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On DESI's DR2 exclusion of $Λ$CDM

Marina Cortês, Andrew R Liddle

TL;DR

The paper analyzes DESI DR2's reported exclusion of LCDM when combining BAO+CMB with three overlapping SN compilations, showing sigmas of $2.8$–$4.2$ depending on the SN dataset. It argues these cannot be naively averaged or simply selected, and presents a principled equal-weight combination yielding a combined exclusion of $\sigma\approx 3.1$, dominated by the weakest constraint. The most robust inference, however, is the $3.1$-sigma exclusion from DESI+CMB alone (omitting SN data), given the strong overlaps and methodological differences among SN compilations. The results highlight the need for standardized, well-validated SN datasets and cross-methodology checks to obtain reliable joint cosmological inferences.

Abstract

The DESI collaboration, combining their Baryon Acoustic Oscillation (BAO) data with cosmic microwave background (CMB) anisotropy and supernovae data, have found significant indication against the $Λ$CDM cosmology. This can also be interpreted as the significance of the detection of the $w_a$ parameter that measures variation of the dark energy equation of state. DESI's DR2 article quotes exclusion of $Λ$CDM for combinations of BAO and CMB data with each of three different and overlapping supernovae compilations (at 2.8-sigma for Pantheon+, 3.8-sigma for Union3, and 4.2-sigma for DESY5). We show that one can neither choose amongst nor average over these three different significances. We demonstrate how a principled statistical combination yields a combined exclusion significance of 3.1-sigma. Further we argue that, faced with these competing significances, the most secure inference from the DESI DR2 results is the 3.1-sigma level exclusion of $Λ$CDM obtained from combining DESI+CMB alone, omitting supernovae.

On DESI's DR2 exclusion of $Λ$CDM

TL;DR

The paper analyzes DESI DR2's reported exclusion of LCDM when combining BAO+CMB with three overlapping SN compilations, showing sigmas of – depending on the SN dataset. It argues these cannot be naively averaged or simply selected, and presents a principled equal-weight combination yielding a combined exclusion of , dominated by the weakest constraint. The most robust inference, however, is the -sigma exclusion from DESI+CMB alone (omitting SN data), given the strong overlaps and methodological differences among SN compilations. The results highlight the need for standardized, well-validated SN datasets and cross-methodology checks to obtain reliable joint cosmological inferences.

Abstract

The DESI collaboration, combining their Baryon Acoustic Oscillation (BAO) data with cosmic microwave background (CMB) anisotropy and supernovae data, have found significant indication against the CDM cosmology. This can also be interpreted as the significance of the detection of the parameter that measures variation of the dark energy equation of state. DESI's DR2 article quotes exclusion of CDM for combinations of BAO and CMB data with each of three different and overlapping supernovae compilations (at 2.8-sigma for Pantheon+, 3.8-sigma for Union3, and 4.2-sigma for DESY5). We show that one can neither choose amongst nor average over these three different significances. We demonstrate how a principled statistical combination yields a combined exclusion significance of 3.1-sigma. Further we argue that, faced with these competing significances, the most secure inference from the DESI DR2 results is the 3.1-sigma level exclusion of CDM obtained from combining DESI+CMB alone, omitting supernovae.

Paper Structure

This paper contains 10 sections, 4 equations, 2 figures.

Table of Contents

  1. Introduction
  2. The nature of the exclusion
  3. Methodologies, or, sigmas from supernovae neither reinforce nor average
  4. Combining exclusion significance over methodological differences
  5. Implications for DESI DR2
  6. The supernovae compilations
  7. DESY5, high- and low-redshift
  8. Pantheon+ versus DESY5
  9. Pantheon+ versus Union3
  10. Conclusion

Figures (2)

  • Figure 1: The combined significance of methodology-dependent analyses. Blue shows the sigma of the weakest exclusion, orange the combined exclusion for three equally-weighted methodologies, and green if the central outcome is overweighted by a factor of 10.
  • Figure 2: Constraint contours for DESI+CMB+DESY5, showing the full sample and with the low-redshift sample excised. [Adapted from Figure 14 of DESIDR2 under Creative Commons BY 4.0 License, original caption "The change to the DESI+CMB+DESY5 posterior (green) when excluding low-redshift SNe at $z<0.1$ from the DESY5 sample (brown dashed). The uncertainties are much larger when excluding these SNe but the shift in the best-fit values of $w_0$ and $w_a$ is small."]