Inhomogeneous Cosmology

Inhomogeneous cosmology is the study of cosmological models that abandon the simplifying assumption of large-scale homogeneity and isotropy embedded in the FLRW metric and the Friedmann equations. Rather than treating the universe as a smoothed fluid whose expansion is governed by a single global scale factor, inhomogeneous approaches account for the real structure of the cosmos — galaxies, clusters, filaments, and voids — and ask how these structures modify the large-scale dynamics that the standard model averages over.

The central question is backreaction: can local inhomogeneities, when averaged, produce an effective expansion history that differs from the smooth solution? General relativity is nonlinear, so the average of the Einstein equations is not the Einstein equation of the average. Some researchers argue that this noncommutativity could mimic dark energy or alter the inferred Hubble constant, potentially resolving the Hubble tension without invoking new physics. Others contend that backreaction effects are quantitatively too small to matter at the percent level required.

Inhomogeneous cosmology occupies a curious position in the field: it is simultaneously conservative — it demands no exotic fields or modified gravity, only a more careful treatment of the geometry we already know — and radical, because it undermines the very approximation scheme on which modern cosmology was built.