  Cosmological consequences of a string-motivated dark energy scenario
featuring a scalar field coupled to the Gauss-Bonnet invariant are
investigated. We study the evolution of the universe in such a model,
identifying its key properties. The evolution of the homogeneous background and
cosmological perturbations, both at large and small scales, are calculated. The
impact of the coupling on galaxy distributions and the cosmic microwave
background is examined. We find the coupling provides a mechanism to viably
onset the late acceleration, to alleviate the coincidence problem, and
furthermore to effectively cross the phantom divide at the present while
avoiding a Big Rip in the future. We show the model could explain the present
cosmological observations, and discuss how various astrophysical and
cosmological data, from the Solar system, supernovae Ia, cosmic microwave
background radiation and large scale structure constrain it.
