Dark energy problem: from phantom theory to modified Gauss-Bonnet   gravity
  The solution of dark energy problem in the models without scalars is presented. It is shown that late-time accelerating cosmology may be generated by the ideal fluid with some implicit equation of state. The universe evolution within modified Gauss-Bonnet gravity is considered. It is demonstrated that such gravitational approach may predict the (quintessential, cosmological constant or transient phantom) acceleration of the late-time universe with natural transiton from deceleration to acceleration (or from non-phantom to phantom era in the last case). 
Shin'ichi Nojiri, Sergei D. Odintsov, and O.G. Gorbunova