A New $\sim 5σ$ Tension at Characteristic Redshift from DESI-DR1 BAO and DES-SN5YR Observations (2503.02880v2)

Abstract: We perform a model-independent reconstruction of the angular diameter distance ($D_{A}$) using the Multi-Task Gaussian Process (MTGP) framework with DESI-DR1 BAO and DES-SN5YR datasets. We calibrate the comoving sound horizon at the baryon drag epoch $r_d$ to the Planck best-fit value, ensuring consistency with early-universe physics. With the reconstructed $D_A$ at two key redshifts, $z\sim 1.63$ (where $D_{A}^{\prime} =0$) and at $z\sim 0.512$ (where $D_{A}^{\prime} = D_{A}$), we derive the expansion rate of the Universe $H(z)$ at these redshifts. Our findings reveal that at $z\sim 1.63$, the $H(z)$ is fully consistent with the Planck-2018 $\Lambda$CDM prediction, confirming no new physics at that redshift. However, at $z \sim 0.512$, the derived $H(z)$ shows a more than $5\sigma$ discrepancy with the Planck-2018 $\Lambda$CDM prediction, suggesting a possible breakdown of the $\Lambda$CDM model as constrained by Planck-2018 at this lower redshift. This emerging $\sim 5\sigma$ tension at $z\sim 0.512$, distinct from the existing ``Hubble Tension'', may signal the first strong evidence for new physics at low redshifts.

A New $\sim 5\sigma$ Tension at Characteristic Redshift from DESI DR1 and DES-SN5YR Observations: An Expert Overview

The paper "A New $\sim 5\sigma$ Tension at Characteristic Redshift from DESI DR1 and DES-SN5YR Observations" by Mukherjee and Sen introduces a novel analysis aimed at probing the expansion history of the universe in a model-independent manner. The authors utilize data from the Dark Energy Spectroscopic Instrument (DESI) and the Dark Energy Survey Supernova Program (DES-SN5YR) to investigate potential deviations from the standard cosmological model, $\Lambda$CDM, particularly focusing on the angular diameter distance.

Methodology

The research employs a Multi-Task Gaussian Process (MTGP) framework to reconstruct the angular diameter distance ($D_A$) and its derivative ($D_A'$) from observational data. This approach leverages the DESI-DR1 Baryon Acoustic Oscillation (BAO) data and the five-year DES-SN5YR supernovae data, providing a robust framework for model-independent analysis. By calibrating the comoving sound horizon $r_d$ to the Planck best-fit value, the paper ensures consistency with early-universe physics.

The authors focus on two characteristic redshifts: $z \sim 1.63$ where $D_A'$ is zero, and $z \sim 0.512$ where $D_A = D_A'$. These characteristic points allow for direct determination of the Hubble parameter $H(z)$ without relying on specific cosmological models, thus providing a stringent test of the $\Lambda$CDM framework constrained by Planck-2018 data.

Key Findings

• Tension at Low Redshift: At $z \sim 0.512$, a significant tension exceeding $5\sigma$ is documented between the reconstructed $H(z)$ from DESI+DESY5 data and the Planck-2018 $\Lambda$CDM predictions. This discrepancy is particularly notable as it suggests potential new physics or systematic errors in the observational data independent of the existing Hubble tension.
• Consistency at High Redshift: The reconstructed $H(z)$ at $z \sim 1.63$ is consistent with Planck-2018 predictions, confirming no deviations from $\Lambda$CDM at this higher redshift.
• Robustness Across Methodological Choices: The results remain stable across different MTGP kernels and reconstructed mean functions, underscoring the robustness of the findings.

Implications and Speculations

The observed tension at $z \sim 0.512$ highlights potential weaknesses or necessary extensions to the $\Lambda$CDM model, especially in the context of low-redshift observations. This could imply the existence of new physics affecting cosmic expansion that becomes apparent only at lower redshifts. Alternatively, the detected tension might suggest unknown systematic biases in the observational datasets used. In particular, if future analyses corroborate these findings, this could point to modifications in the dark sector, such as evolving dark energy equations of state.

With tensions apparent in regions unexplored by the existing model, further studies could investigate alternative dark energy parameterizations or explore modifications to the standard cosmological framework that might better accommodate these deviations. Ultimately, these findings motivate additional targeted studies to verify the robustness and origins of the detected tension.

The paper serves as a call to re-examine fundamental cosmological parameters using diverse observational datasets and methodologies to either confirm or refute the standard cosmological paradigm's current boundaries. Such efforts are crucial for advancing our understanding of the universe and its accelerating expansion.