Unified view of scalar and vector dark matter solitons

Abstract

The existence of solitons -- stable, long-lived, and localized field configurations -- is a generic prediction for ultralight dark matter. These solitons, known by various names such as boson stars, axion stars, oscillons, and Q-balls depending on the context, are typically treated as distinct entities in the literature. This study aims to provide a unified perspective on these solitonic objects for real or complex, scalar or vector dark matter, considering self-interactions and nonminimal gravitational interactions. We demonstrate that these solitons share universal nonrelativistic properties, such as conserved charges, mass-radius relations, stability and profiles. Without accounting for alternative interactions or relativistic effects, distinguishing between real and complex scalar dark matter is challenging. However, self-interactions differentiate real and complex vector dark matter due to their different dependencies on the macroscopic spin density of dark matter waves. Furthermore, gradient-dependent nonminimal gravitational interactions impose an upper bound on soliton amplitudes, influencing their mass distribution and phenomenology in the present-day universe.