Abstract
In-In perturbation theory is a vital tool for cosmology and nonequilibrium physics. Here, we reconcile an apparent conflict between two of its important aspects with particular relevance to De Sitter/inflationary contexts: (i) the need to slightly deform unitary time evolution with an iϵ prescription that projects the free (“Bunch-Davies”) vacuum onto the interacting vacuum and renders vertex integrals well-defined, and (ii) Weinberg’s “nested commutator” reformulation of in-in perturbation theory which makes manifest the constraints of causality within expectation values of local operators, assuming exact unitarity. We show that a modified iϵ prescription maintains the exact unitarity on which the derivation of (ii) rests, while nontrivially agreeing with (i) to all orders of perturbation theory.
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ArXiv ePrint: 2010.10785
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Baumgart, M., Sundrum, R. Manifestly causal in-in perturbation theory about the interacting vacuum. J. High Energ. Phys. 2021, 80 (2021). https://doi.org/10.1007/JHEP03(2021)080
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DOI: https://doi.org/10.1007/JHEP03(2021)080