Abstract
A first-order electroweak phase transition in extensions of the Standard Model can generate a stochastic gravitational-wave (GW) background. If the transition also violates charge-parity (CP) symmetry, it may produce circularly polarized GWs with unequal left- and right-handed components. This thesis investigates the generation of such chiral GW signals across the distinct dynamical stages of a cosmological phase transition: scalar bubble collisions, the acoustic phase of the primordial plasma, and fully developed magnetohydrodynamic (MHD) turbulence.
By explicitly evaluating the scalar energy-momentum tensor, we first demonstrate that bubble collisions sourced purely by scalar-field dynamics remain intrinsically parity-even and therefore do not generate a macroscopic chiral GW background. The CP-violating microphysics localized at the bubble walls must instead be transferred to the surrounding plasma. Extending the Sound Shell Model [1–3] to include parity-odd fluid perturbations, we show that anomalous hydrodynamic transport through the chiral vortical effect imprints helicity onto the vortical fluid motions within the acoustic plasma. Although the interference of these chiral sound waves formally produces a polarized GW component, the resulting signal is found to be strongly suppressed and observationally negligible.
The final and most promising stage arises once the plasma becomes turbulent and a chiral chemical potential activates the chiral magnetic effect. Unlike the purely hydrodynamic case, the magnetic mechanism naturally admits scenarios in which the severe geometric suppression is avoided. This biases the turbulent amplification of opposite magnetic helicity modes and efficiently converts microscopic CP violation into macroscopic magnetic helicity. By analytically deriving the resulting helicity injection rates and mapping them onto direct numerical simulations of helical MHD turbulence, we establish the scaling relation governing the resulting GW polarization. Finally, we discuss the physical conditions under which these chiral signatures could be detected by future space-based interferometers.