Not all adiabatic vacua are physical states

Abstract

Adiabatic vacua are known to be Hadamard states. We show, however that the energy-momentum tensor of a linear Klein-Gordon field on Robertson-Walker spaces developes a generic singularity on the initial hypersurface if the adiabatic vacuum is of order less than four. Therefore, adiabatic vacua are physically reasonable only if their order is at least four. A certain non-local large momentum expansion of the mode functions has recently been suggested to yield the subtraction terms needed to remove the ultraviolet divergences in the energy-momentum tensor. We find that this scheme fails to reproduce the trace anomaly and therefore is not equivalent to adiabatic regularisation.

13 pages, LaTex2e

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• Towards the Theory of Reheating After Inflation
• Reheating after Inflation
• Non-Equilibrium Quantum Fields in the Large N Expansion
• Renormalization of nonequilibrium dynamics in FRW cosmology
• On the choice of initial states in nonequilibrium dynamics

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