Models of dark energy are conveniently characterized by the equation-of-state parameter w = p / \rho , where \rho is the energy density and p is the pressure .
Imposing the Dominant Energy Condition , which guarantees stability of the theory , implies that w \geq - 1 .
Nevertheless , it is conceivable that a well-defined model could ( perhaps temporarily ) have w < -1 , and indeed such models have been proposed .
We study the stability of dynamical models exhibiting w < -1 by virtue of a negative kinetic term .
Although naively unstable , we explore the possibility that these models might be phenomenologically viable if thought of as effective field theories valid only up to a certain momentum cutoff .
Under our most optimistic assumptions , we argue that the instability timescale can be greater than the age of the universe , but only if the cutoff is at or below 100 MeV .
We conclude that it is difficult , although not necessarily impossible , to construct viable models of dark energy with w < -1 ; observers should keep an open mind , but the burden is on theorists to demonstrate that any proposed new models are not ruled out by rapid vacuum decay .