Gravitational waves from collapsing vacuum domains

The breaking of an approximate discrete symmetry, the final stages of a fir st order phase transition, or a postinflationary biased probability distrib ution for scalar fields are possible cosmological scenarios characterized b y the presence of unstable domain wall networks. Combining analytical and n umerical techniques, we show that the nonspherical collapse of these domain s can be a powerful source of gravitational waves. We compute their contrib ution to the stochastic background of gravitational radiation and explore t heir observability by present and future gravitational wave detectors.