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We study two-field bouncing cosmologies in which primordial perturbations arecreated in either an ekpyrotic or a matter-dominated contraction phase. We usea non-singular ghost condensate bounce model to follow the perturbationsthrough the bounce into the expanding phase of the universe. In contrast to theadiabatic perturbations, which on large scales are conserved across the bounce,entropy perturbations can grow significantly during the bounce phase. If theyare converted into adiabatic/curvature perturbations after the bounce, theytypically form the dominant contribution to the observed temperaturefluctuations in the microwave background, which can have several beneficialimplications. For ekpyrotic models, this mechanism loosens the constraints onthe amplitude of the ekpyrotic potential while naturally suppressing theintrinsic amount of non-Gaussianity. For matter bounce models, the mechanismamplifies the scalar perturbations compared to the associated primordialgravitational waves.

Converting entropy to curvature perturbations after a cosmic bounce