Entanglement Entropy and Decoupling in the Universe

TL;DR

This paper explores how quantum entanglement affects entropy calculations in the universe, especially during decoupling, revealing important corrections to traditional thermodynamic entropy in cosmological models.

Contribution

It introduces a perturbative framework to compute entanglement entropy for coupled fields, extending thermodynamics to include quantum entanglement effects in cosmology.

Findings

Entanglement entropy provides significant corrections during instantaneous decoupling.

The formulation is applied to scalar, QED, and Yukawa models.

Quantum corrections to entropy are crucial in early universe scenarios.

Abstract

In the expanding universe, two interacting fields are no longer in thermal contact when the interaction rate becomes smaller than the Hubble expansion rate. After decoupling, two subsystems are usually treated separately in accordance with equilibrium thermodynamics and the thermodynamic entropy gives a fiducial quantity conserved in each subsystem. In this paper, we discuss a correction to this paradigm from quantum entanglement of two coupled fields. The thermodynamic entropy is generalized to the entanglement entropy. We formulate a perturbation theory to derive the entanglement entropy and present Feynman rules in diagrammatic calculations. For specific models to illustrate our formulation, an interacting scalar-scalar system, quantum electrodynamics, and the Yukawa theory are considered. We calculate the entanglement entropy in these models and find a quantum correction to the thermodynamic entropy. The correction is revealed to be important in circumstances of instantaneous decoupling.