Can we remove the systematic error due to isotropic inhomogeneities?

TL;DR

This paper investigates whether large-scale isotropic inhomogeneities in the universe can be distinguished from homogeneous models using combined cosmological observations, highlighting the importance of multiple observables for accurate parameter determination.

Contribution

It demonstrates that combining three key cosmological observables can differentiate between inhomogeneous and homogeneous isotropic universe models under adiabatic perturbations.

Findings

Inhomogeneous and homogeneous models predict different data for all three observables when combined.

Two observables alone are insufficient to distinguish the models, but three can break the degeneracy.

Distinguishing the models depends on the assumption of adiabatic perturbations.

Abstract

Usually, we assume that there is no inhomogeneity isotropic in terms of our location in our uni- verse. This assumption has not been observationally confirmed yet in sufficient accuracy, and we need to consider the possibility that there are non-negligible large-scale isotropic inhomogeneities in our universe. The existence of large-scale isotropic inhomogeneities affects the determination of the cosmological parameters. In particular, from only the distance-redshift relation, we can not dis- tinguish the inhomogeneous isotropic universe model from the homogeneous isotropic one, because of the ambiguity in the cosmological parameters. In this paper, in order to avoid such ambiguity, we consider three observables, the distance-redshift relation, the fluctuation spectrum of the cosmic microwave background radiation(CMBR) and the scale of the baryon acoustic oscillation(BAO), and compare these observables in two universe models; One is the inhomogeneous isotropic uni- verse model with the cosmological constant and the other is the homogeneous isotropic universe model with the dark energy other than the cosmological constant. We show that these two universe models can not predict the same observational data of all three observables but the same ones of only two of three, as long as the perturbations are adiabatic. In principle, we can distinguish the inhomogeneous isotropic universe from the homogeneous isotropic one through appropriate three observables, if the perturbations are adiabatic.