satisfy the dominant energy condition and that the slice be asymptotically flat and regular at the origin. This bound can be used to derive new conditions for the formation of apparent horizons. The bounds hold even when the matter has a distribution on a shell or blows up at the origin so as to give a conical singularity. 
E. Malec and N. \'O Murchadha (Physics Department, University College,   Cork, Ireland and Uniwersytet Jagiello\'nski, Insty...