  We discuss possibilities to observe stochastic gravitational wave backgrounds
produced by the electroweak phase transition in the early universe. Once the
first-order phase transition occurs, which is still predicted in a lot of
theories beyond the standard model, collisions of nucleated vacuum bubbles and
induced turbulent motions can become significant sources of the gravitational
waves. Detections of such gravitational wave backgrounds are expected to reveal
the Higgs sector physics. In particular, through pulsar timing experiments
planned in Square Kilometre Array (SKA) under construction, we will be able to
detect the gravitational wave in near future and distinguish particle physics
models by comparing the theoretical predictions to the observations.
