QUANTUM TO CLASSICAL TRANSITION OF DENSITY-FLUCTUATIONS IN THE INFLATIONARY MODEL

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