A ug 2 01 3 Hybrid modified gravity unifying local tests , galactic dynamics and late - time cosmic acceleration ∗

The non-equivalence between the metric and Palatini formalisms of f(R) gravity is an intriguing feature of these theories. However, in the recently proposed hybrid metric-Palatini gravity, consisting of the superposition of the metric Einstein-Hilbert Lagrangian with an f(R) term constructed à la Palatini, the “true” gravitational field is described by the interpolation of these two non-equivalent approaches. The theory predicts the existence of a light long-range scalar field, which passes the local constraints and affects the galactic and cosmological dynamics. Thus, the theory opens new possibilities for a unified approach, in the same theoretical framework, to the problems of dark energy and dark matter, without distinguishing a priori matter and geometric sources, but taking their dynamics into account under the same standard. keywords: modified gravity, late-time cosmic acceleration, dark matter, Solar System tests PACS numbers: 04.50.Kd, 04.20.Cv Electronic address: [email protected] Electronic address: [email protected] Electronic address: [email protected] Electronic address: [email protected]