Seminarium Teorii Względności i Grawitacji

We will show that the Penrose limit enables one to distinguish a quantum state for any massless field on a background of plane pp-waves. Having introduced necessary geometrical tools, we shall investigate properties of the obtained state (including an energy--momentum tensor and particle production) finding that it can really be seen as the bona fide vacuum. Our results are independent of the dimension, coupling between field and curvature and, to some extent, its spin. We also comment on the limitations one encounters when working with a quantum field theory in a non-globally hyperbolic spacetime. Those states can be seen as a generalization of states previously considered by Gibbons for massive theories. Finally, we can use our vacua to find exact solutions to the semiclassical Einstein equations with quantized scalar field. In four dimensions, those are the first explicit non-Ricci-flat solutions with less than ten Killing vectors. We will discuss backreaction due to the presence of non-classical correlations and to the source being a mixed state. Joint work with W. Kamiński. [Zoom link ask wkaminsk at fuw.edu.pl]