TL;DR
This paper discusses the concept of back-reaction in relativistic cosmology, examining how inhomogeneities influence the universe's large-scale evolution and summarizing recent progress in understanding this effect.
Contribution
It provides a critical overview of various approaches to back-reaction in cosmology and highlights recent advances in the field.
Findings
Different methods to quantify back-reaction are summarized.
Recent progress has clarified the impact of inhomogeneities on cosmic evolution.
The paper identifies key challenges and future directions in the study of back-reaction.
Abstract
We introduce the concept of back-reaction in relativistic cosmological modeling. Roughly speaking, this can be thought of as the difference between the large-scale behaviour of an inhomogeneous cosmological solution of Einstein's equations, and a homogeneous and isotropic solution that is a best-fit to either the average of observables or dynamics in the inhomogeneous solution. This is sometimes paraphrased as `the effect that structure has of the large-scale evolution of the universe'. Various different approaches have been taken in the literature in order to try and understand back-reaction in cosmology. We provide a brief and critical summary of some of them, highlighting recent progress that has been made in each case.
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