Can averaging over cosmic inhomogeneities mimic dark energy?
In plain words
The standard model computes the expansion as if the universe were perfectly smooth, but it is lumpy, with large voids and dense walls. Some researchers argue that averaging over the lumps, including clocks running at different rates in voids and walls, produces apparent acceleration without dark energy.
Precise statement
In general relativity the volume-averaged expansion of an inhomogeneous dust universe obeys the Buchert equations, whose kinematical backreaction term $Q_D$ acts as an effective energy component. Determine (a) whether $Q_D$ in a universe with realistic nonlinear structure is large enough to give an apparent deceleration parameter $q < 0$ at $z < 0.5$, and (b) whether the timescape model fits type Ia supernovae, BAO and CMB jointly as well as LCDM. Seifert et al. (2025, MNRAS Lett. 537, L55) report Bayesian evidence $\ln B > 5$ for timescape over LCDM with Pantheon+ supernovae alone.
What would settle it
Fully relativistic numerical cosmology simulations giving $Q_D$ for realistic structure, plus a joint timescape fit to supernovae, BAO and CMB with a shared systematics model.
Status in the literature
Unverified note
The 2025 supernova-only timescape preference has no independent joint-data confirmation as of 2026; most groups regard backreaction as small.