Observational Phase-Coherent Residual Modulation in Type Ia Supernova Hubble Diagrams

Abstract

We analyze a reported nonlinear-phase modulation in Type Ia supernovaHubble-diagram residuals, emphasizing visualization, cross-compilation phasecoherence, look-elsewhere effects, waveform choice, endpoint artifacts, andpassband systematics. In a reconstructed 7687-entry diagnostic sample from sixcompilations, the normalized coordinate $u=(z+qz^2)/(1+q)$ with one Fourierharmonic gives $q=0.080$, $T=0.05263$, 19 cycles over $0\leq z\leq1$,$A=0.0160\pm0.0028$ mag, and $\Delta\chi^2=31.91$. Five compilation phases liewithin 0.056 cycles of the joint phase; DES is weaker and offset by 0.140cycles. A cubic coordinate improves $\Delta\chi^2$ by only 1.05, while secondand third harmonics add only 1.19 and 1.53. Repeating the fullcoordinate-period-waveform search in 10,000 null realizations gives globalprobabilities $3.0\times10^{-4}$, $4.0\times10^{-4}$, and $2.9\times10^{-3}$for raw, AIC-adjusted, and BIC-adjusted statistics. Paired MODEL009 light-curverefits cover 1529/1580 Pantheon+ entries; despite a 26.2 mmag weighted-rmsdistance perturbation, the fitted phase and amplitude remain stable. Themodulation is robust to this spectral-model perturbation.