Instabilities and the null energy condition

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.

Comments: 5 pages, 1 figure, revtex, presentation improved, minor changes