Phase transitions in the early Universe give rise to effective masses for massless fields in the symmetry-broken phase. We perform lattice simulations to study the dynamical impact of a mass-acquiring spectator field on the evolution of first-order phase transitions and the associated gravitational-wave production, while keeping the effective potential responsible for bubble nucleation fixed. In addition to the well-known friction effects, we identify a novel effect that significantly enhances the strength of first-order phase transitions. In contrast to the general scenario, although the effective potential governs the tunneling rate, the amplitude of the $\chi$ field is strongly suppressed inside the true vacuum bubble, resulting in a faster bubble expansion than predicted by the effective potential alone. The amplitude of the mass-acquiring field is highly suppressed in the true vacuum bubbles, resulting in additional release of vacuum energy that concentrate on the bubble walls. We further develop an analytical framework that not only explains our numerical results but can also be used to improve the estimation of gravitational-wave signals in related phase-transition scenarios.

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• Gravitational waves
• Phase transitions
• Astrophysical & cosmological simulations