Exact solutions for the interacting tachyonic-dark matter system
  We find exact solutions leading to power law accelerated expansion for a homogeneous, isotropic and spatially flat universe, dominated by an interacting mixture of cold dark matter and a tachyonic field such that the ratio of the energy densities of both components at late times is constant and no coincidence problem arises. 
Ramon Herrera, Diego Pavon and Winfried Zimdahl