Decoherence of Primordial Perturbations

We study the quantum decoherence of curvature perturbations at superhorizon scales induced by gravitational nonlinearities. We show that cubic gravitational couplings, constrained by spatial diffeomorphism invariance, generate both infrared (IR) and ultraviolet (UV) divergences in the decoherence rate at one loop. These divergences arise from fluctuations of deep IR modes, which appear as a background for a local observer, and from violent zero-point fluctuations in the deep UV. We argue that these divergences are unobservable, as they vanish when expressed in terms of proper observables. Specifically, we consider correlators defined using the geodesic distance to regulate IR divergences and time-averaged correlators to regulate UV divergences. To incorporate the observer’s perspective, we propose defining an effective quantum state in terms of actual observables, providing a more appropriate probe of the system’s quantum coherence as measured by an observer. Using this framework, we evaluate the finite decoherence rate induced by the superhorizon environment both during inflation and in the late universe. This talk is based on arXiv:2504.10472.