Nonlinear perturbations in scalar-tensor cosmologies

Can local fluctuations of a âquintessenceâ scalar field play a dynamical role in the gravitational clustering and cosmic structure formation process? We address this question in the general framework of scalar-tensor theories of gravity. Nonlinear energy density perturbations, both in the scalar field and matter component, and linear metric perturbations are accounted for in the perturbed Einstein equations. We derive the Newtonian limit of the relevant equations for clustering in scalar-tensor cosmologies. We then specialize to nonlinear perturbations of the âextended quintessenceâ model of dark energy; in such a model, a nonminimally coupled scalar field is thought to be responsible for driving the present accelerated phase of the Universe expansion. The interplay between dark energy and dark matter is displayed in the equations governing the growth of structure in the Universe. © 2004 The American Physical Society