Accelerating expansion or inhomogeneity? Part 2: Mimicking acceleration with the energy function in the Lema\^{\i}tre-Tolman model

TL;DR

This paper demonstrates how the Lemaître-Tolman model without a cosmological constant can mimic the luminosity distance-redshift relation of the ΛCDM model by appropriately choosing the energy function E(r), providing insights into inhomogeneous cosmologies.

Contribution

It introduces a method to replicate accelerated expansion effects in the Lemaître-Tolman model using the energy function E(r), without requiring a cosmological constant.

Findings

E > 0 is necessary for mimicking acceleration.

The functions z(r) and E(r) are numerically calculated and matched at key points.

The model can be matched to a Friedmann background to avoid shell crossings.

Abstract

This is a continuation of the paper published in {\it Phys. Rev.} {\bf D89}, 023520 (2014). It is investigated here how the luminosity distance -- redshift relation $D_L(z)$ of the $\Lambda$CDM model is duplicated in the Lema\^{\i}tre -- Tolman (L--T) model with $\Lambda = 0$, constant bang-time function $t_B$ and the energy function $E(r)$ mimicking accelerated expansion on the observer's past light cone ($r$ is a uniquely defined comoving radial coordinate). Numerical experiments show that $E > 0$ necessarily. The functions $z(r)$ and $E(r)$ are numerically calculated from the initial point at the observer's position; then backward from the initial point at the apparent horizon (AH). Reconciling the results of the two calculations allows one to determine the values of $E/r^2$ at $r = 0$ and at the AH. The problems connected with continuing the calculation through the AH are discussed in detail and solved. Then $z(r)$ and $E(r)$ are continued beyond the AH, up to the numerical crash that signals the contact of the light cone with the Big Bang. Similarly, the light cone of the L--T model is calculated by proceeding from the two initial points, and compared with the $\Lambda$CDM light cone. The model constructed here contains shell crossings, but they can be removed by matching the L--T region to a Friedmann background, without causing any conflict with the type Ia supernovae observations. The mechanism of imitating the accelerated expansion by the $E(r)$ function is explained in a descriptive way.