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DESIgning concordant distances in the age of precision cosmology: the impact of density fluctuations

David Camarena, Kylar Greene, John Houghteling, Francis-Yan Cyr-Racine

TL;DR

This study tests the FLRW homogeneity assumption by comparing line-of-sight and transverse BAO distances, finding hints of an anisotropic expansion that could arise from intermediate-scale density fluctuations rather than phantom dark energy. It then analyzes a simple late-time inhomogeneous extension, the ΛLTB model, which yields two directional expansion rates and can fit BAO, SNeIa, and CMB data at levels comparable to phantom $w_0w_a$ models; Pantheon+ data prefer modest inhomogeneities (2.8σ from FLRW), while DESY5 data prefer more extended overdensities (5.2σ), illustrating dataset sensitivity. The results motivate exploring beyond-FLRW cosmologies and developing more realistic inhomogeneous descriptions, with future surveys like Euclid and LSST poised to decisively test whether large-scale homogeneity remains tenable in precision cosmology. In short, deviations from homogeneity at intermediate scales offer a plausible alternative to phantom dark energy for explaining distance–redshift observations, warranting further theoretical and observational investigation.

Abstract

Discrepancies between distance measurements and $Λ$CDM predictions reveal notable features in the distance-redshift relation, possibly suggesting the presence of an evolving dark energy component. Given the central role of the Friedmann-Lemaître-Robertson-Walker (FLRW) metric in modeling cosmological distances, we investigate here whether these features instead point to a possible departure from the fundamental FLRW symmetries. Exploiting the transverse and line-of-sight distances provided by baryonic acoustic oscillations (BAO) observations, we demonstrate that observed distances hint at a slight but systematic preference for an anisotropic expansion rate emerging regardless of the dark energy model considered. Leveraging this non-FLRW feature, we investigate an inhomogeneous extension of the $Λ$CDM model that naturally provides an anisotropic expansion rate. Our analysis demonstrates that models featuring spherical overdensities can explain BAO, supernova, and cosmic microwave background data, providing fits statistically indistinguishable from those obtained with a phantom dark energy scenario. When Pantheon+ data is considered, our analysis challenges the FLRW framework at $2.8σ$ and yields scenarios that can be interpreted as subtle but non-negligible deviations from the FLRW metric. When DESY5 supernovae are considered instead, deviations are notably more significant, yielding scenarios that mildly violate the Copernican principle and exclude the FLRW assumption at $5.2σ$. Overall, our results motivate a more in-depth investigation of whether the perfectly homogeneous and isotropic FLRW paradigm can still be assumed to accurately predict cosmological distances in the era of precision cosmology.

DESIgning concordant distances in the age of precision cosmology: the impact of density fluctuations

TL;DR

This study tests the FLRW homogeneity assumption by comparing line-of-sight and transverse BAO distances, finding hints of an anisotropic expansion that could arise from intermediate-scale density fluctuations rather than phantom dark energy. It then analyzes a simple late-time inhomogeneous extension, the ΛLTB model, which yields two directional expansion rates and can fit BAO, SNeIa, and CMB data at levels comparable to phantom models; Pantheon+ data prefer modest inhomogeneities (2.8σ from FLRW), while DESY5 data prefer more extended overdensities (5.2σ), illustrating dataset sensitivity. The results motivate exploring beyond-FLRW cosmologies and developing more realistic inhomogeneous descriptions, with future surveys like Euclid and LSST poised to decisively test whether large-scale homogeneity remains tenable in precision cosmology. In short, deviations from homogeneity at intermediate scales offer a plausible alternative to phantom dark energy for explaining distance–redshift observations, warranting further theoretical and observational investigation.

Abstract

Discrepancies between distance measurements and CDM predictions reveal notable features in the distance-redshift relation, possibly suggesting the presence of an evolving dark energy component. Given the central role of the Friedmann-Lemaître-Robertson-Walker (FLRW) metric in modeling cosmological distances, we investigate here whether these features instead point to a possible departure from the fundamental FLRW symmetries. Exploiting the transverse and line-of-sight distances provided by baryonic acoustic oscillations (BAO) observations, we demonstrate that observed distances hint at a slight but systematic preference for an anisotropic expansion rate emerging regardless of the dark energy model considered. Leveraging this non-FLRW feature, we investigate an inhomogeneous extension of the CDM model that naturally provides an anisotropic expansion rate. Our analysis demonstrates that models featuring spherical overdensities can explain BAO, supernova, and cosmic microwave background data, providing fits statistically indistinguishable from those obtained with a phantom dark energy scenario. When Pantheon+ data is considered, our analysis challenges the FLRW framework at and yields scenarios that can be interpreted as subtle but non-negligible deviations from the FLRW metric. When DESY5 supernovae are considered instead, deviations are notably more significant, yielding scenarios that mildly violate the Copernican principle and exclude the FLRW assumption at . Overall, our results motivate a more in-depth investigation of whether the perfectly homogeneous and isotropic FLRW paradigm can still be assumed to accurately predict cosmological distances in the era of precision cosmology.

Paper Structure

This paper contains 24 sections, 20 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Early hints of an anisotropic expansion
  3. Transverse and line-of-sight BAO distances
  4. Results of a geometric test: $D_\mathrm{M}$ vs $D_\mathrm{H}$
  5. Inhomogeneous cosmology
  6. Motivation for inhomogeneous models
  7. Distance measures in the LTB metric
  8. Description of our $\Lambda$LTB model
  9. Constraints on an inhomogeneous Universe
  10. Dynamic dark energy vs inhomogeneities
  11. Discussion
  12. The local expansion rate
  13. How rare are such structures?
  14. Beyond a simple $\Lambda$LTB approach
  15. Conclusions
  16. ...and 9 more sections

Figures (5)

  • Figure 1: Marginalized $\Omega_\mathrm{m}$ posterior obtained from the geometrical test, $D_\mathrm{M}$ vs. $D_\mathrm{H}$, when considering the $\Lambda$CDM model (upper panel) and $w_0w_a$ parametrization (lower panel).
  • Figure 2: Constraints on the $\Lambda$LTB model obtained by profiling the likelihood in one (left) and two dimensions (right). In the left panel, horizontal dotted lines---color-coded by data combination---indicate the $\chi^2_\mathrm{min}$ values for the $w_0w_a$CDM model relative to the $\Lambda$CDM best fit. In the right panel, filled contours represent the (frequentist) 68$\%$ and 95$\%$ confidence regions for the different data combinations. The dark and light dashed red lines denote the (Bayesian) 68$\%$ and 95$\%$ credible regions, respectively, obtained in Ref. Camarena:2021mjr by applying a Copernican prior convolved with CMB observations.
  • Figure 3: The transverse (left column) and line-of-sight (middle column) BAO distances, along with the SNe Ia distance modulus (right column), predicted for the best-fit $w_0w_a$ and $\Lambda$LTB models. These are compared to the fiducial $\Lambda$CDM cosmology used by DESI in their BAO reconstruction analysis. Results are presented separately for each supernova dataset, with upper and lower panels corresponding to DESY5 and Pantheon+, respectively. The light green dashed line in the lower panels denotes the secondary local minimum identified in the Pantheon+ analysis. For clarity, the SNeIa data are shown in binned form; the binning procedure is described in Appendix \ref{['sub:SNe_binning']}.
  • Figure 4: Marginalized constraints in $w_\mathrm{pivot}$ and $w_a$ at $68\%$ and $95\%$ credible intervals obtained from the geometrical test $D_\mathrm{M}$ vs. $D_\mathrm{H}$.
  • Figure 5: Marginalized $\Omega_\mathrm{m}$ posterior obtained from the geometrical test, $D_\mathrm{M}$ vs. $D_\mathrm{H}$, when employing Pantheon$+$ data and considering both the $\Lambda$CDM model (upper panel) and $w_0w_a$ parametrization (lower panel).