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In the context of "two-metric" theories of gravity there is the possibility that cosmic defects will produce a spectrum of primordial adiabatic density perturbations, when the speed characterizing the defect-producing scalar field is much larger than the speed characterizing gravity and standard model particles. This model will exactly mimic the standard predictions of inflationary models, with the exception of a small non-Gaussian signal which could be detected by future experiments.
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Avelino et al. (2000) studied this question.
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