Fermi-ball dark matter from a first-order phase transition

TL;DR

This paper introduces a new dark matter model where macroscopic Fermi-balls form during a first-order phase transition, potentially explaining dark matter abundance and producing detectable gravitational waves.

Contribution

It proposes a novel mechanism for dark matter formation via phase transition trapping, highlighting the role of Fermi-balls and their potential gravitational wave signatures.

Findings

Fermi-balls can account for dark matter abundance across various models.

The scenario predicts gravitational wave signals at the electroweak scale.

Fermi-balls are stable due to quantum pressure balancing free energy release.

Abstract

We propose a novel dark matter (DM) scenario based on a first-order phase transition in the early universe. If dark fermions acquire a huge mass gap between true and false vacua, they can barely penetrate into the new phase. Instead, they get trapped in the old phase and accumulate to form macroscopic objects, dubbed Fermi-balls. We show that Fermi-balls can explain the DM abundance in a wide range of models and parameter space, depending most crucially on the dark-fermion asymmetry and the phase transition energy scale (possible up to the Planck scale). They are stable by the balance between fermion's quantum pressure against free energy release, hence turn out to be macroscopic in mass and size. However, this scenario generally produces no detectable signals (which may explain the null results of DM searches), except for detectable gravitational waves (GWs) for electroweak scale phase transitions; although the detection of such stochastic GWs does not necessarily imply a Fermi-ball DM scenario.