Inflationary Decoherence from the Gravitational Floor

Abstract

We re-examine the decoherence rate of primordial fluctuations within minimal inflationary models, using only the gravitational interactions required for the underlying fluctuation-generation mechanism itself. Since gravity provides the weakest interactions the result provides a plausible floor on the rate of primordial decoherence. Previous calculations ({\tt arXiv:2211.11046}) did so using only a subset of these interactions, motivated by assuming both system and environment were super-Hubble. We extend this by including the effects on super-Hubble modes of {\it all} gravitational interactions amongst scalar fluctuations at leading order in $H/\Mp$ (and so need not restrict the decohering environment to being super-Hubble). We show how the decohering evolution becomes Markovian for super-Hubble modes, without the need to appeal to truncations (like the `rotating wave' approximation) that are often used in optics but can be inapprorpriate for cosmology. We find that the dominant contribution comes from the nonlocal cubic interactions obtained by solving the constraints. We find a decoherence rate that grows in the super-Hubble regime {\it faster} than found earlier and identify its leading divergent and finite parts. We argue why the divergent parts must cancel with other competing contributions -- such as decoherence due to environmental tensor modes -- that are partially computed elsewhere (and for which a complete calculation is in progress). We discuss the steps required to resum this result to late times and briefly discuss why they are more complicated than for earlier calculations.