Cover Story (Issue 2, 2026) |The images of Brans-Dicke-Kerr type naked singularities
Author: Yu-Xiao Liu (Lanzhou University, Lanzhou, China)
The first black hole images captured by the Event Horizon Telescope have profoundly advanced theoretical and experimental investigations in strong-gravity astronomy. Astrophysical images shaped by strong gravitational lensing and accretion flow emission encode crucial information about the spacetime geometry near central compact objects, serving as a vital observational probe for testing gravity in the strong-field regime. A pivotal question arises: are current observations fully consistent with the predictions of general relativity, or do they hint at more general gravitational theories? Driven by this inquiry, a growing body of research has explored the shadow structures of rotating compact objects in modified gravity theories and their potential to distinguish between different gravitational frameworks.
A recent study [1] explores the imaging properties of Brans-Dicke-Kerr (BDK) spacetimes within the framework of Brans-Dicke gravity—a representative scalar-tensor theory motivated by Mach’s principle, in which the gravitational constant is elevated to a dynamical quantity governed by a scalar field. Through an extensive parameter space analysis, the study reveals a groundbreaking finding: even in the naked singularity regime, BDK spacetimes can produce images containing a dark shadow. This implies that BDK-type naked singularities could be viable astrophysical compact objects from an observational perspective. Notably, the shadow may develop a distinctive "jellyfish-like" shape with a self similar fractal structure, and the image may also feature a prominent gray region consisting of two separated patches. These unique characteristics provide observational signatures that can differentiate BDK spacetimes from Kerr naked singularities and other related geometries.
In conclusion, this work enriches our understanding of strong-gravity phenomenology in scalar-tensor theories and provides a useful framework for future high-precision black hole imaging to test scalar–tensor gravity and further constrain deviations from general relativity. These results significantly advance gravitational theory and enhance the interpretation of strong-gravity observations.
References[1] F. Long, W. K. Deng, X. Qin et al., Chin. Phys. C 50, 025108 (2026), arXiv: 2511.01478 [gr-qc]
