Instabilities and the null energy condition

TL;DR

This paper demonstrates that violating the null energy condition leads to instabilities across various models, suggesting that dark energy with an equation of state parameter less than -1 is unlikely.

Contribution

It establishes a general link between null energy condition violations and instabilities, impacting dark energy models with w < -1.

Findings

Null energy condition violation causes instability in classical gauge theories and perfect fluids.

Dark energy models with w < -1 are likely unstable due to null energy condition violations.

Supports the idea that w < -1 dark energy scenarios are physically implausible.

Abstract

We show that violation of the null energy condition implies instability in a broad class of models, including classical gauge theories with scalar and fermionic matter as well as any perfect fluid. When applied to the dark energy, our results imply that $w = p / \rho$ is unlikely to be less than -1.