A New ~ 5σ Tension at Characteristic Redshift from DESI-DR1 BAO and DES-SN5YR Observations

We perform a model-independent reconstruction of the angular diameter distance (DA) 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 rd to the Planck best-fit value, ensuring consistency with early-universe physics. With the reconstructed DA at two key redshifts, z~ 1.63 (where DAprime =0) and at z~ 0.512 (where DAprime = DA), we derive the expansion rate of the Universe H(z) at these redshifts. Our findings reveal that at z~ 1.63, the H(z) is fully consistent with the Planck-2018 LambdaCDM prediction, confirming no new physics at that redshift. However, at z ~ 0.512, the derived H(z) shows a more than 5σ discrepancy with the Planck-2018 LambdaCDM prediction, suggesting a possible breakdown of the LambdaCDM model as constrained by Planck-2018 at this lower redshift. This emerging ~ 5σ tension at z~ 0.512, distinct from the existing ``Hubble Tension'', may signal the first strong evidence for new physics at low redshifts.