Manifestly causal in-in perturbation theory about the interacting vacuum

Matthew Baumgart, Raman Sundrum

Research output: Contribution to journalArticlepeer-review

13 Scopus citations


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.

Original languageEnglish (US)
Article number80
JournalJournal of High Energy Physics
Issue number3
StatePublished - Mar 2021


  • Beyond Standard Model
  • Cosmology of Theories beyond the SM

ASJC Scopus subject areas

  • Nuclear and High Energy Physics


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