Abstract
We argue that gravitational wave 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. Though we study a specific string construction involving warped throats, we argue that our conclusions are more general. Many such transitions could have occurred in the post-inflationary Universe, as a large number of throats with exponentially different mass scales can be present in the string landscape, leading to several signals of widely different frequencies – a soundscape connected to the landscape of vacua. Detectors such as aLIGO/VIRGO, LISA, and pulsar timing observations with SKA and EPTA have the sensitivity to detect such signals. A distribution of primordial black holes is also a likely consequence, though reliable estimates of masses and their abundance require dedicated numerical simulations, as do the fine details of the gravitational wave spectrum due to the unusual nature of the transition.
Highlights
The recent direct detection of gravitational waves (GWs) [1,2] opens a new mode of physical exploration
We argue that GW detectors provide a powerful tool to interrogate the nature of short-distance physics, string theory, in a way unrelated to inflation: GW signals from postinflationary vacuum decay are a natural feature of the type IIB string landscape
We have shown how GW can be produced in the context of string theory, due to a process of vacuum decay linked to the landscape of vacua characteristic of string constructions
Summary
The recent direct detection of gravitational waves (GWs) [1,2] opens a new mode of physical exploration. We explore early-Universe vacuum decay taking place via zero-temperature quantum nucleation of bubbles of true, locallySUSY-preserving, vacuum within a given throat, and argue that the resulting ultra-relativistic bubble wall collisions can lead to an observable stochastic ‘background’ of GWs. The peak frequency sensitively depends on the throat characteristics, most of all on the gravitational warp factor w I R 1, which sets the relation between the infra-red energy scale of the throat tip and the string scale Ms Since a large number of throats with exponentially different warp factors can be present in the string landscape [25], GW signals with very different frequencies can be produced – a soundscape of possible signals that can be potentially discovered by detectors such as aLIGO [26] and LISA [27], and pulsar timing arrays [28,29]. The possible production of pBHs in those mass ranges where they may account for (part of) the dark matter provides further motivation for detailed studies of the rich physics of the string soundscape
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