Can the coincidence problem be solved by a cosmological model of coupled
  dark energy and dark matter?
  Motivated by the cosmological constant and the coincidence problems, we
consider a cosmological model where the dark sectors are interacting together
through a phenomenological decay law $\dot{\rho}_{\Lambda}=Q\rho_{\Lambda}^n$
in a FRW spacetime with spatial curvature. We show that the only value of $n$
for which the late-time matter energy density to dark energy density ratio
($r_m=\rho_m/\rho_{\Lambda}$) is constant (which could provide an explanation
to the coincidence problem) is $n=3/2$. For each value of $Q$, there are two
distinct solutions. One of them involves a spatial curvature approaching zero
at late times ($\rho_k\approx0$) and is stable when the interaction is weaker
than a critical value ${Q_0=-\sqrt{32\pi G/c^2}}$. The other one allows for a
non-negligible spatial curvature ($\rho_k\napprox0$) at late times and is
stable when the interaction is stronger than $Q_0$. We constrain the model
parameters using various observational data (SNeIa, GRB, CMB, BAO, OHD). The
limits obtained on the parameters exclude the regions where the cosmological
constant problem is significantly ameliorated and do not allow for a completely
satisfying explanation for the coincidence problem.
