Dark Temperature Hierarchies and Gravitational Waves from the Electroweak Phase Transition

TL;DR

This paper explores how a hotter dark sector influences the electroweak phase transition and enhances gravitational wave signals, making them more detectable by future space-based interferometers.

Contribution

It introduces a minimal Higgs-portal model with a dark sector at higher temperature, showing significant effects on gravitational wave amplitude and frequency.

Findings

Gravitational wave amplitude increases by over an order of magnitude.

Peak frequency shifts to the millihertz range.

Enhanced signals are within reach of future detectors.

Abstract

We investigate the impact of a semi-decoupled dark sector with a temperature hierarchy relative to the Standard Model plasma on the electroweak phase transition and its associated gravitational wave signal. Working within a minimal Higgs-portal extension, we allow the dark sector to possess a higher temperature at the electroweak epoch while remaining consistent with cosmological bounds on additional relativistic degrees of freedom. The temperature hierarchy modifies the thermal structure of the effective potential and alters nucleation dynamics without requiring large portal couplings or extreme supercooling. Within the cosmologically allowed window, we find a monotonic enhancement of the gravitational wave amplitude by more than an order of magnitude compared to the standard thermal case, accompanied by a shift of the peak frequency within the millihertz regime. The resulting stochastic background moves substantially closer to the projected sensitivity of future space-based interferometers. Our results demonstrate that hidden-sector temperature hierarchies can leave observable imprints on electroweak-scale phase transitions even in minimal and perturbative frameworks.