Dark range of ω in Brans-Dicke gravity

The variational field equations of Brans-Dicke scalar-tensor theory of gravitation are presented in a Riemannian and non-Riemannian setting in the language of exterior differential forms over 4-dimensional spacetime. In Rosen coordinates, the equations of motion of non-spinning test masses are considered in a gravitational plane wave setup and detailed to interpret a scalar field $\alpha$ as a candidate for dark energy for negative values of the Brans-Dicke parameter $-3/2 < \omega < 0$ .