A Search for Nonlinear ISW Signals in Cosmic Voids Using DESI DR1 and Planck Data: Constraints, Systematic Biases, and Lessons from Stress Testing

This research package contains two papers, analysis code, and results for an empirical test of nonlinear density-dependent scaling in the integrated Sachs-Wolfe (ISW) signal of cosmic voids. Using 2,891,109 spectroscopic galaxies from DESI Data Release 1 (z = 0.4-0.8) cross-correlated with the Planck PR3 SMICA CMB temperature map, we identify 222 cosmic voids and test whether the void-ISW relationship is linear (as predicted by standard LCDM) or nonlinear (as predicted by density-dependent gravitational modifications). Key result: Delta-BIC = 6.55 favoring the quadratic model, with F-test p = 0.014 and null-test z-score = 3.69. All results are fully reproducible using only publicly available data from DESI DR1 (CC BY 4.0) and the Planck Legacy Archive. Data acquired on February 28, 2026.