Causal Horizon Cosmology: A Unified Framework for Inflation, Dark Energy, and Hubble Tension Resolution via Five-Dimensional Bulk Dynamics

We present Causal Horizon Cosmology (CHC), a unified theoretical framework that simultaneously addresses the inflationary fine-tuning problem, the nature of dark energy, and the Hubble tension. Our model embeds a (3+1)-dimensional brane within a (4+1)-dimensional Schwarzschild-Anti-de Sitter bulk spacetime. The key mechanism is "anti-friction" inflation: during bulk collapse following higher-dimensional black hole formation, the inflaton field experiences negative effective friction, naturally driving it toward the Starobinsky plateau without fine-tuned initial conditions. The framework makes three falsifiable predictions: (1) dark radiation from bulk Weyl curvature projection yielding ΔNeff = 0.18 ± 0.06, testable by CMB-S4 around 2030; (2) frequency-dependent gravitational wave echoes with reflectivity |R|² ∝ ω^(-0.3), detectable by LISA; and (3) slowly evolving dark energy with w₀ ≈ -0.98 and wₐ ≈ +0.02, distinguishable from ΛCDM by Roman Space Telescope. The predicted dark radiation reduces the Hubble tension from >5σ to approximately 2.6σ by decreasing the sound horizon, yielding H₀ = 69.8 ± 1.1 km/s/Mpc. All predictions remain consistent with current Planck, BICEP/Keck, and LIGO constraints while offering clear falsification criteria within the next decade.