Scalar-tensor gravity and quintessence - art. no. 023518

Scalar fields with inverse power-law effective potentials may provide a neg ative pressure component to the energy density of the universe today, as re quired by cosmological observations. Ln order to be cosmologically relevant today, the scalar field should have a mass m(phi) = O(10(-33) eV), thus po tentially inducing sizable violations of the equivalence principle and spac e-time variations of the coupling constants. Scalar-tensor theories of grav ity provide a framework for accommodating phenomenologically acceptable ult ralight scalar fields. We discuss nonminimally coupled scalar-tensor theori es in which the scaler-matter coupling is a dynamical quantity. Two attract or mechanisms are operative at the same time: one towards the tracker solut ion, which accounts for the accelerated expansion of the Universe, and one towards general relativity, which makes the ultralight scalar field phenome nologically safe today. As in usual tracker-field models, the late-time beh avior is largely independent of the initial conditions. Strong distortions in the cosmic microwave background anisotropy spectra as well as in the mat ter power spectrum are expected.