arXiv subjects

Shoulong Li

Publications and source records attributed to Shoulong Li.

13 recordsLinked to original sources

arXiv2026

Geodesics and shadows of the spindle-deformed Kerr black hole

Recently, a new exact Ricci-flat rotating black-hole solution was constructed in four-dimensional general relativity, in which an additional parameter $B$ characterizes a spindle deformation of the Kerr geometry. We study geodesic motion and black-hole shadows in this spacetime. The Hamilton-Jacobi equation is not exactly separable for either timelike or null geodesics. Remarkably, however, at the leading nontrivial order, ${\cal O}(B^2)$, null but not timelike geodesics become separable. In the timelike sector, the spindle deformation shifts the innermost stable circular orbit and can give rise to an outermost stable circular orbit. In the null sector, exploiting the perturbatively separated equations, we analytically determine the photon region, equatorial photon orbits, and black-hole shadow, and compare the resulting predictions with direct ray tracing in the exact spacetime. The numerical results validate the perturbative treatment and quantify the deviations from Kerr.

gr-qc↗

arXiv2026

Solar-System Bounds on Ricci-flat Spindle Deformations of Schwarzschild

Recently, a new class of deformed black-hole exact solutions was constructed in four-dimensional general relativity. The deformation is controlled by a parameter $B$, which survives after demagnetizing a black hole immersed in an external Bertotti-Robinson magnetic field and changes the global structure of the spacetime into a non-asymptotically flat spindle geometry. Although no astrophysical mechanism for generating such a deformation is currently known, it is natural to ask phenomenologically how large such a geometric deformation could be if it extended over the weak-field solar exterior. Using two classical Solar-System tests, we derive the leading corrections to planetary perihelion precession and to the light travel time in a Shapiro-type configuration. Requiring the $B$-induced perihelion advance to be smaller than the observational uncertainties in the supplementary perihelion precessions of planets gives the strongest bounds, $ |B|\lesssim 10^{-24}\text{--}10^{-23}\ {\rm cm}^{-1}$, while a Cassini time delay estimate gives a complementary null-geodesic sensitivity at the level $ |B|\lesssim 10^{-21}\ {\rm cm}^{-1}$. These results show that any such spindle deformation, if extended to the solar exterior geometry, must be extremely suppressed on Solar-System scales.

gr-qc↗

arXiv2026

Neutron stars more compact than black holes in quasi-topological gravity: Equilibrium configurations and radial stability

Within general relativity, black holes are widely regarded as the ultimate benchmark for compactness in the Universe. Recently, however, neutron star models have been constructed in a higher-curvature theory -- quasi-topological gravity (QTG) -- whose compactness can exceed the black-hole limit~ [S. Li, H. L\"u, Y. Gao, R. Xu, L. Shao, and H. Yu, companion Letter, Neutron stars more compact than black holes as a probe of strong-field gravity, Phys. Rev. D 114, L021504 (2026).]. Here we present a detailed analysis of both the equilibrium structure and radial stability of such configurations in QTG. By examining several representative equations of state and different values of the gravitational coupling constant, we find that in the high-central-density regime the compactness exceeding the black-hole bound exhibits a universal behavior in QTG. We further show that QTG corrections grow increasingly significant at large central densities and can stabilize configurations that are radially unstable in general relativity over a broad parameter range. These results establish ultra-compact neutron stars in QTG as theoretically viable strong-field configurations and provide a foundation for further investigations of their dynamical and phenomenological implications.

gr-qc↗

arXiv2026

Black holes and neutron stars in massive Hellings-Nordtvedt theory

Hellings-Nordtvedt theory is a vector-tensor theory in which a vector field $A_\mu$ is nonminimally coupled to curvature through two independent interactions $A^2{\cal R}$ and $A^\mu A^\nu{\cal R}_{\mu\nu}$. When supplemented by a potential whose zero-energy minimum occurs at nonzero $A^2$, the restricted $A^\mu A^\nu{\cal R}_{\mu\nu}$ sector is known to admit black-hole and neutron-star solutions with a monopole-like asymptotic vacuum structure. We examine whether this structure is a generic consequence of the nonzero vector vacuum or instead relies on the special Ricci-tensor coupling. By analyzing the field equations near spatial infinity, we show that the asymptotic vacuum condition is incompatible with generic nonzero values of both couplings and instead selects two allowed single-coupling sectors. The $A^\mu A^\nu{\cal R}_{\mu\nu}$ sector reproduces the known monopole-like asymptotics, whereas the $A^2{\cal R}$ sector admits an asymptotically flat Schwarzschild metric with a nontrivial radial vector field. We further compute the Noether mass in the $A^2{\cal R}$ sector, derive the corresponding Solar-System constraints, and construct neutron-star configurations. Although the weak-field deviation is constrained to be small, neutron stars can still show appreciable departures from both general relativity and the Ricci-tensor-coupling sector in their masses, radii, and moments of inertia. Our results identify that the $A^2{\cal R}$ sector of massive Hellings-Nordtvedt theory as a viable and useful framework for studying strong-field compact objects with a nonzero vector vacuum while remaining compatible with weak-field tests.

gr-qc↗

arXiv2026

Self-consistent neutron stars in a class of massive vector-tensor gravity

Einstein-bumblebee gravity, as a class of massive non-minimally coupled vector-tensor theories, provides a useful framework for constraining Lorentz symmetry breaking through astrophysical observations, largely due to the existence of exact static and spherically symmetric black hole solutions. These solutions are typically obtained under the assumption that the vector-field potential vanishes everywhere once the vector field acquires a nonzero radial vacuum expectation value. However, imposing this assumption globally obstructs the construction of self-consistent compact-star solutions. In this work, we elucidate the origin of this inconsistency through a detailed analysis of the field equations and construct neutron-star configurations by abandoning the global vanishing-potential assumption. Crucially, we show that even without enforcing this condition everywhere, it is violated only in the strong-field interior region and is dynamically restored in the weak-field regime by asymptotic boundary conditions at spatial infinity. As a result, consistency with existing black-hole solutions and observational constraints is preserved. Our results establish massive vector-tensor gravity as a unified, natural, and self-consistent framework for compact objects, significantly extending its astrophysical viability beyond black holes and Solar System tests.

gr-qc↗

arXiv2025

Revisiting black holes and their thermodynamics in Einstein-Kalb-Ramond gravity

Einstein-Kalb-Ramond (EKR) gravity is an alternative theory in which a rank-two antisymmetric tensor field, the Kalb-Ramond field, is nonminimally coupled to gravity, potentially generating Lorentz-violating backgrounds. In this work, we revisit black hole solutions and thermodynamics in EKR gravity, addressing subtleties overlooked in previous studies. We obtain two distinct classes of exact static black hole solutions with general topological horizons in diverse dimensions, both with and without a cosmological constant, corresponding to different coupling sectors dictated by the field equations. We analyze their thermodynamic properties and, using the Wald formalism, compute the Noether mass and entropy, establishing the first law and clarifying the role of the Noether mass. Finally, we discuss the implications of this definition of mass for observational constraints in EKR gravity.

gr-qc↗

arXiv2025

Dyonic RN-like and Taub-NUT-like black holes in Einstein-bumblebee gravity

Einstein-bumblebee gravity is one of the simplest vector-tensor theories that realizes spontaneous Lorentz symmetry breaking. In this work, we first construct an exact dyonic Reissner-Nordstr\"om-like black hole solution in four dimensions, carrying both electric and magnetic charges and admitting general topological horizons. We then study its thermodynamic properties, and employ the Wald formalism to compute the conserved mass and entropy, thereby establishing the first law of black hole thermodynamics. Furthermore, we generalize these results to Taub-Newman-Unti-Tamburino case and higher dimensions case.

gr-qc↗

arXiv2025

Radial oscillations of neutron stars in Starobinsky gravity and its Gauss-Bonnet extension

Starobinsky gravity, as one of the simplest and best-behaved higher-curvature gravity theories, has been extensively studied in the context of neutron stars over the past few decades. In this work, we investigate the adiabatic radial oscillation stability of neutron stars within the framework of Starobinsky gravity. We find that gravitational modifications can significantly impact stellar stability. Specifically, the higher-derivative nature of the theory causes the exterior spacetime to dynamically respond to fluid oscillations, in contrast to general relativity where Birkhoff's theorem ensures a static exterior. For stellar models with low central densities, the fundamental frequency becomes nearly independent of the central density when the coupling constant is large. For stellar models with high central densities, the transition from stability to instability still approximately occurs near the maximum-mass configuration, similar to the case in general relativity. Our main analysis is conducted in the Jordan frame of the scalar-tensor gravity equivalent to Starobinsky gravity, and we explicitly verify consistency with results obtained in the Einstein frame. We further extend our study to a class of Gauss-Bonnet extensions of Starobinsky gravity.

gr-qc↗

arXiv2024

Neutron stars in Gauss-Bonnet extended Starobinsky gravity

Recently, a class of Gauss-Bonnet extended Starobinsky gravity was proposed, allowing black holes to carry ghost-free massive scalar hair for the first time without requiring additional matter fields. This intriguing feature offers a new perspective for understanding higher-curvature pure gravity and highlights the importance of further studying the potential effects of Gauss-Bonnet extensions in gravitational systems beyond black holes. In this study, we investigate the properties of neutron stars within this model, focusing on how the higher-curvature terms, particularly the coupling between the Gauss-Bonnet term and the curvature-squared term, impact the stellar structure. We present a detailed analysis of these effects and compute the moment of inertia for rotating neutron stars under the slow-rotation approximation. The substantial differences in the moment of inertia between general relativity and Gauss-Bonnet extended Starobinsky gravity are expected to be detectable through future high-precision observations.

gr-qc↗

arXiv2023

Neutron stars more compact than black holes as a probe of strong-field gravity

Probing gravity in its strongest regime is a central goal of modern physics, as the nature of the most compact objects reflects fundamental aspects of Einstein's theory of general relativity (GR). In GR, black holes are regarded as the most compact objects in the Universe. Here, for the first time, we demonstrate that stable stellar configurations more compact than black holes can arise when neutron-star equations of state are embedded in quasi-topological gravity, a class of higher-curvature extensions of GR. We construct such ultra-compact stars, analyze their macroscopic properties, and establish their stability against radial perturbations, confirming their physical plausibility. We further identify potential observational signatures to distinguish these stars from black holes, most notably gravitational-wave echoes whose detectability could provide direct evidence of physics beyond Einstein's GR in the strong-field regime.

gr-qc↗

arXiv2016

Stability of Differentially Rotating Disks in $f(T)$ Theory

To explain the accelerated expansion of our universe, many dark energy models and modified gravity theories have been proposed so far. It is argued in the literature that they are difficult to be distinguished on the cosmological scales. Therefore, it is well motivated to consider the relevant astrophysical phenomena on (or below) the galactic scales. In this work, we study the stability of self-gravitating differentially rotating galactic disks in $f(T)$ theory, and obtain the local stability criteria in $f(T)$ theory, which are valid for all $f(T)$ theories satisfying $f(T=0)=0$ and $f_T (T=0)\not=0$, if the adiabatic approximation and the weak field limit are considered. The information of the function $f(T)$ is mainly encoded in the parameter $α\equiv 1/f_T(T=0)$. We find that the local stability criteria in $f(T)$ theory are quite different from the ones in Newtonian gravity, general relativity, and other modified gravity theories such as $f(R)$ theory. We consider that this might be a possible hint to distinguish $f(T)$ theory from general relativity and other modified gravity theories on (or below) the galactic scales.

gr-qc↗

arXiv2016

Dyonic (A)dS Black Holes in Einstein-Born-Infeld Theory in Diverse Dimensions

We study Einstein-Born-Infeld gravity and construct the dyonic (A)dS planar black holes in general even dimensions, that carry both the electric charge and magnetic fluxes along the planar space. In four dimensions, the solution can be constructed with also spherical and hyperbolic topologies. We study the black hole thermodynamics and obtain the first law. We also classify the singularity structure.

hep-th↗

arXiv2015

Thermodynamics of static dyonic AdS black holes in the $ω$-deformed Kaluza-Klein gauged supergravity theory

We study thermodynamical properties of static dyonic AdS black holes in four-dimensional $ω$-deformed Kaluza-Klein gauged supergravity theory, and find that the differential first law requires a modification via introducing a new pair of thermodynamical conjugate variables (X, Y). To ensure such a modification, we then apply the quasi-local ADT formalism developed in Ref. [20] to calculate the quasi-local conserved charge and identify that the new pair is precisely the one previously introduced to modify the differential form of first law.

hep-th↗