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Dark energy candidates for which the equation-of-state parameter w is lessthan -1 violate the dominant energy condition, and are typically unstable. Inscalar-tensor theories of gravity, however, the expansion of the universe canmimic the behavior of general relativity with w<-1 dark energy, withoutviolating any energy conditions. We examine whether this possibility isphenomenologically viable by studying Brans-Dicke models and characterizingboth the naturalness of the models themselves, and additional observationalconstraints from limits on the time-dependence of Newton's constant. We findthat only highly contrived models would lead observers to measure w<-1.Comment: 20 pages, 4 figures, uses RevTe

physical review. d. particles, fields, gravitation, and cosmology/physical review. d, particles, fields, gravitation, and cosmology

Can we be tricked into thinking that<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>w</mml:mi></mml:math>is less than<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:math>?

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