Thermodynamics of Quantum Fields in Nonstationary Spacetimes

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Quantum field theory (QFT) on non-stationary spacetimes is well understood from the side of the algebra of observables. The state space, however, is largely unexplored, due to the non-existence of distinguished states (vacuum, scattering states, thermal states). Project C7 of the SFB 676 was focused on characterisations of states by asymptotic conditions, e.g. holography (in case the boundary has sufficiently many symmetries), on a precise version of an approximate particle interpretation (for instance in Robertson--Walker spacetimes) and on the determination in terms of expectation values of locally covariant fields. Additionally, the backreaction of quantum matter fields on the curvature as well as the perturbative quantisation of the Einstein--Klein--Gordon system in the case of a cosmological background have been investigated. Finally, a detailed analysis and construction of equilibrium states for interacting field theories has been performed.