Gravitational waves from domain walls and their implications
  We evaluate the impact of domain-wall annihilation on the currently ongoing
and planned gravitational wave experiments, including a case in which domain
walls experience a frictional force due to interactions with the ambient
plasma. We show the sensitivity reach in terms of physical parameters, namely,
the wall tension and the annihilation temperature. We find that a Higgs portal
scalar, which stabilizes the Higgs potential at high energy scales, can form
domain walls whose annihilation produces a large amount of gravitational waves
within the reach of the advanced LIGO experiment (O5). Domain wall annihilation
can also generate baryon asymmetry if the scalar is coupled to either SU(2)$_L$
gauge fields or the $(B-L)$ current. This is a variant of spontaneous
baryogenesis, but it naturally avoids the isocurvature constraint due to the
scaling behavior of the domain-wall evolution. We delineate the parameter space
where the domain-wall baryogenesis works successfully and discuss its
implications for the gravitational wave experiments.
