Secondary GWs and PBHs in string inflation: formation and detectability

TL;DR

This paper explores how string inflation models with an ultra slow-roll phase can produce detectable secondary gravitational waves linked to primordial black holes, offering potential observational signatures independent of dark matter contributions.

Contribution

It demonstrates that secondary GWs from string inflation with an ultra slow-roll phase are detectable regardless of PBH dark matter abundance, highlighting a new observational window.

Findings

Secondary GWs can be detected by LISA, ET, or BBO.

PBH formation is linked to an ultra slow-roll phase in inflation.

Detectability of GWs is independent of PBH dark matter contribution.

Abstract

We derive the spectrum and analyse the detectability prospects of secondary gravity waves (GWs) associated to primordial black hole (PBH) production in a class of string inflationary models called Fibre Inflation. The inflationary potential features a near inflection point that induces a period of ultra slow-roll responsible for an enhancement of the scalar perturbations which can lead to PBHs with different masses and contributions to dark matter (DM) in agreement with current observational bounds, including CMB constraints on the scalar spectral index and the tensor-to-scalar ratio. This enhancement of the curvature perturbations sources secondary GWs which can be detected by either LISA, ET or BBO, depending on the GW frequency but regardless of the amount of PBH DM since secondary GWs remain detectable even if the PBH contribution to DM is exponentially suppressed. The possibility to see a secondary GW signal is instead due to the presence of an ultra slow-roll epoch between CMB horizon exit and the end of inflation.