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.