The effect of structure formation on the expansion of the universe

TL;DR

This paper investigates how non-linear structure formation in the universe might explain the observed discrepancy in late-time expansion rate measurements, suggesting that inhomogeneities could increase the expansion rate compared to homogeneous models.

Contribution

It introduces a model incorporating non-linear structures in a dust universe to evaluate their impact on the expansion rate, providing a potential explanation for observational discrepancies.

Findings

Expansion rate increases by a factor of 1.1-1.3 due to structure formation.

The timescale for this effect is derived from the cold dark matter transfer function.

No additional free parameters are needed for the model.

Abstract

Observations of the expansion rate of the universe at late times disagree by a factor of 1.5-2 with the prediction of homogeneous and isotropic models based on ordinary matter and gravity. We discuss how the departure from linearly perturbed homogeneity and isotropy due to structure formation could explain this discrepancy. We evaluate the expansion rate in a dust universe which contains non-linear structures with a statistically homogeneous and isotropic distribution. The expansion rate is found to increase relative to the exactly homogeneous and isotropic case by a factor of 1.1-1.3 at some tens of billion of years. The timescale follows from the cold dark matter transfer function and the amplitude of primordial perturbations without additional free parameters.