Gravitational waves from the fragmentation of axion-like particle dark
  matter
  The misalignment mechanism allows for the efficient, and usually very cold,
production of light scalar bosons, such as axion-like particles (ALPs), making
them an appealing dark matter candidate. However, in certain cases, such as in
the presence of a monodromy, the self-interactions of ALPs can be sufficiently
strong such that the homogeneous field fragments soon after the onset of
oscillations. The resulting large inhomogeneities can lead to the production of
gravitational waves (GWs). We investigate the nonlinear dynamics of
fragmentation, as well as of the subsequent turbulent regime, and calculate the
stochastic GW background that is produced from this process. The GW background
can be enhanced if the time evolution features an extended intermediate phase
of ultrarelativistic dynamics due to a small mass at the bottom of the
potential. Yet, this enhancement is limited by the requirement that the dark
matter remains sufficiently cold. In some cases the resulting GWs may be within
reach of future GW detectors, allowing a complementary probe of this type of
dark matter.
