Unitarity in Quantum Fields Theory in Curved Spacetimes

The goal of this talk is to describe an aspect of quantum field theory in time dependent, globally hyperbolic spacetimes which is not commonly appreciated: dynamics is not unitary in the standard sense. This point will be illustrated with a simple cosmological spacetime. I will then shown that a generalized notion of unitarity does hold. This generalized notion allows one to correctly pass to the Schr\"{o}dinger picture starting from the Heisenberg picture used in the textbook treatments.