Vacuum polarization in the Szekeres Class of colliding plane wave   space-times
  We study the quantization of a scalar field on a classical background given by the Szekeres Class of solutions, which represent the collision of two gravitational plane waves with constant polarization. These solutions consist of two approaching gravitational plane waves moving in a flat background and an interaction region which always contains a curvature singularity. Following a suitable approximate procedure, introduced in a previous paper, we propose a way to compute the vacuum expectation value of the stress-energy tensor throughout the causal past region of the collision center in the quantum state which corresponds to the vacuum before the arrival of the waves. 
Miquel Dorca (Enrico Fermi Institute, The University of Chicago)