  We show how the concept of quantum open system and the methods in
non-equilibrium statistical mechanics can be usefully applied to studies of
quantum statistical processes in the early universe. We first sketch how noise,
fluctuation, dissipation and decoherence processes arise in a wide range of
cosmological problems. We then focus on the origin and nature of noise in
quantum fields and spacetime dynamics. We introduce the concept of
geometrodynamic noise and suggest a statistical mechanical definition of
gravitational entropy. We end with a brief discussion of the theoretical
appropriateness to view the physical universe as an open system.
