Weak Lensing Magnification of Type Ia Supernovae from a Predicted External-Convergence Model: DES-SN5YR, Union3, and Pantheon+

Abstract — v1

We detect weak gravitational lensing magnification of Type Ia supernovae (SNe Ia) with a lensing signal predicted, not fitted, from the foreground galaxy and cluster distribution, applied uniformly to DES-SN5YR (1450 SNe), Union3 (1213), and Pantheon+ (901). Using only public data, we compute the external convergence and shear along each sight line from a stellar-mass–halo-mass chain calibrated to the DESI DR1 spectrophotometric mass scale and validated against measured galaxy–galaxy lensing. The predicted convergence correlates with the Hubble residuals of all three compilations, with slopes consistent with the theoretical $d\Delta\mu/d\kappa_{ext} = -5/\ln 10 = -2.17$. Over the 2920 unique sight lines the correlation is detected at $p = 4 \times 10^{-6}$ ($4.4\sigma$) against a null that shuffles residuals within source-redshift bins and so preserves the spatial correlations between overlapping apertures. A heteroscedastic latent-variable regression, which corrects each sight line for the noise in its own prediction, gives an observed-to-predicted amplitude $A = 1.17 \pm 0.44\ (\mathrm{stat}) \pm 0.27\ (\mathrm{sys})$, consistent with unity; the inverse-variance fit gives the noise-attenuated $A = 0.82 \pm 0.18$, or $1.20 \pm 0.30$ after an independent mock calibration of the attenuation. The signal does not depend on the SN standardization pipeline, is flat in source redshift as predicted, and is unchanged when per-galaxy spectroscopic masses replace the photometric ones. De-lensing changes the DESI DR2 evolving-dark-energy $\Delta\chi^2$ by at most 0.4 for any of the three SN samples. We release the per-supernova $\kappa_{ext}$ catalogs.