Crossing the phantom divide barrier with scalar tensor theories

Abstract

There is accumulating observational evidence (based on SnIa data) that the dark energy equation of stateparameter w may be evolving with time and crossing the phantom divide barrierw = −1 at recent times. The confirmation of these indications by future data would indicate thatminimally coupled quintessence cannot reproduce the observed expansion rateH(z) for any scalar field potential. Here we explicitly demonstrate that scalar tensortheories of gravity (extended quintessence) can predict crossing of the phantomdivide barrier. We reconstruct phenomenologically viable scalar–tensor potentialsF(Φ) and U(Φ) that can reproduce a polynomial best fit expansion rateH(z) and the corresponding dark energy equation of state parameterw(z) crossingthe w = −1 line. The form of the reconstructed scalar tensor potentials is severely constrained but isnot uniquely determined. This is due to observational uncertainties in the form ofH(z) and also because asingle observed function H(z) does not suffice for the reconstruction of the two potential functionsF(Φ) and U(Φ).