Foundation I: The Metric Universe - Extended Version 1

The standard ΛCDM model currently faces persistent tensions, most notably in the Hubble constant H0 and in the amplitude of late-time structure growth quantified by S8, while the initial singularity remains a conceptual limitation of the standard cosmological picture. In this work, Foundation I, we investigate a minimal cosmological extension of General Relativity based on the Einstein-Cartan-Kibble-Sciama framework. Within an effective spin-fluid description, a primordial spin component induces a transient stiff phase prior to recombination. In the representative calibrated solution studied here, this mechanism reduces the comoving sound horizon to rs ≈ 135.8 Mpc, thereby allowing a CMB-consistent calibration compatible with a local Hubble value near H0 ≈ 73.0 km/s/Mpc. The same framework also yields a scale-dependent suppression of structure growth, with a representative value S8 ≈ 0.766, in agreement with weak-lensing-preferred ranges. At the theoretical level, the spin-squared correction acts as a repulsive contribution at extreme density and allows a non-singular bounce in place of the initial singularity in the effective Friedmann dynamics. Beyond the resolution of the current tensions, the framework leads to several falsifiable signatures: a possible cosmic birefringence angle of order β ≈ 0.35°, a residual CMB damping-tail deficit at high multipoles, an independently testable contracted acoustic scale in future 21 cm measurements, and a blue-tilted primordial gravitational-wave background associated with the bounce.