Open Effective Field Theory and Inflation

Abstract: Effective field theories (EFTs) are very useful ways to exploit the simplicity that comes when systems have a hierarchy of scales. This simplicity is normally summarized in terms of an effective lagrangian, which systematically captures the most important effects of short-distance states for long-distance observers. A long-standing problem asks: What is the EFT for extra-Hubble physics during inflation? This talk shows that the answer is a new kind of effective field theory, Open EFT, such as would apply to open systems like a particle moving through an environment like a fluid. In such a system the ability to exchange information with the environment implies pure states can evolve to mixed states, precluding a description in terms of an effective lagrangian. The simplicity associated with a hierarchy of scale can nonetheless be captured, using a Lindblad equation. This talk provides a quick introduction to Open EFTs, and uses them to describe the effective field theory appropriate to super-Hubble modes during inflation in the presence of an environment of sub-Hubble modes. The resulting Lindblad equation explains why primordial quantum fluctuations quickly decohere while outside the Hubble scale, re-entering as classical fluctuations in the field eigen-basis. It also explains why fluctuation probabilities are described by a stochastic framework, and provides a first-principles derivation of Starobinsky's stochastic inflation formalism (plus, in principle, allowing the computation of sub-dominant corrections to it).