PERTURBATION-THEORY FOR NONEQUILIBRIUM QUANTUM-FIELDS

Standard perturbation theory fails when applied to nonequilibrium proc esses. I explain why. To achieve sensible results, it is necessary to resum the absorptive parts of real-time self-energy corrections. Using the closed-time-path formalism, and in the context of scalar-field th eory in an expanding universe, I show how a generalized renormalizatio n technique can achieve this.