Inverse Construction of the $\Lambda$LTB Model from a Distance-redshift Relation

TL;DR

This paper presents a method to reconstruct the $ ext{Lambda}$LTB cosmological model from a given distance-redshift relation, enabling the study of inhomogeneities in the universe with a positive cosmological constant.

Contribution

The authors develop a set of differential equations to uniquely determine the $ ext{Lambda}$LTB model from observational data, including treatment of singular points, and demonstrate the method with a flat $ ext{Lambda}$CDM example.

Findings

Constructed $ ext{Lambda}$LTB models for various $ ext{Lambda}$ values.

Identified void structures for lower $ ext{Lambda}$ values.

Established relation between inhomogeneity amplitude and $ ext{Lambda}$ ratio.

Abstract

Spherically symmetric dust universe models with a positive cosmological constant $\Lambda$, known as $\Lambda$-Lema\^itre-Tolman-Bondi($\Lambda$LTB) models, are considered. We report a method to construct the $\Lambda$LTB model from a given distance-redshift relation observed at the symmetry center. The spherical inhomogeneity is assumed to be composed of growing modes. We derive a set of ordinary differential equations for three functions of the redshift, which specify the spherical inhomogeneity. Once a distance-redshift relation is given, with careful treatment of possible singular points, we can uniquely determine the model by solving the differential equations for each value of $\Lambda$. As a demonstration, we fix the distance-redshift relation as that of the flat $\Lambda$CDM model with $(\Omega^{\rm dis}_{\rm m0}, \Omega^{\rm dis}_{\rm \Lambda 0})=(0.3,0.7)$, where $\Omega^{\rm dis}_{\rm m0}$ and $\Omega^{\rm dis}_{\rm \Lambda 0}$ are the normalized matter density and the cosmological constant, respectively. Then, we construct the $\Lambda$LTB model for several values of $\Omega_{\rm \Lambda 0}:=\Lambda/(3H_0^2)$, where $H_0$ is the present Hubble parameter observed at the symmetry center. We obtain void structure around the symmetry center for $\Omega_{\Lambda 0}<\Omega^{\rm dis}_{\Lambda 0}$. We show the relation between the ratio $\Omega_{\Lambda0}/\Omega^{\rm dis}_{\Lambda 0}$ and the amplitude of the inhomogeneity.