Green functions for the scalar, spinor and vector fields in a plane wave
geometry arising as a Penrose limit of AdS×S are obtained. The
Schwinger-DeWitt technique directly gives the results in the plane wave
background, which turns out to be WKB-exact. Therefore the structural
similarity with flat space results is unveiled. In addition, based on the local
character of the Penrose limit, it is claimed that for getting the correct
propagators in the limit one can rely on the first terms of the direct geodesic
contribution in the Schwinger-DeWitt expansion of the original propagators .
This is explicitly shown for the Einstein Static Universe, which has the same
Penrose limit as AdS×S with equal radii, and for a number of other
illustrative cases.Comment: 18 pages, late