The Possibility of Formation of Compact Boson Stars via Cosmological Evolution of a Background Scalar Field

TL;DR

This paper proposes a cosmological scenario where two coupled scalar fields evolve over time, enabling the formation of compact boson stars within the universe's age, expanding understanding of exotic compact object formation.

Contribution

It introduces a novel formation mechanism for boson stars involving background scalar field evolution, not achievable in single-field models.

Findings

Non-relativistic boson clouds can evolve into boson stars via background field dynamics.

Formation is possible if the background field variation reaches Planck scale.

Further research needed due to initial state complexities.

Abstract

Boson stars, hypothetical astrophysical objects bound by the self-gravity of a scalar field, have been widely studied as a type of exotic compact object that is horizonless and provides a testing ground for physics beyond the Standard Model. In particular, many previous works have demonstrated methods for distinguishing compact boson stars from black holes in general relativity through gravitational wave observations. However, the formation scenario of compact boson stars within the age of the universe remains unclear. In this paper, we explore a possible scenario for the formation of compact boson stars. The model we consider requires two coupled scalar fields: a complex scalar field that forms a boson star and a spatially homogeneous background field, as formation of a compact boson star cannot be achieved in a single filed model. Using the adiabatic approximation, we show that non-relativistic boson clouds can evolve into compact boson stars through the cosmological time-evolution of the background field. In our model the background field evolves to increase the effective mass of the scalar field, and as a result compact boson stars can form within the cosmological timescale, if the variation of the background field is as large as the Planck scale. However, further investigation is required because the required initial states are not the configurations that can be described by the well-studied Schr\"odinger-Poisson system.