Dark energy candidates for which the equation-of-state parameter w is less than -1 violate the dominant energy condition , and are typically unstable .
In scalar-tensor theories of gravity , however , the expansion of the universe can mimic the behavior of general relativity with w < -1 dark energy , without violating any energy conditions .
We examine whether this possibility is phenomenologically viable by studying Brans-Dicke models and characterizing both the naturalness of the models themselves , and additional observational constraints from limits on the time-dependence of Newton ’ s constant .
We find that only highly contrived models would lead observers to measure w < -1 .