Scaling cosmology

We show that with the help of a suitable coupling between dark energy and cold dark matter it is possible to reproduce any scaling solution rho(X) proportional to rho(M)a(xi), where rho(X) and rho(M) are the densities of dark energy and dark matter, respectively. We demonstrate how the case xi = 1 alleviates the coincidence problem. Future observations of supernovae at high redshift as well as quasar pairs which are planned to discriminate between different cosmological models will also provide direct constraints on the coupling between dark matter and dark energy.