Scalar-tensor theories of gravity provide a consistent framework to accommodate an ultra-light quintessence scalar field. While the equivalence principle is respected by construction, deviations from General Relativity and standard cosmology may show up at nucleosynthesis, CMB, and solar system tests of gravity. After imposing all the bounds coming from these observations, we consider the expansion rate of the universe at WIMP decoupling, showing that it can lead to an enhancement of the dark matter relic density up to few orders of magnitude with respect to the standard case. This effect can have an impact on supersymmetric candidates for dark matter.PACS numbers: 98.80. Cq, 95.35.+d

We develop a framework to study the phase space of a system consisting of a scalar field rolling down an arbitrary potential with varying slope and a background fluid, in a cosmological setting. We give analytical approximate solutions of the field evolution and discuss applications of its features to the issues of quintessence, moduli stabilisation and quintessential inflation.

Recent data point in the direction of a cosmological constant dominated universe. We investigate the role of supersymmetric QCD with N f ϽN c as a possible candidate for a dynamical cosmological constant ͑''quintessence''͒. When N f Ͼ1, the multiscalar dynamics is fully taken into account, showing that a certain degree of flavor symmetry in the initial conditions is necessary in order to attain the required late-time ''tracking'' attractor behavior. A possible solution to the problem of long-range interactions of gravitational strength is proposed. Finally we show how, as a consequence of the interaction with the standard model fields, the early cosmological evolution of the scalar fields may be dramatically altered, and the approach to the tracking behavior may become much more efficient.PACS number͑s͒: 98.80. Cq, 95.35.ϩd