TOPICAL REVIEW: General relativistic boson stars

Abstract: There is accumulating evidence that (fundamental) scalar fields may exist in Nature. The gravitational collapse of such a boson cloud would lead to a boson star (BS) as a new type of a compact object. Similarly as for white dwarfs and neutron stars, there exists a limiting mass, below which a BS is stable against complete gravitational collapse to a black hole. According to the form of the self-interaction of the basic constituents and the spacetime symmetry, we can distinguish mini-, axidilaton, soliton, charged, oscillating and rotating BSs. Their compactness prevents a Newtonian approximation, however, modifications of general relativity, as in the case of Jordan-Brans-Dicke theory as a low energy limit of strings, would provide them with gravitational memory. In general, a BS is a compact, completely regular configuration with structured layers due to the anisotropy of scalar matter, an exponentially decreasing 'halo', a critical mass inversely proportional to constituent mass, an effective radius, and a large particle number. Due to the Heisenberg principle, there exists a completely stable branch, and as a coherent state, it allows for rotating solutions with quantised angular momentum. In this review, we concentrate on the fascinating possibilities of detecting the various subtypes of (excited) BSs: Possible signals include gravitational redshift and (micro-)lensing, emission of gravitational waves, or, in the case of a giant BS, its dark matter contribution to the rotation curves of galactic halos.

• The paper establishes that boson stars are stable, self-gravitating configurations of scalar fields that mimic compact objects like neutron stars.
• The paper analyzes various theoretical models, including complex scalar fields and non-minimal couplings, to explore critical masses and stability criteria.
• The paper highlights potential observational implications, such as gravitational redshifts and dark matter contributions, guiding future experimental research.

General Relativistic Boson Stars: A Comprehensive Review

The paper "General Relativistic Boson Stars" by Schunck and Mielke offers an exhaustive review of the theoretical framework, background, and potential implications of boson stars (BSs) within the context of general relativity and scalar field theory. The discourse establishes the foundation of BSs as intriguing compact objects analogous to neutron stars but formed from scalar fields instead of fermionic matter. This review aims to synthesize the profound complexities and the diverse research avenues associated with these theoretical constructs.

Theoretical Framework

Boson stars are envisioned as stable, self-gravitating configurations of scalar fields, which might be either real or complex depending on the chosen model. A key aspect of their stability against gravitational collapse lies within the quantum properties of their constituents, adhering to constraints imposed by the Heisenberg uncertainty principle. Several theoretical models exist for these configurations, contingent on the self-interaction potential of the scalar field and whether the scalar field forms a complex or real structure.

From a theoretical standpoint, the paper touches on varying approaches, such as using complex scalar fields or invoking Jordan-Brans-Dicke modifications for non-minimal coupling. Schunck and Mielke take an extensive look into numerous potential fields and configurations, establishing a clear distinction between models with and without self-interactions.

Key Findings and Results

The authors present a comprehensive review of models that further substantiate the viability of BSs within different gravitational frameworks. They explore studies showing rotating BSs, the gravitational memory of these stars in scalar-tensor gravity, and their observational impacts, such as gravitational redshifts and lensing effects. The paper makes clear numerically-backed claims concerning the critical masses and sizes of these constructs and highlights distinctions between mini-boson stars and standard BS configurations, emphasizing how different mass and interaction terms can result in significantly varied phenomenological consequences.

Observational Implications

One of the focal points in exploring the hypothesis of boson stars is their potential to explain astrophysical observations that exceed current theoretical expectations of neutron star capabilities. Schunck and Mielke suggest that BSs could potentially contribute to the galactic rotation curves and even serve as candidates for gallant halo dark matter, theorizing how these objects could account for observed gravitational effects presently attributed to more traditional structures, such as neutron stars or black holes.

Future Directions and Speculations

The review opens avenues for future exploration in multiple aspects, including the identification of possible observational signatures, such as gravitational waves emanating from these hypothesized stars. The potential coupling of scalar fields with astronomical radiation could offer experimental pathways to confirm or refute the existence of boson stars. Moreover, improving experimental constraints on fundamental interactions and scalar particles, such as the Higgs boson, might pave the way for establishing the practical existence of BSs beyond theoretical speculation.

Moreover, the intriguing possibility of boson stars as a form of non-baryonic dark matter provides an exciting frontier in cosmology and particle physics intersection, demanding more refined models and observational analysis to bridge theoretical predictions with empirical data.

Conclusion

This review by Schunck and Mielke thoroughly traverses the landscape of theoretical and observational research on boson stars, emphasizing their potential implications in astrophysics and fundamental physics. Despite being primarily a theoretical construct, the exploration of BSs expands our understanding of high-density matter configurations under the general relativity framework and informs ongoing inquiries into the nature of dark matter and the potential existence of exotic astrophysical objects. By integrating insights from scalar field theory and general relativity, this review underscores the significance of continuing investigations into the viability and detection of boson stars within the astrophysical community.