Species Scale and Primordial Gravitational Waves

The species scale is a field-dependent UV cut-off for any effective field theory weakly coupled to gravity. In this letter, it is shown that in the context of inflationary cosmology, a detection of primordial gravitational waves will set an upper bound on the decay rate (Formula presented.) of the species scale. Specifically, this is derived in terms of the tensor-to-scalar ratio (Formula presented.) of power spectra of primordial perturbations. Given the targets of current and next generation experiments, it is shown that any successful detection would signify that this upper limit is of the order of unity, which is consistent with recent discussions in the literature.