Can we be tricked into thinking thatwis less than−1?

Abstract

Dark energy candidates for which the equation-of-state parameter $w$ is less than $\ensuremath{-}1$ 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 $w<\ensuremath{-}1$ 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 $w<\ensuremath{-}1$.