Gravitational Waves from Broken Cosmic Strings: The Bursts and the Beads
  We analyze the gravitational wave signatures of a network of metastable
cosmic strings. We consider the case of cosmic string instability to breakage,
with no primordial population of monopoles. This scenario is well motivated
from GUT and string theoretic models with an inflationary phase below the
GUT/string scale. The network initially evolves according to a scaling
solution, but with breakage events resulting from confined monopoles (beads)
being pair produced and accelerated apart. We find these ultra-relativistic
beads to be a potent source of gravitational waves bursts, detectable by
Initial LIGO, Advanced LIGO, and LISA. Indeed, Advanced LIGO could observe
bursts from strings with tensions as low as $G\mu \sim 10^{-12}$. In addition,
we find that ultra-relativistic beads produce a scale-invariant stochastic
background detectable by LIGO, LISA, and pulsar timing experiments. The
stochastic background is scale invariant up to Planckian frequencies. This
phenomenology provides new constraints and signatures of cosmic strings that
disappear long before the present day.
