Aspects of the cosmological "coincidence problem"
  The observational fact that the present values of the densities of dark
energy and dark matter are of the same order of magnitude,
$\rho_{de0}/\rho_{dm0} \sim \mathcal{O}(1)$, seems to indicate that we are
currently living in a very special period of the cosmic history. Within the
standard model, a density ratio of the order of one just at the present epoch
can be seen as coincidental since it requires very special initial conditions
in the early Universe. The corresponding "why now" question constitutes the
cosmological "coincidence problem". According to the standard model the
equality $\rho_{de} = \rho_{dm}$ took place "recently" at a redshift $z \approx
0.55$. The meaning of "recently" is, however, parameter dependent. In terms of
the cosmic time the situation looks different. We discuss several aspects of
the "coincidence problem", also in its relation to the cosmological constant
problem, to issues of structure formation and to cosmic age considerations.
