Crossing the Phantom Divide Barrier with Scalar Tensor Theories

There is accumulating observational evidence (based on SnIa data) that the dark energy equation of state parameter $w$ may be evolving with time and crossing the phantom divide barrier $w=-1$ at recent times. The confirmation of these indications by future data would indicate that minimally coupled quintessence can not reproduce the observed expansion rate $H(z)$ for any scalar field potential. Here we explicitly demonstrate that scalar tensor theories of gravity (extended quintessence) can predict crossing of the phantom divide barrier. We reconstruct phenomenologically viable scalar-tensor potentials $F(\Phi)$ and $U(\Phi)$ that can reproduce a polynomial best fit expansion rate $H(z)$ and the corresponding dark energy equation of state parameter $w(z)$ crossing the $w=-1$ line. The form of the reconstructed scalar tensor potentials is severely constrained but is not uniquely determined. This is due to observational uncertainties in the form of $H(z)$ and also because a single observed function $H(z)$ does not suffice for the reconstruction of the two potential functions $F(\Phi)$ and $U(\Phi)$.