Gravitational wave spectra for cosmological phase transitions with non-linear decay of the fluid motion

Abstract: We summarize the theoretical framework of gravitational wave (GW) production by bulk fluid motion induced by expanding broken-phase bubbles during a first-order phase transition. Using a locally stationary unequal-time correlator (UETC) to model the decay of the source due to non-linearities, we provide templates for the resulting GW background that have been validated against data from Higgsless simulations. This UETC generalizes the stationary one considered in the sound-shell model, appropriate for linear sound waves whose kinetic-energy decay is negligible, to encompass the non-linear evolution of the compressional fluid motion beyond the sound-wave regime. We demonstrate the implementation of templates based on this theoretical description and the results from the Higgsless simulations in the public Python package CosmoGW, facilitating their use in experimental forecasts and parameter-estimation studies. The GW spectrum is delivered as a function of the key phase transition parameters: the wall velocity $v_w$, the strength $\alpha$, the nucleation rate $\beta$, and the source duration $\delta \eta_{\rm fin}$.