The String Soundscape at Gravitational Wave Detectors

TL;DR

This paper explores the potential for gravitational wave signals originating from bubble wall collisions in string theory, suggesting these signals could form a detectable soundscape across various frequencies, linked to the string landscape.

Contribution

It introduces the idea that vacuum decay in string theory can produce a diverse range of gravitational wave signals, expanding the scope of potential observable phenomena in the universe.

Findings

Multiple signals across different frequencies are possible due to various throat scales.

Detectors like eLISA, AEGIS, BBO, SKA, and EPTA could detect these signals.

Primordial black holes may also be produced as a consequence.

Abstract

We argue that gravitational wave (GW) signals due to collisions of ultra-relativistic bubble walls may be common in string theory. This occurs due to a process of post-inflationary vacuum decay via quantum tunnelling within (Randall-Sundrum-like) warped throats. Though a specific example is studied in the context of type IIB string theory, we argue that our conclusions are likely more general. Many such transitions could have occurred in the post-inflationary Universe, as a large number of throats with exponentially different IR scales can be present in the string landscape, potentially leading to several signals of widely different frequencies -- a soundscape connected to the landscape of vacua. Detectors such as eLISA and AEGIS, and observations with BBO, SKA and EPTA (pulsar timing) have the sensitivity to detect such signals, while at higher frequency aLIGO is not yet at the required sensitivity. A distribution of primordial black holes is also a likely consequence, though reliable estimates of masses and $\Omega_{\rm pBH} h^2$ require dedicated numerical simulations, as do the fine details of the GW spectrum due to the unusual nature of both the bubble walls and transition.