In ΛCDM, late-time cosmic acceleration causes large-scale gravitational potentials to decay, imprinting a specific positive cross-correlation between CMB temperature fluctuations and foreground large-scale structure via the Integrated Sachs–Wolfe (ISW) effect, but measurements of this correlation are generally weaker and more inconsistent than the standard model predicts (Crittenden & Turok 1996; Giannantonio et al. 2012). This mismatch forces ΛCDM either to invoke uncomfortably low signal amplitudes, subtle selection and analysis systematics, or modified gravity and dark-energy behavior on large scales, none of which provides a fully satisfactory, self-consistent explanation across all surveys and scales (Planck Collaboration 2016; Hang et al. 2021).