A New $\sim 5σ$ Tension at Characteristic Redshift from DESI-DR1 BAO and DES-SN5YR Observations
This work addresses a critical issue in cosmology by identifying a new tension that could indicate new physics at low redshifts, though it is incremental as it builds on existing datasets and methods.
The study tackled the problem of testing the ΛCDM model by reconstructing the angular diameter distance using DESI-DR1 BAO and DES-SN5YR data, finding a more than 5σ discrepancy in the expansion rate at redshift z∼0.512 compared to Planck-2018 predictions, suggesting a possible breakdown of the model.
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 $Λ$CDM prediction, confirming no new physics at that redshift. However, at $z \sim 0.512$, the derived $H(z)$ shows a more than $5σ$ discrepancy with the Planck-2018 $Λ$CDM prediction, suggesting a possible breakdown of the $Λ$CDM model as constrained by Planck-2018 at this lower redshift. This emerging $\sim 5σ$ tension at $z\sim 0.512$, distinct from the existing ``Hubble Tension'', may signal the first strong evidence for new physics at low redshifts.