TL;DR
This paper reviews the backreaction hypothesis suggesting that inhomogeneities in the universe may explain observed cosmic acceleration without new physics, highlighting recent progress and unresolved questions.
Contribution
It synthesizes recent developments in backreaction research, emphasizing progress and open issues in understanding cosmic inhomogeneities' effects.
Findings
Backreaction could account for cosmic acceleration.
Progress has been made in modeling inhomogeneities.
Open questions remain about the quantitative impact of backreaction.
Abstract
Homogeneous and isotropic cosmological models with ordinary matter and gravity predict slower expansion and shorter distances than observed. It is possible that this failure is due the known breakdown of homogeneity and isotropy related to structure formation, rather than new fundamental physics. We review this backreaction conjecture, concentrating on topics on which there has been progress as well as open issues.
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