Running of the scalar spectral index in bouncing cosmologies

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We calculate the running of the scalar index in the ekpyrotic and matter bounce cosmological scenarios, and find that it is typically negative for ekpyrotic models, while it is typically positive for realizations of the matter bounce where multiple fields are present. This can be compared to inflation, where the observationally preferred models typically predict a negative running. The magnitude of the running is expected to be between 10⁻⁴ and up to 10⁻², leading in some cases to interesting expectations for near-future observations.