The Cosmic Refrigerator: Gradient-Driven Diffusion of Zero-Point Energy Across the Cosmological Horizon as Dark Energy

I propose that cosmic acceleration arises from the natural thermodynamic diffusion of zero-point vacuum energy across the particle horizon. Our observable universe occupies a slightly higher-vacuum-energy inflationary patch; the super-horizon region beyond carries a lower density imprinted by eternal inflation. This minute gradient (relative difference δ ≲ 10^{-30}) drives a net outward flux of vacuum fluctuations, described by an effective Fickian current on cosmic scales. The mechanism uses only established physics — QFT in curved spacetime, semi-classical gravity, and the second law — without exotic fields or absolute vacuum-energy fine-tuning. It naturally reproduces the observed dark-energy density, the linear Hubble law, and w ≈ −1. Numerical integration confirms the model matches the standard expansion history to better than 0.01%. Testable predictions are made for upcoming surveys including Euclid, Roman, and LiteBIRD.