arXiv subjects

Jianhui Qiu

Publications and source records attributed to Jianhui Qiu.

11 recordsLinked to original sources

arXiv2026

Geodesic Focusing Conditions in $f(Q)$ Gravity

We study the geodesic deviation equation in symmetric teleparallel geometry (STG), where the relative acceleration is defined with respect to the STG connection. We analyze the modified Raychaudhuri equation along a geodesic congruence in $f(Q)$ gravity under the Weyl-type ansatz, together with an additional assumption under which the metric variation term along the congruence is converted into a disformation-induced acceleration term. In contrast to the purely geometrical Raychaudhuri equation obtained in general metric-affine settings, the equation derived here contains matter-source contributions through the trace equation of $f(Q)$ gravity. Different from general relativity, focusing in $f(Q)$ gravity is not automatic, and one must impose an appropriate focusing condition. We collect the model-dependent terms in the modified Raychaudhuri equation into an effective energy-momentum trace $T_{\text{eff}}$, so that the focusing condition can be written as the inequality $T\leq T_{\text{eff}}$, where $T$ is the trace of the matter energy-momentum tensor. We also apply this condition to the flat Friedmann--Lemaître--Robertson--Walker (FLRW) background. The homogeneous and isotropic STG connection admits three branches, each characterized by a single connection function $γ_i$, with $i=1,2,3$. Only the first branch with the coincident gauge is compatible with the Weyl-type ansatz. We obtain the resulting effective trace $T_{\text{eff}}=T$ for any form of $f(Q)$ satisfying $f_Q>0$ in the flat FLRW universe. The focusing inequality is saturated and imposes no additional constraint on the matter content.

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arXiv2025

Teleparallel gravity at null infinity

Four-dimensional asymptotically flat spacetimes have been central to recent developments in infrared physics. Gravitational waves reaching the asymptotic boundary reveal an infinite-dimensional symmetry group known as the Bondi-Metzner-Sachs (BMS) group. The vacuum structure breaks this symmetry, giving rise to Goldstone modes that play a pivotal role in the analysis of scattering amplitudes. However, these modes must be added to the phase space of the metric formulation. In this work, we explore an alternative formulation of general relativity, teleparallel gravity, which is dynamically equivalent to the standard metric description in the bulk. This framework relies on a tetrad field to encode the gravitational degrees of freedom and a flat connection to represent inertial effects. Leveraging this decomposition, we propose a novel encoding of the Goldstone modes within the flat connection. We examine the implications of this approach in the covariant phase space framework, focusing on the symplectic potential and its connection to the Wald-Zoupas prescription.

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arXiv2025

Regularization of Gauss-Bonnet Gravity in Riemann-Cartan Geometry

We extend the conformal dimensional-derivative regularization of four-dimensional Gauss- Bonnet gravity to Riemann-Cartan geometry, obtaining a regularized action whose torsionless limit equals the well-known regularized four-dimensional Einstein-Gauss-Bonnet model. Varying independently with respect to the scalar, tetrad, and spin connection yields field equations that remain strictly second order in covariant derivatives, thereby avoiding Ostrogradsky-type instabil- ities. Within this framework we obtain static, spherically symmetric black holes carrying torsion hair, showing that the regularized Gauss-Bonnet interaction can support long-range torsion hair without invoking extra dimensions.

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arXiv2024

From the Janis-Newman-Winicour naked singularities to Einstein-Maxwell phantom wormholes

The Janis-Newman-Winicour spacetime corresponds to a static spherically symmetric solution of Einstein equations with the energy momentum tensor of a massless quintessence field. It is understood that the spacetime describes a naked singularity. The solution has two parameters, $b$ and $s$. To our knowledge, the exact physical meaning of the two parameters is still unclear. In this paper, starting from the Janis-Newman-Winicour naked singularity solution, we first obtain a wormhole solution by a complex transformation. Then let the parameter $s$ approaching infinity, we obtain the well-known exponential wormhole solution. After that, we embed both the Janis-Newman-Winicour naked singularity and its wormhole counterpart in the background of de Sitter or anti-de Sitter Universe with the energy momentum tensor of massive quintessence and massive phantom fields, respectively. To our surprise, the resulting quintessence potential is actually the dilaton potential found by one of us. It hints us that, by modulating the parameters in the charged dilaton black hole solutions we can get the Janis-Newman-Winicour solution. Furthermore, a charged wormhole solution is obtained by performing complex transformation on the charged dilaton black hole solutions in the background of de Sitter or anti-de Sitter Universe. We eventually find that $s$ is actually related to the coupling constant of dilaton field to Maxwell field and $b$ is related to a negative mass for the dilaton black holes. A negative black hole mass is physically forbidden. Therefore, we conclude that the Janis-Newman-Winicour naked singularity solution is not physically allowed.

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arXiv2021

Slowly rotating black holes in the Einstein-Maxwell-scalar theory

We investigate a slowly rotating black hole solution in a novel Einstein-Maxwell-scalar theory, which is prompted by the classification of general Einstein-Maxwell-scalar theories. The gyromagnetic ratio of this black hole is calculated, and it increases as the second free parameter $β$ increases, but decreases with the increasing parameter $γ\equiv \frac{2 α^{2}}{1+α^2}$. In the Einstein-Maxwell-dilaton (EMD) theory, the parameter $β$ vanishes, but the free parameter $α$ governing the strength of the coupling between the dilaton and the Maxwell field remains. The gyromagnetic ratio is always less than $2$, the well-known value for a Kerr-Newman (KN) black hole as well as for a Dirac electron. Scalar hairs reduce the magnetic dipole moment in dilaton theory, resulting in a drop in the gyromagnetic ratio. However, we find that the gyromagnetic ratio of two can be realized in this Einstein-Maxwell-scalar theory by increasing $β$ and the charge-to-mass ratio $Q/M$ simultaneously (recall that the gyromagnetic ratio of KN black holes is independent of $Q/M$). The same situation also applies to the angular velocity of a locally non-rotating observer. Moreover, we analyze the period correction for circular orbits in terms of charge-to-mass ratio, as well as the correction of the radius of the innermost stable circular orbits. It is found the correction increases with $β$ but decreases with $Q/M$. Finally, the total radiative efficiency is investigated, and it can vanish once the effect of rotation is considered.

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arXiv2021

On black holes with scalar hairs

By using the Taylor series method and the solution-generating method, we construct exact black hole solutions with minimally coupled scalar field. We find that the black hole solutions can have many hairs except for the physical mass. These hairs come from the scalar potential. Unlike the mass, there is no symmetry corresponding to these hairs, thus they are not conserved and one cannot understand them as Noether charges. They arise as coupling constants. Although there are many hairs, the black hole has only one horizon. The scalar potential becomes negative for sufficient large $ϕ$ (or in the vicinity of black hole singularity). Therefore, the no-scalar-hair theorem does not apply to our solutions since the latter does not obey the dominant energy condition. Although the scalar potential becomes negative for sufficient large $ϕ$, the black holes are stable to both odd parity perturbations and scalar perturbations. As for even parity perturbations, we find there remains parameter space for the stability of the black holes. Finally, the black hole thermodynamics are developed.

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arXiv2021

Extending Bekenstein's theorem in order to search exact solutions of Einstein-Maxwell-conformal-scalar equations

The Bekenstein's theorem allows us to generate a Einstein-conformal scalar solution from a single Einstein-ordinary scalar solution. In this article, we extend this theorem to Einstein-Maxwell-scalar (EMS) theory with a non-minimal coupling between the scalar and Maxwell field. As applications of this extended theorem, the well-known static dilaton solution and rotating solution with a specific coupling between dilaton and Maxwell field are considered, and new conformal dilaton black hole solutions are found. The Noether charges such as the mass, electric charge, angular momentum are compared between the old and new black hole solutions connected by conformal transformations, and they are found conformally invariant. We speculate that the theorem may be helpful in the computations of metric perturbations and spontaneous scalarization of black holes in the Einstein-Maxwell-conformal-scalar theory since they can be mapped to the corresponding EMS theories, which have been investigated in detail.

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arXiv2021

Constructing black holes in Einstein-Maxwell-scalar theory

Exact black hole solutions in the Einstein-Maxwell-scalar theory are constructed. They are the extensions of dilaton black holes in de Sitter or anti de Sitter universe. As a result, except for a scalar potential, a coupling function between the scalar field and the Maxwell invariant is present. Then the corresponding Smarr formula and the first law of thermodynamics are investigated.

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arXiv2020

Nonsingular black holes and nonsingular universes in the regularized Lovelock gravity

It is found that, when the coupling constants $α_p$ in the theory of regularized Lovelock gravity are properly chosen and the number of Lovelock tensors $p\rightarrow \infty$, there exist a fairly large number of nonsingular (singularity free) black holes and nonsingular universes. Some nonsingular black holes have numerous horizons and numerous energy levels (a bit like atom) inside the outer event horizon. On the other hand, some nonsingular universes start and end in two de Sitter phases. The ratio of energy densities for the two phases are $120$ orders. It is thus helpful to understand the cosmological constant problem.

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arXiv2020

When the regularized Lovelock tensors are kinetically coupled to scalar field

The recently proposed regularized Lovelock tensors are kinetically coupled to the scalar field. The resulting equation of motion is second order. In particular, it is found that when the $p=3$ regularized Lovelock tensor is kinetically coupled to the scalar field, the scalar field is the potential candidate of cosmic dark energy.

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arXiv2020

Constructing higher dimensional exact black holes in Einstein-Maxwell-scalar-theory

We construct higher dimensional and exact black holes in Einstein-Maxwell-scalar-theory. The strategy we adopted is to extend the known, static and spherically symmetric black holes in the Einstein-Maxwell-dilaton gravity and Einstein-Maxwell-scalar theory. Then we investigate the black hole thermodynamics. Concretely, the generalized Smarr formula and the first law of thermodynamics are derived.

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