ON THE TREATMENT OF INITIAL CORRELATIONS IN QUANTUM-FIELD THEORY OF NONEQUILIBRIUM STATES

Wick's theorem is discussed for correlated states. The non-zero expectation values of normal-ordered operators give rise to new types of contractions. They can be incorporated into the diagrammatic Feynman rules as additional vertices, which couple the time evolution of the system to its initial correlations. It is shown that the perturbation series for Green functions is at most linear in these correlations. They can be expressed as linear combinations of initial free-field Wightman functions, thereby making a normal ordering procedure unnecessary. The connection of the formalism to the standard description is discussed using the generating functional for correlations. © 1990.