Phase transition gravitational waves from pseudo-Nambu-Goldstone dark
  matter and two Higgs doublets
  We investigate the potential stochastic gravitational waves from first-order
electroweak phase transitions in a model with pseudo-Nambu-Goldstone dark
matter and two Higgs doublets. The dark matter candidate can naturally evade
direct detection bounds, and can achieve the observed relic abundance via the
thermal mechanism. Three scalar fields in the model obtain vacuum expectation
values, related to phase transitions at the early Universe. We search for the
parameter points that can cause first-order phase transitions, taking into
account the existed experimental constraints. The resulting gravitational wave
spectra are further evaluated. Some parameter points are found to induce strong
gravitational wave signals, which have the opportunity to be detected in future
space-based interferometer experiments LISA, Taiji, and TianQin.
