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 we show that, in the context of inflationary cosmology, a detection of primordial gravitational waves will set an upper bound on the decay rate $|\Lambda'_s/\Lambda_s|$ of the species scale. Specifically, we derive this in terms of the tensor-to-scalar ratio $r$ of power spectra of primordial perturbations. Given the targets of current and next generation experiments, we show 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.