Elementary Processes in Dissipative Cosmic Medium

Quantum dynamics of a finite degrees of freedom is often much affected by interaction with the larger environment of cosmic medium. In this lecture we first review some recent developments of the theory of quantum dissipation in the linear open system. In the second part we discuss two classes of applications: decay of unstable particle in medium, and environmental effect on the parametric particle production. The first subject of particle decay may have important consequences on the scenario of baryogenesis due to a GUT heavy particle. On the other hand, the parametric resonant particle production is related to the reheating problem after inflation, and its environmental effect is related also, perhaps more importantly, to the decay of moduli fields in supergravity models. † Lecture given at Workshop on The Cosmological Constant and Evolution of the Universe, held at Tokyo, December 25-27, 1996. To appear in the Proceedings.