Tensions in Cosmology: Interpreting Them Through Inhomogeneous Models

TL;DR

This paper reviews how inhomogeneous cosmological models, specifically the $ ext{Lambda}$LTB model, can reinterpret current tensions in cosmology, such as the Hubble constant discrepancy and dark energy anomalies, by introducing spatial gradients and anisotropies.

Contribution

It demonstrates how the $ ext{Lambda}$LTB model can account for multiple cosmological tensions without requiring new physics, highlighting the need for a well-founded baseline model.

Findings

Inhomogeneous models can mimic dark energy evolution effects.

Spatial gradients can explain local Hubble constant variations.

Current data constraints limit the model's ability to resolve all tensions.

Abstract

We review a subset of the current tensions affecting the standard $\Lambda$CDM cosmological model, emphasizing the role of chronic systematics and significance inflation in shaping their interpretation. As a unifying framework, we consider the spherically symmetric inhomogeneous $\Lambda$LTB model and use it as a set of "glasses" through which to reinterpret the Hubble, dipole, and dark-energy tensions. Large-scale spatial gradients in this model introduce anisotropic expansion and position-dependent observables, allowing local estimates of $H_{0}$ to shift, dipolar signatures to arise, and an apparently evolving dark-energy equation of state to be mimicked without invoking genuinely dynamical dark energy. We discuss how these effects are constrained once the full supernova, CMB, and large-scale-structure data sets are included, and argue that it remains unclear whether any single $\Lambda$LTB configuration can simultaneously account for all major anomalies. More broadly, we highlight that cosmology currently lacks a widely accepted baseline model that is both theoretically well founded and capable of accommodating the Hubble and dark-energy tensions, leaving us without a true concordance framework for forecasting future surveys.