Aspects of the cosmological "coincidence problem"

TL;DR

This paper examines the cosmological coincidence problem, exploring why dark energy and dark matter densities are comparable today, and discusses its relation to the cosmological constant, structure formation, and cosmic age.

Contribution

It provides a comprehensive discussion of various aspects of the coincidence problem and its connections to other fundamental cosmological issues.

Findings

The density ratio of dark energy to dark matter is approximately one today.

The coincidence problem is sensitive to the choice of parameters and epoch definitions.

Multiple facets of the problem are interconnected with the cosmological constant and structure formation.

Abstract

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.