Informal Phenomenology Seminar

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. As another application, Lorentzian (traversable) wormholes and time machines with semi-classical spacetimes are unstable to small perturbations.