Universal signature of quantum entanglement across cosmological distances

Abstract

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. Although this signal is too weak for direct detection in the foreseeable future, we discuss the importance of its theoretical implications.

Minimal decoherence from inflation

“…In summary, as expected gravitational mode coupling leaves only a tenuous imprint on the power spectrum: an extremely small correction to the its tilt that is suppressed by (H/M p ) 2 , and a correction to its amplitude that is suppressed by ε 1 (H/M p ) 2 (see also [40]).…”

“…In this paper we follow up on earlier work adapting to gravity well-developed tools from the quantum theory of open systems, 1 whose use to explore late-time evolution we call Open Effective Field Theory (Open EFT) [17,27,[34][35][36][37][38][39][40][41]. These tools show how the evolution equation for the reduced density matrix in the interaction picture can be brought into an approximate (schematic) form…”

The Warm Inflation Story

“…Moreover, of the myriad of possible interesting effects that might emerge from density perturbations, it is a model-building game and one can concoct various such features both from warm and cold inflation models. More recently there has been work to look for intrinsic features that signal a quantum origin or in its absence a classical origin [72][73][74][75]. However such tests appear to be extremely difficult and, if the CMB data remain without significant features, maybe impossible to decisively measure.…”

Looking inside the Swampland from Warm Inflation: Dissipative Effects in De Sitter Expansion

“…That is a challenging task, given the technical complications of dealing with quantum fields in curved spacetimes, as well as additional complexities inherent to de Sitter spacetimes [1][2][3]. One can tackle these problems from either a top-down approach or a bottom-up approach [4][5][6] (see [7][8][9][10] for a new perspective). With the former method, one would begin with a UVcomplete theory of gravity and investigate the implications it has on infrared (IR) degrees of freedom and the respective EFTs that describe their dynamic.…”