Dynamical fixed points in holography

TL;DR

This paper uses holography to classify and analyze dynamical fixed points in a driven strongly coupled quantum field theory, revealing conditions for stability and the possibility of unbounded entropy growth.

Contribution

It identifies perturbatively stable and unstable dynamical fixed points in a holographic model of a driven QFT, and explores their non-perturbative stability and entropy behavior.

Findings

Stable DFPs can be non-perturbatively unstable.

Entanglement entropy density signals non-perturbative stability.

Driven systems may lack fixed points, with entropy growing unbounded.

Abstract

Typically, an interactive system evolves towards thermal equilibrium, with hydrodynamics representing a universal framework for its late-time dynamics. Classification of the dynamical fixed points (DFPs) of a driven Quantum Field Theory (with time dependent coupling constants, masses, external background fields, etc.) is unknown. We use holographic framework to analyze such fixed points in one example of strongly coupled gauge theory, driven by homogeneous and isotropic expansion of the background metric - equivalently, a late-time dynamics of the corresponding QFT in Friedmann-Lemaitre-Robertson-Walker Universe. We identify DFPs that are perturbatively stable, and those that are perturbatively unstable, computing the spectrum of the quasinormal modes in the corresponding holographic dual. We further demonstrate that a stable DFP can be unstable non-perturbatively, and explain the role of the entanglement entropy density as a litmus test for a non-perturbative stability. Finally, we demonstrated that a driven evolution might not have a fixed point at all: the entanglement entropy density of a system can grow without bounds.