We investigate a minimal cosmological extension of General Relativitybased on the Einstein-Cartan-Kibble-Sciama (ECKS) framework. A primordialspin component — with stiff equation of state w=1, geometrically distinctfrom scalar kination — induces a transient phase reducing the comovingsound horizon to rₛ = 135. 8 Mpc, enabling H₀ = 73. 0 km/s/Mpc consistentwith local measurements. The same stiffness parameter Fᵢon = 1. 2765governs both sound-horizon reduction and scale-dependent suppression ofstructure growth (S₈ = 0. 766), unifying H₀ and S₈ under a single degreeof freedom. A global MCMC fit yields Δχ² = −39. 5 vs ΛCDM (AIC = −31. 5, BIC = −8. 2). Resolving H₀ natively exposes an irreducible H₀–Age–BAOtrilemma, predicting t₀ = 12. 74 Gyr — now corroborated by H0DN 2026 (H₀ = 73. 50 ± 0. 81 km/s/Mpc, >6σ tension). Additional falsifiablesignatures: cosmic birefringence βECF ≈ 0. 35° (LiteBIRD), JWSThigh-redshift seeds, and a blue-tilted primordial GW background. ── Version notes (v3) ──────────────────────────────────- Paper body: Modified Friedmann equation and bounce condition (H=0 at finite aₘin) made explicit; bounce robustness against spin inhomogeneities (Poplawski 2010, 2012) added. - Weak-field decoupling: ρₛpin (z=0) /ρ_Λ ~ 10⁻²² demonstrated. - CMB peaks: positions vs. heights distinction clarified. - DESI: comparison qualified as a priori; DR2 added. - Birefringence: Planck PR4 vs. ACT DR6 quantified (Eskilt 2022). - New: MCMC check vs. DESI DR1 (3 H₀ priors) ; 5 new figures. - 6 new references. Numerical results unchanged (Δχ²=−39. 5, H₀=73. 04, rₛ=135. 8 Mpc). GitHub: https: //github. com/pfichant/spin-torsion-cosmology
Pascal Fichant (Sun,) studied this question.