QCD-sourced tachyonic phase transition in a supercooled Universe

TL;DR

This paper introduces a new gravitational wave production mechanism triggered by a tachyonic phase transition during supercooling in the early universe, with potential detectability by future observatories.

Contribution

It demonstrates that a tachyonic phase transition driven by a scalar field can generate observable gravitational waves in supercooled cosmological scenarios, expanding understanding of early universe dynamics.

Findings

Gravitational wave signals are detectable in most parameter space regions.

A tachyonic phase transition can occur during supercooling due to a scalar field crossing a negative mass region.

The mechanism is demonstrated within a classically conformal $U(1)_{B-L}$ model.

Abstract

We propose a novel gravitational wave production mechanism in the context of quasi-conformal Standard Model extensions, which provide a way to dynamically generate the electroweak scale. In these models, the cosmic thermal history is modified by a substantial period of thermal inflation, potentially supercooling the Universe below the QCD scale. The exit from supercooling is typically realized through a strong, first-order phase transition. By employing the classically conformal $U(1)_{\tiny\rm B-L}$ model as a representative example, we show that a large parameter space exists where bubble percolation is inefficient. In this case, the top quark condensate triggers a tachyonic phase transition driven by classical rolling of the new scalar field towards the true vacuum. As the field crosses a region where its effective mass is negative, long-wavelength scalar field fluctuations are exponentially amplified, preheating the supercooled Universe. We study the dynamics of this scenario and estimate the peak of the associated gravitational wave signal, which is detectable by future observatories in almost the entire available parameter space.