Neutrino Horizon Dynamics and the Hubble Tension

This work develops a structurally minimal late–time cosmology in which the vacuum energy density scales with the square of the Hubble rate. The normalization is fixed by principles rather than data fits, established independently through three complementary routes: (i) a four–form variational derivation, (ii) horizon thermodynamics, and (iii) a probabilistic energy–probability argument. A distinctive physical interpretation also emerges: relic neutrinos provide the natural carrier of the late-time drift. In the early Universe, when the cosmological horizon was much smaller, their Compton wavelength was larger than the horizon scale, making them behave as effectively heavier. As the horizon expanded, the same neutrinos became lighter in their gravitational influence. This simple but overlooked mechanism offers an intuitive and testable explanation for the late-time Hubble uplift, without introducing new fields or exotic interactions. The framework differs from common running-vacuum and holographic models by: · enforcing exact early-time agreement with the ΛCDM background, · preserving local General Relativity and standard matter conservation (with no exchange term Q), · introducing no new propagating degrees of freedom, and · confining all departures to a slow, monotonic modulator. From this construction follow fit-free, falsifiable predictions: · a compact relation linking the Hubble uplift to the late-time drift, · a mild, directed suppression of structure growth, and · a reduced redshift–drift amplitude. Preliminary empirical checks (Planck vs. SH0ES/TRGB, cosmic chronometer H(z), DESI Lyα BAO, DES weak lensing) already support these signatures, while future BAO, redshift–drift, and growth measurements provide clear falsification routes.