Gravitational waves from vacuum first-order phase transitions. II. From thin to thick walls

In a vacuum first-order phase transition, gravitational waves are generated from collision of bubbles of the true vacuum. The spectrum from such collisions takes the form of a broken power law. We consider a toy model for such a phase transition, where the dynamics of the scalar field depends on a single parameter (lambda) over bar, which controls how thin the bubble wall is at nucleation and how close to degenerate the vacua are relative to the barrier. We extend on our previous work by performing a series of simulations with a range of (lambda) over bar. The peak of the gravitational-wave power spectrum varies by up to a factor of 1.3, which is probably an unobservable effect. We find that the UV power law in the gravitational-wave spectrum becomes steeper as (lambda) over bar -> 0, varying between k(-1.4) and k(-2.2) for the (lambda) over bar. considered. This provides some evidence that the form of the underlying effective potential of a vacuum first-order phase transition could be determined from the gravitational-wave spectrum it produces.Peer reviewe