# Comment on "Hubble flow variations as a test for inhomogeneous cosmology"

## TL;DR

This paper critiques a previous study that tested inhomogeneous cosmology models, showing that the simulation used did not accurately incorporate key features of the timescape model, thus questioning the validity of their conclusions.

## Contribution

The paper identifies and explains the omission of essential geometrical features of the timescape cosmology in the simulation used by Saulder et al (2019).

## Key findings

- The simulation lacked key geometrical features of the timescape model.
- The differential expansion model tested does not accurately represent the timescape cosmology.
- The critique questions the validity of Saulder et al's conclusions based on their simulation approach.

## Abstract

Saulder et al [2019, A&A, 622, A83; arXiv:1811.11976] have performed a novel observational test of the local expansion of the Universe for the standard cosmology as compared to an alternative model with differential cosmic expansion. Their analysis employs mock galaxy samples from the Millennium Simulation, a Newtonian $N$-body simulation on a $\Lambda$CDM background. For the differential expansion case the simulation has been deformed in an attempt to incorporate features of a particular inhomogeneous cosmology: the timescape model. It is shown that key geometrical features of the timescape cosmology have been omitted in this rescaling. Consequently, the differential expansion model tested by Saulder et al (2019) cannot be considered to approximate the timescape cosmology.