The String Soundscape at Gravitational Wave Detectors
  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.
