Testing $\Lambda$CDM cosmology in a binned universe: anomalies in the deceleration parameter

TL;DR

This study investigates potential anomalies in the deceleration parameter of the universe using supernova data, revealing tensions and possible deviations from the standard $ ext{Lambda}$CDM model, especially at very low redshifts.

Contribution

It introduces a binning approach to analyze the deceleration parameter, highlighting tensions and potential deviations from $ ext{Lambda}$CDM, influenced by low-redshift supernova data.

Findings

Tensions of 2-3 sigma in the full sample that vanish when low-redshift data are removed.

A 3.8 sigma tension observed between redshift bins when splitting data by hemisphere.

Posterior distribution analysis suggests a positive slope for $q_0$, contrary to $ ext{Lambda}$CDM expectations.

Abstract

We study the reconstructed deceleration parameter splitting the data in different redshift bins, fitting both a cosmographic luminosity distance and also assuming a flat $\Lambda$CDM model, using the Pantheon+ sample of type Ia supernova data (SNIA). We observe tensions $\sim 2\sigma-3\sigma$ for different redshift and distance indicators if the full sample is used. However, those tensions disappear when the SNIA at $z<0.008$ are removed. If the data is splitted in 2 hemispheres according to our movement w.r.t CMB, a strange $3.8 \sigma$ tension appears in one of the samples between particular redshift bins. Finally, considering posterior distribution as Gaussian, general linear model prefers a positive slope for $q_0$ across redshift bins opposed to a zero slope expected in a $\Lambda$CDM universe. We discuss possible explanations for our results and the influence of lowest redshift SNIA data in cosmological analysis.