Universal signature of quantum entanglement across cosmological
  distances
  Although the paradigm of inflation has been extensively studied to
demonstrate how macroscopic inhomogeneities in our universe originate from
quantum fluctuations, most of the established literature ignores the crucial
role that entanglement between the modes of the fluctuating field plays in its
observable predictions. In this paper, we import techniques from quantum
information theory to reveal hitherto undiscovered predictions for inflation
which, in turn, signals how quantum entanglement across cosmological scales can
affect large scale structure. Our key insight is that observable
long-wavelength modes must be part of an open quantum system, so that the
quantum fluctuations can decohere in the presence of an environment of
short-wavelength modes. By assuming the simplest model of single-field
inflation, and considering the leading order interaction term from the
gravitational action, we derive a universal lower bound on the observable
effect of such inescapable entanglement.
