  We consider a scalar field theory in Minkowski spacetime and define a coarse
grained Closed Time Path (CTP) effective action by integrating quantum
fluctuations of wavelengths shorter than a critical value. We derive an exact
CTP renormalization group equation and solve it using a derivative expansion
approach. Explicit calculation is performed for the $\lambda \phi^4$ theory. We
discuss the relevance of the CTP average action in the study of non-equilibrium
aspects of phase transitions in quantum field theory.
