Scalar-tensor gravity and quintessence

Scalar fields with inverse power-law effective potentials may provide a negative pressure component to the energy density of the universe today, as required by cosmological observations. In order to be cosmologically relevant today, the scalar field should have a mass m_=O (10^-33eV), thus potentially inducing sizable violations of the equivalence principle and space-time variations of the coupling constants. Scalar-tensor theories of gravity provide a framework for accommodating phenomenologically acceptable ultralight scalar fields. We discuss nonminimally coupled scalar-tensor theories in which the scalar-matter coupling is a dynamical quantity. Two attractor mechanisms are operative at the same time: one towards the tracker solution, which accounts for the accelerated expansion of the Universe, and one towards general relativity, which makes the ultralight scalar field phenomenologically safe today. As in usual tracker-field models, the late-time behavior is largely independent of the initial conditions. Strong distortions in the cosmic microwave background anisotropy spectra as well as in the matter power spectrum are expected.