Electroweak First-Order Phase Transition Triggered by Non-Gaussian Fluctuations of a $\mathbb{Z}_2$-Symmetric Spectator Scalar

Abstract

We propose a novel mechanism to trigger a first-order cosmological electroweak phase transition using non-Gaussian primordial fluctuations of a $\mathbb{Z}_2$-symmetric spectator scalar field. We show that the large fluctuations of the spectator field can modify the Higgs thermal mass and enhance the thermal barrier, thereby enabling a strong first-order phase transition. Non-Gaussianities in the primordial fluctuation spectrum significantly increase the probability of large-amplitude fluctuations, allowing a substantial fraction of the Universe to undergo the transition. The spectator field also naturally serves as a cold dark matter candidate through its coherent oscillations, reproducing the observed relic abundance. The resulting stochastic gravitational wave background peaks in the $10^{-3}$-$10^{-1}$ Hz band, making it detectable by future space-based interferometers.