Unimodular gravity with energy diffusion eases Hubble tension

Unimodular gravity replaces the fixed cosmological constant with a dynamical energy exchange between matter and geometry. Adding a power-law diffusion term and allowing spatial curvature, the authors fit the Pantheon+ supernova and BAO datasets jointly, finding a mild preference for a closed universe and a diffusion exponent near 0.5. The framework satisfies the second law of thermodynamics by construction and produces an effective dark-energy equation of state around −0.83, closer to quintessence than a cosmological constant. The result shifts H₀ to the high end favored by local distance-ladder measurements while keeping the cosmic age near 13.6 Gyr.