A hybrid simulation of gravitational wave production in first-order
  phase transitions
  The LISA telescope will provide the first opportunity to probe the scenario
of a first-order phase transition happening close to the electroweak scale. By
now, it is evident that the main contribution to the GW spectrum comes from the
sound waves propagating through the plasma. Current estimates of the GW
spectrum are based on numerical simulations of a scalar field interacting with
the plasma or on analytical approximations -- the so-called sound shell model.
In this work we present a novel setup to calculate the GW spectra from sound
waves. We use a hybrid method that uses a 1d simulation (with spherical
symmetry) to evolve the velocity and enthalpy profiles of a single bubble after
collision and embed it in a 3d realization of multiple bubble collisions,
assuming linear superposition of the velocity and enthalpy. The main advantage
of our method compared to 3d hydrodynamic simulations is that it does not
require to resolve the scale of bubble wall thickness. This makes our
simulations more economical and the only two relevant physical length scales
that enter are the bubble size and the shell thickness (that are in turn
enclosed by the box size and the grid spacing). The reduced costs allow for
extensive parameter studies and we provide a parametrization of the final GW
spectrum as a function of the wall velocity and the fluid kinetic energy.
