Anomalous Negative Integrated Sachs-Wolfe Effect from Evolving Metric Signature Boundaries in Cosmic Voids

Hansen et al. (2025) reported a highly significant achromatic CMB temperature anomaly in the local universe (z < 0.03): a temperature excess of 2.7σ–3.6σ in cosmic voids and a deficit exceeding 5σ in dense filaments, with an amplitude approximately ten times larger than standard ΛCDM Integrated Sachs-Wolfe (ISW) predictions. We show that this anomaly arises naturally if the metric signature of spacetime varies with local energy density, as proposed in a companion paper [Henderson, 2026a]. In regions where the phase parameter φ governing the metric component g₀₀ = −(1 − 2φ) evolves in time—specifically, at advancing phase boundaries where the Lorentzian signature weakens as voids expand and cool—the time derivative of the gravitational potential Φ̇ is enhanced relative to the standard ISW prediction. This enhanced Φ̇ produces a localised, anomalous negative ISW effect: CMB photons traversing the evolving boundary are achromatically blueshifted (heated) in voids and redshifted (cooled) in filaments. The predicted signal is inherently achromatic, preserving the blackbody spectrum, consistent with the Hansen et al. observations. The mechanism is supported by recent work of Alexandre, Gielen, and Magueijo (2024), who demonstrated that metric signature change across degenerate boundaries implies a change in the effective cosmological constant, providing the physical driver for the enhanced potential evolution. We identify specific observational tests, including the redshift dependence of the anomaly amplitude and its correlation with void expansion rate, capable of confirming or excluding this mechanism.