Equilateral Non-Gaussianity and the Effective Sound Speed of Perturbations at the Bounce

Equilateral non-Gaussianity arises from derivative interactions among perturbations of comparable wavelength, and in most early-universe models it is controlled by the sound speed of the scalar perturbations. This paper computes it for perturbations crossing a torsion-regulated bounce, where the torsion contribution modifies the effective sound speed during the crossing epoch. The prediction is fNL = 1.3 - 22.0 (1/cs squared - 1). At the fiducial sound speed cs = 0.72 the result is fNL = -19, deviating from the Planck 2018 equilateral value of -26 plus or minus 47 by 0.15 standard deviations. The dependence is steep: at cs = 0.5 the prediction falls to -65, and at cs = 0.3 to -2.2e+02, the latter outside the Planck two-standard-deviation band. The equilateral measurement therefore already excludes strongly subluminal perturbation propagation at the bounce, while leaving the mildly subluminal regime, including the fiducial point, fully viable. The study is computed from stated assumptions, not measured.