A New $\sim 5\sigma$ Tension at Characteristic Redshift from DESI-DR1 BAO and DES-SN5YR Observations

TL;DR

This study reconstructs the universe's expansion rate using DESI-DR1 and DES-SN5YR data, finding a significant tension at redshift ~0.512 with Planck-2018 $ m extLambda$CDM predictions, indicating potential new physics at low redshift.

Contribution

It introduces a model-independent method using Multi-Task Gaussian Processes to analyze BAO and supernova data, revealing a significant deviation from standard cosmology at low redshift.

Findings

No deviation at $z\,\sim 1.63$ from Planck predictions.

A more than $5\sigma$ discrepancy at $z\sim 0.512$ with $ m \Lambda$CDM.

Potential evidence for new physics at low redshift.

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.