Speaker
Description
First-order phase transitions in a dark sector can produce an observable stochastic gravitational-wave background, and many models predicting such signals have been studied in the context of current and future gravitational-wave experiments. For moderately strong transitions, hydrodynamical simulations suggest that sound waves generated by the expansion and collisions of true-vacuum bubbles are the dominant source of gravitational waves. It is often assumed that these sound waves propagate only within the dark-sector fluid, while the possible impact of the visible sector is usually neglected. In this talk, we consider a linear hydrodynamical setup that includes the coupling between the dark and visible fluids. We show that, even if the bubble expansion initially affects only the dark fluid, sound waves can later be induced in the visible sector as well. Using the sound-shell model, we find that this transfer of acoustic energy suppresses the gravitational-wave signal compared to the case of a completely decoupled dark sector.