  First order phase transitions in the early Universe generate gravitational
waves, which may be observable in future space-based gravitational wave
observatiories, e.g. the European eLISA satellite constellation. The
gravitational waves provide an unprecedented direct view of the Universe at the
time of their creation. We study the generation of the gravitational waves
during a first order phase transition using large-scale simulations of a model
consisting of relativistic fluid and an order parameter field. We observe that
the dominant source of gravitational waves is the sound generated by the
transition, resulting in considerably stronger radiation than earlier
calculations have indicated.
