Dark energy problem: from phantom theory to modified Gauss-Bonnet gravity

The solution of the dark energy problem in models without scalars is presented. It is shown that a late-time accelerating cosmology may be generated by an ideal fluid with some implicit equation of state. The evolution of the universe within modified Gauss-Bonnet gravity is considered. It is demonstrated that such a gravitational approach may predict the (quintessential, cosmological constant or transient phantom) acceleration of the late-time universe with a natural transition from deceleration to acceleration (or from non-phantom to phantom era in the last case).