For a quantum field living on a nonstatic space–time no instantaneous Hamiltonian is definable, for this generically necessitates a choice of inequivalent representation of the canonical commutation relations at each instant of time. This fact suggests a description in terms of time-dependent Hilbert spaces, a concept that fits naturally in a (consistent) histories framework. Our primary tool for the construction of the quantum theory in a continuous-time histories format is the recently developed formalism based on the notion of the history group. This we employ to study a model system involving a scalar field in a cavity with moving boundaries. The instantaneous (smeared) Hamiltonian and a decoherence functional are then rigorously defined so that finite values for the time-averaged particle creation rate are obtainable through the study of energy histories. We also construct the Schwinger–Keldysh closed-time-path generating functional as a “Fourier transform” of the decoherence functional and evaluate the corresponding n-point functions.
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