Dark Energy from structure: a status report

TL;DR

This paper reviews how inhomogeneities in the universe might explain Dark Energy through backreaction effects, discussing current understanding, challenges, and the potential need to revise standard cosmological models.

Contribution

It provides a comprehensive overview of backreaction effects in relativistic cosmology and formulates strategies for their quantitative evaluation in theory and observations.

Findings

Backreaction effects suggest a global instability in the standard cosmological model.

Exact solutions and perturbative results support the potential of inhomogeneities to explain Dark Energy.

Current models and simulations may be insufficient to fully describe the late universe.

Abstract

The effective evolution of an inhomogeneous universe model in any theory of gravitation may be described in terms of spatially averaged variables. In Einstein's theory, restricting attention to scalar variables, this evolution can be modeled by solutions of a set of Friedmann equations for an effective volume scale factor, with matter and backreaction source terms. The latter can be represented by an effective scalar field (`morphon field') modeling Dark Energy. The present work provides an overview over the Dark Energy debate in connection with the impact of inhomogeneities, and formulates strategies for a comprehensive quantitative evaluation of backreaction effects both in theoretical and observational cosmology. We recall the basic steps of a description of backreaction effects in relativistic cosmology that lead to refurnishing the standard cosmological equations, but also lay down a number of challenges and unresolved issues in connection with their observational interpretation. The present status of this subject is intermediate: we have a good qualitative understanding of backreaction effects pointing to a global instability of the standard model of cosmology; exact solutions and perturbative results modeling this instability lie in the right sector to explain Dark Energy from inhomogeneities. It is fair to say that, even if backreaction effects turn out to be less important than anticipated by some researchers, the concordance high-precision cosmology, the architecture of current N-body simulations, as well as standard perturbative approaches may all fall short in correctly describing the Late Universe.