Isocurvature Perturbations Through a Bounce: Amplification of Entropy Modes and the Planck Fraction Bound

Isocurvature perturbations, fluctuations in the relative composition of the cosmic fluids at fixed total density, are generated whenever more than one field or fluid carries the primordial fluctuations. This paper computes their amplification through a torsion-regulated bounce and the resulting isocurvature fraction. Entropy modes grow by a power factor G = 1.36 across the transition. Starting from a pre-bounce entropy-to-curvature power ratio of 4.3e-3, the post-bounce isocurvature fraction is beta_iso = 0.005817, a factor 6.5 below the Planck bound of 0.038 for cold dark matter isocurvature. The correlation fraction between the adiabatic and isocurvature modes is 0.12. The bound converts into a limit on the entropy growth index: mu above 0.792 would overproduce isocurvature. The bounce therefore neither erases pre-existing entropy perturbations nor amplifies them dangerously at the fiducial point, but the margin is less than an order of magnitude, so multi-field bounce constructions with stronger entropy growth are already constrained. The study is computed from stated assumptions, not measured.