Two Matching Prescriptions at the Bounce Give Two Spectral Tilts: A Difference Planck Can Resolve

Calculations of perturbations through a bounce require a prescription for matching the perturbations across the transition, and the literature uses more than one. This paper computes the spectral index under the two standard prescriptions and finds that the choice is not a technicality: it moves the prediction by more than the observational error. Matching on a constant-density surface, with transfer exponent p = 0.482, gives ns = 0.9658, a deviation of 0.21 standard deviations from the Planck value. Matching on a comoving surface, with p = 0.790, gives ns = 0.9574, deviating by -1.8 standard deviations. The difference between the prescriptions is 0.008378, or 2 times the Planck uncertainty. Data therefore already discriminate between matching prescriptions that parts of the literature treat as interchangeable: the constant-density prescription is compatible with the measured tilt while the comoving prescription, applied to this background, is disfavoured. Any bounce calculation that does not state its prescription is withholding a choice worth two standard deviations. The study is computed from stated assumptions, not measured.