The null energy condition and instability

TL;DR

This paper demonstrates that violating the null energy condition leads to instability in various physical models, discusses causality implications, and suggests dark energy's equation of state parameter w is unlikely below -1.

Contribution

It generalizes the link between null energy condition violation and instability across multiple models, including fluids and gauge theories.

Findings

Violation of null energy condition implies instability.

Causality considerations are integral to the analysis.

Dark energy models with w < -1 are unlikely based on stability constraints.

Abstract

We extend previous work showing that violation of the null energy condition implies instability in a broad class of models, including gauge theories with scalar and fermionic matter as well as any perfect fluid. Simple examples are given to illustrate these results. The role of causality in our results is discussed. Finally, we extend the fluid results to more general systems in thermal equilibrium. When applied to the dark energy, our results imply that w is unlikely to be less than -1.