Evolution from Pure States into Mixed States in de Sitter Space

An attempt is made to clarify realization of a classical distribution from quantum fluctuations of the order parameter in the inflationary universe. We discuss destruction of quantum coherence associated with a state of the order parameter in models where it interacts with the environment. For that purpose, the time evolution of the reduced density matrix ρ, which is obtained by coarse-graining of the environment, is investigated. It is shown that off-diagonal elements of ρ decreased as the phase transition proceeds.