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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.
In this paper we investigate possible solutions to the coincidence problem in flat phantom dark energy models with a constant dark energy equation of state and quintessence models with a linear scalar field potential. These models are representative
The cosmic coincidence problem is a serious challenge to dark energy model. We suggest a quantitative criteria for judging the severity of the coincidence problem. Applying this criteria to three different interacting models, including the interactin
We investigate the cosmology of massive spinor electrodynamics when torsion is non-vanishing. A non-minimal interaction is introduced between the torsion and the vector field and the coupling constant between them plays an important role in subsequen
The field equations in FRW background for the so called C-theories are presented and investigated. In these theories the usual Ricci scalar is substituted with $f(mathcal{R})$ where $mathcal{R}$ is a Ricci scalar related to a conformally scaled metri
We analyze the global behaviour of the growth index of cosmic inhomogeneities in an isotropic homogeneous universe filled by cold non-relativistic matter and dark energy (DE) with an arbitrary equation of state. Using a dynamical system approach, we