Gauss-Bonnet dark energy

We propose the Gauss-Bonnet dark energy model inspired by string/M-theory where standard gravity with scalar contains additional scalar-dependent coupling with Gauss-Bonnet invariant. It is demonstrated that effective phantom (or quintessence) phase of late universe may occur due to presence of such term which is relevant even at small curvature. However, with the increase of the curvature the scalar potential may become dominant so that phantom phase is transient and $w=-1$ barrier may be passed. Hence, the current acceleration of the universe may be caused by stringy effects. It is remarkable that scalar-Gauss-Bonnet coupling acts against the Big Rip occurence also in phantom cosmology realized by scalar with negative kinetic energy.