The radial BAO scale and Cosmic Shear, a new observable for Inhomogeneous Cosmologies

TL;DR

This paper introduces the normalized cosmic shear as a new observable to test inhomogeneous cosmologies, updates observational constraints on LTB models, and provides tools for future analysis to determine the universe's homogeneity.

Contribution

It proposes a novel observable, the normalized cosmic shear, applicable to inhomogeneous models, and updates constraints on LTB models using recent data, including supernovae, CMB, and BAO measurements.

Findings

LTB models remain consistent with current observations.

New data sets slightly increase the size of permissible voids.

The paper provides an updated code for model calculations and comparisons.

Abstract

As an alternative explanation of the dimming of distant supernovae it has recently been advocated that we live in a special place in the Universe near the centre of a large spherical void described by a Lemaitre-Tolman-Bondi (LTB) metric. In this scenario, the Universe is no longer homogeneous and isotropic, and the apparent late time acceleration is actually a consequence of spatial gradients. We propose in this paper a new observable, the normalized cosmic shear, written in terms of directly observable quantities, and calculable in arbitrary inhomogeneous cosmologies. This will allow future surveys to determine whether we live in a homogeneous universe or not. In this paper we also update our previous observational constraints from geometrical measures of the background cosmology. We include the Union Supernovae data set of 307 Type Ia supernovae, the CMB acoustic scale and the first measurement of the radial baryon acoustic oscillation scale. Even though the new data sets are significantly more constraining, LTB models -- albeit with slightly larger voids -- are still in excellent agreement with observations, at chi^2/d.o.f. = 307.7/(310-4)=1.005. Together with the paper we also publish the updated easyLTB code used for calculating the models and for comparing them to the observations.