Speaker
Description
Given their weak interaction with different degrees of freedom, gravitational waves (GWs) offer fresh opportunities to probe the earliest moments of the Universe, and physics beyond the Standard Model. For instance, a first-order phase transition (FOPT) in the primordial plasma at the electroweak scale could emit a recognizable signal in the frequency range of the upcoming detectors like LISA. During a FOPT, bubbles of the stable phase nucleate, expand, and collide, generating perturbations that result in GWs. Thus, to model the consequent GW spectrum realistically, it is crucial to understand the fluid perturbations of the primordial plasma. Given the high energy and the relativistic speeds involved in strong enough phase transitions, the shape and amplitude of the power spectrum could be significantly affected by nonlinearities and the generated turbulence. While these topics have been generally studied in fluid dynamics for decades, their impact on GW spectra from FOPTs remains unclear. We investigate the production of vorticity and turbulence in the relativistic regime beyond linear theory. Using a semi-analytical approach, we report estimates of the GW spectrum, as well as the time scales and the strength of vorticity production when the initial field is purely compressional, as in the case of sound waves. This analysis helps constrain the applicability of models that approximate the fluid perturbations induced by bubble collisions as a superposition of the longitudinal modes, such as sound waves. Furthermore, it lays the foundation for more detailed numerical studies of the relativistic regime in the future.
