Gravitational effects on fluid dynamics in cosmological first-order phase transitions

Abstract Cosmological first-order phase transition (FOPT) sources the stochastic gravitational wave background (SWGB) through bubble collisions, sound waves, and turbulence. So far, most studies on the fluid profile of an expanding bubble are limited to transitions that complete in a much shorter time scale than the cosmic expansion. In this study, we investigate gravitational effects on the fluid profile beyond the self-similar regime. For this purpose we combine a hydrodynamic scheme in the presence of gravity with a fluid computation scheme under energy injection from the bubble wall. By performing (1+1)d simulations of spherical bubble for constant wall velocities, we find that the fluid generally develops a thinner shell in our cosmological setup, which qualitatively agrees with previous studies discussing the late-time behavior of fluid in expanding spacetime. We also observe reduction in the energy budget for the fluid kinetic energy. Furthermore, we find that the fluid profile develops sub-structure for accelerating bubble walls. We also comment on the possible broadening of the SGWB spectral plateau.