Interacting quintessence and the coincidence problem
  We investigate the role of a possible coupling of dark matter and dark energy. In particular, we explore the consequences of such an interaction for the coincidence problem, i.e., for the question, why the energy densities of dark matter and dark energy are of the same order just at the present epoch. We demonstrate, that, with the help of a suitable coupling, it is possible to reproduce any scaling solution $\rho_X \propto \rho_M a^\xi$, where $a$ is the scale factor of the Robertson-Walker metric and $\xi$ is a constant parameter. $\rho_X$ and $\rho_M$ are the densities of dark energy and dark matter, respectively. Furthermore, we show that an interaction between dark matter and dark energy can drive the transition from an early matter dominated era to a phase of accelerated expansion with a stable, stationary ratio of the energy densities of both components. 
W. Zimdahl (Koeln), D. Pavon (Barcelona), L.P. Chimento (Buenos   Aires), A.S. Jakubi (Buenos Aires)