A novel way to determine the scale of inflation

Abstract

We show that in the Feebly Interacting Massive Particle (FIMP) model of Dark Matter (DM), one may express the inflationary energy scale $H_{\ast }$ as a function of three otherwise unrelated quantities, the DM isocurvature perturbation amplitude, its mass and its self-coupling constant, independently of the tensor-to-scalar ratio. The FIMP model assumes that there exists a real scalar particle that alone constitutes the DM content of the Universe and couples to the Standard Model via a Higgs portal. We consider carefully the various astrophysical, cosmological and model constraints, accounting also for variations in inflationary dynamics and the reheating history, to derive a robust estimate for $H_{\ast }$ that is confined to a relatively narrow range. We point out that, within the context of the FIMP DM model, one may thus determine $H_{\ast }$ reliably even in the absence of observable tensor perturbations.

Comments: 25 pages, 11 figures. v2: Discussion and references added, matches the published version