Probing the gravitational wave background from cosmic strings with LISA
  Cosmic string networks offer one of the best prospects for detection of
cosmological gravitational waves (GWs). The combined incoherent GW emission of
a large number of string loops leads to a stochastic GW background (SGWB),
which encodes the properties of the string network. In this paper we analyze
the ability of the Laser Interferometer Space Antenna (LISA) to measure this
background, considering leading models of the string networks. We find that
LISA will be able to probe cosmic strings with tensions $G\mu \gtrsim
\mathcal{O}(10^{-17})$, improving by about $6$ orders of magnitude current
pulsar timing arrays (PTA) constraints, and potentially $3$ orders of magnitude
with respect to expected constraints from next generation PTA observatories. We
include in our analysis possible modifications of the SGWB spectrum due to
different hypotheses regarding cosmic history and the underlying physics of the
string network. These include possible modifications in the SGWB spectrum due
to changes in the number of relativistic degrees of freedom in the early
Universe, the presence of a non-standard equation of state before the onset of
radiation domination, or changes to the network dynamics due to a string
inter-commutation probability less than unity. In the event of a detection,
LISA's frequency band is well-positioned to probe such cosmic events. Our
results constitute a thorough exploration of the cosmic string science that
will be accessible to LISA.
