David Weir (University of Stavanger, Norway)
We present large-scale numerical simulations of the gravitational radiation produced by a first order phase transition in the early universe. We show that the dominant source of gravitational waves is sound waves generated by the expanding bubbles of the low-temperature phase. The sound waves have a power spectrum with power-law form between scales set by the average bubble separation and the bubble wall width. In turn, the sound waves generate gravitational waves whose power spectrum also has a power-law form. These gravitational waves are generated at a rate proportional to the fluid length scale and the square of the fluid kinetic energy density. The general form of the gravitational wave power spectrum is different from that predicted by the envelope approximation, and the gravitational wave energy density is at least two orders of magnitude larger.