Quantum to classical transition of density fluctuation in the inflationary model

Quantum-classical transition of density fluctuation in an inflationary model is studied. As the environment, short wave length mode of the scalar field is taken. We derived the master equation for the reduced density matrix which describes quantum mechanical time evolution of long wave fluctuation of the scalar field. We show that quantum coherence is lost and classical correlation in phase space appears. Relation to the stochastic treatment of inflation is also discussed.