The Strong Cosmological Coincidence Problem

The standard cosmological coincidence problem asks why matter and dark energy become dynamically comparable only close to the present epoch. I will discuss a stronger version of this problem, motivated by the observation that the growing relative importance of dark energy coincides with the nonlinear development of cosmic structure.

In several inhomogeneous and coarse-grained descriptions, quantities characterising structure formation evolve in a way that resembles the effective dark energy contribution inferred within the standard cosmological model.

In this context, I will introduce Buchert's scalar-averaging framework and its partitioning formalism, which provide a natural way to separate the effects of internal regional dynamics from the variance of expansion rates between different regions. I will then discuss how the relativistic Zel'dovich approximation can be used to close the averaged scalar equations.

I will conclude by outlining how the framework of inhomogeneous cosmology may provide a resolution of the strong cosmological coincidence problem.