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 is unlikely to be less than .
- Received 16 June 2006
DOI:https://doi.org/10.1103/PhysRevD.74.063518
©2006 American Physical Society