Semi-classical Einstein equations:descend to the ground state

Abstract

The time-dependent cosmological term arises from the energy-momentum tensor calculated in a state different from the ground state. We discuss the expectation value of the energy-momentum tensor on the rhs of Einstein equations in various (approximate)pure as well as mixed states. We apply the classical slow-roll field evolution as well as the Starobinsky and warm inflation stochastic equations in order to calculate the expectation value. We show that in a state concentrated at the local maximum of the double-well potential the expectation value is decreasing exponentially. We confirm the descend of the expectation value in the stochastic inflation model. We calculate the cosmological constant $\Lambda$ at large time as the expectation value of the energy density with respect to the stationary probability distribution. We show that $\Lambda\simeq \gamma^{\frac{4}{3}} where $\gamma$ is the thermal dissipation rate.