  We calculate the spectrum of gravitational waves originated from strongly
first order electroweak phase transition in the extended Higgs model with a
real singlet field. In order to calculate the bubble nucleation rate, we
perform a two-field analysis to evaluate bounce solutions connecting the true
and the false vacua using the one-loop effective potential at finite
temperatures. Imposing the Sakharov condition of the departure from thermal
equilibrium for baryogenesis, we survey allowed regions of parameters of the
model. We then investigate the gravitational waves produced at electroweak
bubble collisions in the early Universe, such as the sound wave, the bubble
wall collision and the plasma turbulence. We find that the strength at the peak
frequency can be large enough to be detected at future space-based
gravitational interferometers such as eLISA, DECIGO and BBO. Predicted
deviations in the various Higgs boson couplings are also evaluated at the zero
temperature, and are shown to be large enough too. Therefore, in this model
strongly first order electroweak phase transition can be tested by the
combination of the precision study of various Higgs boson couplings at LHC, the
measurement of the triple Higgs boson coupling at future lepton colliders and
the shape of the spectrum of gravitational wave detectable at future
gravitational interferometers.
