As the universe cooled after the hot Big Bang, such a phase transition would have released huge amounts of energy, both as heat and as the kinetic energy of growing bubbles. In a strongly first-order phase transition, the bubble walls may even grow at near the speed of light. This, in turn, would lead to the production of a stochastic background of gravitational waves. Experiments such as NANOGrav and LISA may be sensitive to this signal.