VACUUM FLUCTUATIONS FOR SPHERICAL GRAVITATIONAL IMPULSIVE WAVES

We propose a method for calculating vacuum fluctuations on the backgro und of a spherical impulsive gravitational wave which results in a fin ite expression for the vacuum expectation value of the stress energy t ensor. The method is based on first including a cosmological constant as an auxiliary constant. We show that the result for the vacuum expec tation value of the stress-energy tensor in second-order perturbation theory is finite if both the cosmological constant and the infrared pa rameter tend to zero at the same rate.