Apparent Supernova Time Dilation from Radiation Propagation in a Realistic Intergalactic Plasma

Observed time dilation in Type Ia supernova light curves is widely interpreted as direct evidence of cosmological metric expansion. In this study, we present a constrained numerical investigation demonstrating that apparent time dilation can arise from long-distance radiation propagation through a realistic intergalactic plasma environment, without invoking expanding spacetime. Using a stability-preserving simulation framework that enforces phase coherence, image sharpness, and global energy conservation, we model broadband optical–UV transient signals propagating through a filamentary, warm ionized intergalactic medium over distances up to 1000 Mpc. The results show a smooth, monotonic broadening of transient light curves with distance, closely resembling observed supernova time-dilation phenomenology, while maintaining spectral coherence and angular fidelity. These findings establish that supernova time dilation is not logically exclusive to metric expansion and therefore cannot be treated as a unique diagnostic without additional independent observational constraints. The results motivate more integrated analyses of time-domain cosmological data that jointly consider propagation effects alongside geometric interpretations.