Matching conditions for cosmological perturbations

Abstract
We derive the matching conditions for cosmological perturbations in a Friedman universe where the equation of state undergoes a sharp jump, for instance, as a result of a phase transition. The physics of the transition which is needed to follow the fate of the perturbations is clarified. We dissipate misleading statements made recently in the literature concerning the predictions of the primordial fluctuations from inflation and confirm standard results. Applications to string cosmology are considered.
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