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Tensions in Cosmology: Interpreting Them Through Inhomogeneous Models

Valerio Marra

TL;DR

The paper investigates tensions in the ΛCDM cosmology and proposes using the ΛLTB inhomogeneous model as a unifying lens to interpret them. By introducing large-scale spatial gradients, the framework can produce apparent shifts in $H_0$, dipole signatures, and an evolving $w(z)$ without requiring dynamical dark energy, but it remains unclear whether a single ΛLTB configuration can reconcile all anomalies when full data sets are considered. The authors emphasize significant roles for chronic systematics and the look-elsewhere effect in inflating apparent tensions, and they conclude that there is no widely accepted concordance model yet. They advocate for comprehensive multi-probe analyses and upcoming surveys to decisively test inhomogeneous cosmologies and to establish a robust baseline for future forecasts.

Abstract

We review a subset of the current tensions affecting the standard $Λ$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 $Λ$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 $Λ$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.

Tensions in Cosmology: Interpreting Them Through Inhomogeneous Models

TL;DR

The paper investigates tensions in the ΛCDM cosmology and proposes using the ΛLTB inhomogeneous model as a unifying lens to interpret them. By introducing large-scale spatial gradients, the framework can produce apparent shifts in , dipole signatures, and an evolving without requiring dynamical dark energy, but it remains unclear whether a single ΛLTB configuration can reconcile all anomalies when full data sets are considered. The authors emphasize significant roles for chronic systematics and the look-elsewhere effect in inflating apparent tensions, and they conclude that there is no widely accepted concordance model yet. They advocate for comprehensive multi-probe analyses and upcoming surveys to decisively test inhomogeneous cosmologies and to establish a robust baseline for future forecasts.

Abstract

We review a subset of the current tensions affecting the standard 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 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 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 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.
Paper Structure (8 sections, 4 equations, 7 figures)

This paper contains 8 sections, 4 equations, 7 figures.

Table of Contents

  1. Introduction
  2. Chronic systematics
  3. Significance inflation and the look-elsewhere effect
  4. The $\Lambda$LTB model
  5. The Hubble Tension
  6. Dipole Anomalies
  7. Dark-Energy Tension
  8. Conclusions

Figures (7)

  • Figure 1: Growth of structures in an inhomogeneous background. From https://valerio-marra.github.io/BEHOMO-project/.
  • Figure 2: Density parameters (top) and Hubble rates (bottom) for an illustrative underdensity (left) and overdensity (right).
  • Figure 3: An observer near the center of an underdensity measures an enhanced local expansion rate, as illustrated in Figure \ref{['fig:example-t0']}.
  • Figure 4: $\Lambda$LTB model fitted to CMB and calibrated supernova data. The green points indicate the subset used to measure the local $H_{0}$. The full supernova sample does not permit the luminosity jump required to explain away the $H_{0}$ tension. From Camarena:2022iae.
  • Figure 5: Velocity estimates inferred from dipoles shown in chronological order with $1\sigma$ uncertainties. Top: previous measurements based on X-ray sources, radio galaxies (RG), quasars (QSO), CMB non-diagonal correlations, and SNe Ia. Bottom: results from daSilveiraFerreira:2024ddn, combining multiple tracers over $0.4<z<2.2$. Dashed line: velocity implied by the CMB dipole under the peculiar-velocity interpretation. From daSilveiraFerreira:2024ddn.
  • ...and 2 more figures