Challenging the Constancy of the Speed of Light: Evidence for a Vacuum-Density-Dependent Exponential Gradient from Cosmic Structures and Supernova Data

The constancy of the speed of light in vacuum (c0 = 299,792,458m/s) underpins special relativity and modern cosmology. However, this assumption, based on Earthbound measurements, may not hold across cosmological scales where vacuum density varies by orders of magnitude. We present two independent analyses to test this hypothesis. Track 1 uses vacuum density estimates from cosmic structures (Earth lab UHV, intergalactic medium, cosmic filaments, voids) to infer an effective speed of light (ceff), finding an increase from c0 to 1.4677c0 at z = 3.0. Track 2 analyzes a subset of the Pantheon+ Type Ia supernova dataset, deriving ceff(z) from luminosity distances in a matter-only universe, revealing a consistent rise. Both tracks indicate ceff increases exponentially with decreasing vacuum density, a trend mirroring universal exponential laws (e.g., RC time constants). This finding suggests the speed of light is not a cosmic constant, but a dynamic field—a shift that may unify multiple cosmological tensions under a single physical principle.