Cosmic VoidFilament Boundaries as Adiabatic MSW Resonance Layers for Ultra-Light Axion-Like Particles

We identify cosmic voidlament boundaries as adiabatic MikheyevSmirnovWolfenstein (MSW) resonance layers for photonaxion-like particle (ALP) conver-sion. Using published electron density proles from the IllustrisTNG and EAGLEhydrodynamical simulations Galarraga2021,Tuominen2021, we compute the MSWadiabaticity parameter γMSW for the voidlament transition and nd γMSW ∼ 10301031 across the full range of VHE energies and ALP couplings permitted by currentexperiments.The transition is strongly adiabatic: the photon follows its mixing eigenstatethroughout the density gradient, and the conversion probability at resonance ap-proaches P (γ → a) → sin2(2θvac) potentially large for ALP masses in the rangema ∼ 10−1510−13 eV, corresponding to the electron densities of voidlament in-terfaces in the cosmic web.This mass range coincides with the sensitivity of next-generation atom interfer-ometers (AION, MAGIS, AEDGE), providing a concrete observational programme.A falsiable prediction for blazar spectral populations classied by the GeodesicLOS Tension TLOS ArizaXIII is derived and deferred to a companion analysis.