Stochastic Dynamics of Coarse-Grained Quantum Fields in Inflationary Universe

It is shown how coarse-graining of quantum field ^M theory in de Sitter space leads to the emergence of a classical ^M stochastic description as an effective theory in the infra-red regime. ^M The quantum state of the coarse-grained scalar field is found to be a highly^M squeezed coherent state, whose center performs a random walk on a^M bundle of classical trajectories.^M