Gravitational waves from strong first-order phase transitions in an expanding Universe

We investigate the gravitational waves generated by first-order phase transitions (PTs) in an expanding Universe. Using a scalar field-fluid coupled model, we perform three-dimensional lattice numerical simulations of slow first-order PTs with strong strength. We observe that the contribution from sound waves to the gravitational wave (GW) signal significantly dominates over that from bubble collisions. The sound wave spectra exhibit two peaks located at the mean bubble separation and the sound-shell width. Rotational modes are generated in the fluid during the bubble collisions, and their magnitude grows with the PT strength. The lifetime of the sound wave source increases with decreasing α: for weaker transitions the source can persist for longer than a Hubble time, while for stronger transitions it decays within a Hubble time. When the walls propagate as deflagrations, the GW spectral shape changes drastically, with steeper ultraviolet tails and a shorter-lived sound wave source. We also observe that the magnitude of the normalized GWs is roughly an order of magnitude smaller than obtained in the Minkowski spacetime simulations.