Yan Peng
In the black hole spacetime, a no short hair theorem was proved, which states that the effective radius of black hole hairs must extend beyond the null circular orbit. In the present paper, in the horizonless gravity, we find a similar no short hair behavior that the effective radius of matter fields must also extend beyond the null circular orbit of ultra-compact stars.
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No long hair behaviors of ultra-compact objects
在渐近平直背景下研究球对称超致密天体外部的物质场分布，基于主能量条件和非负迹条件，解析发现物质场毛的有效半径不能超出最外层零圆轨道的无长毛行为。
Non-existence of short hairs for static black holes
本文将黑洞无短毛定理推广至静态轴对称有毛黑洞解，证明其“毛发层”必须延伸至最内层光环径向范围之外。
We present spherically symmetric black hole solutions for Einstein gravity coupled to anisotropic matter. We show that these black holes have arbitrarily short hair, and argue for stability by showing that they can arise from dynamical collapse. We also show that a recent `no short hair' theorem does not apply to these solutions.
Hairy Black Holes: Non-existence of Short Hairs and Bound on Light Ring Size
本文将“无短毛”定理推广到任意维度且不依赖引力场方程，证明毛层必须延伸至黑洞最内侧光环，并讨论其观测意义。
The No-Short Hair Theorem for Black Holes and Wormholes in Extra-Dimension
本研究将无短发定理推广到额外维度中的黑洞和虫洞，探讨其在探测额外维度和量子引力理论中的应用。
Hypothetical ultralight bosonic fields will spontaneously form macroscopic bosonic halos around Kerr black holes, via superradiance, transferring part of the mass and angular momentum of the black hole into the halo. Such process, however, is only efficient if resonant: when the Compton wavelength of the field approximately matches the gravitational scale of the black hole. For a complex-valued field, the process can form a stationary, bosonic field-black hole equilibrium state - a black hole with synchronised hair. For sufficiently massive black holes, such as the one at the centre of the M87…
The recently proved `no short hair' theorem asserts that, if a spherically-symmetric static black hole has hair, then this hair (the external fields) must extend beyond the null circular geodesic (the "photonsphere") of the corresponding black-hole spacetime: $r_{\text{field}}>r_{\text{null}}$. In this paper we provide compelling evidence that the bound can be {\it violated} by {\it non}-spherically symmetric hairy black-hole configurations. To that end, we analytically explore the physical properties of cloudy Kerr-Newman black-hole spacetimes -- charged rotating black holes which support lin…
If a black hole has hair, how short can this hair be? A partial answer to this intriguing question was recently provided by the 'no-short hair' theorem which asserts that the external fields of a spherically-symmetric electrically neutral hairy black-hole configuration must extend beyond the null circular geodesic which characterizes the corresponding black-hole spacetime. One naturally wonders whether the no-short hair inequality $r_{\text{hair}}>r_{\text{null}}$ is a generic property of all electrically neutral hairy black-hole spacetimes? In this paper we provide evidence that the answer to…
We show that in all theories in which black hole hair has been discovered, the region with non-trivial structure of the non-linear matter fields must extend beyond $3/ 2$ the horizon radius, independently of all other parameters present in the theory. We argue that this is a universal lower bound that applies in every theory where hair is present. This {\it no short hair conjecture} is then put forward as a more modest alternative to the now debunked {\it no hair conjecture}.
Radiative properties of thin accretion disks around rotating short-hairy black holes
本文研究旋转短毛黑洞周围薄吸积盘的辐射性质，分析了旋转参数、短毛参数和幂参数对测地线运动及盘辐射通量密度、有效温度、微分和光谱光度的影响。
We show that in all theories in which black hole hair has been discovered, the region with non-trivial structure of the non-linear matter fields must extend beyond 3/2 the horizon radius, independently of all other parameters present in the theory. We argue that this is a universal lower bound that applies in every theory where hair is present. This {\it no short hair conjecture} is then put forward as a more modest alternative to the original {\it no hair conjecture}, the validity of which now seems doubtful.
No-hair and almost-no-hair results for static axisymmetric black holes and ultracompact objects in astrophysical environments
本文在静态轴对称情形下推广无毛定理，证明与给定外部引力场相容的黑洞几何仅构成单参数族，并证明在曲率有界条件下，近黑洞的无视界天体的形状偏离必须随趋近黑洞极限而消失。
Periodic orbits and gravitational wave radiation in short hair black hole spacetimes for an extreme mass ratio system
本文研究极端质量比系统中短发黑洞的发参数和结构参数对粒子周期轨道及引力波辐射的影响，发现参数变化会改变轨道特性并显著增强引力波信号，为区分短发黑洞与经典黑洞提供新观测依据。
Novel exact ultra-compact and ultra-sparse hairy black holes emanating from regular and phantom scalar fields
本研究在含最小耦合标量场的简单引力理论中，推导了源自正则与幻影标量场的渐近平直超致密、超稀疏带毛黑洞静态及慢旋转解析解，通过轴向扰动分析证明两类黑洞在参数空间中存在共存区域。
It has been shown recently that extreme Reissner-Nordström black holes perturbed by a minimally coupled, free, massless scalar field have permanent scalar hair. The hair - a conserved charge calculated at the black hole's event horizon - can be measured by a certain expression at future null infinity: the latter approaches the hair inversely in time. We generalize this newly discovered hair also for extreme Kerr black holes. We study the behavior of nearly extreme black hole hair and its measurement at future null infinity as a transient phenomenon. For nearly extreme black holes the measureme…
Testing The Weak Cosmic Censorship Conjecture in Short Haired Black Holes
本文通过测试粒子和标量场入射旋转带毛黑洞，研究弱宇宙监督猜想，发现毛发强度参数κ的不同取值范围决定事件视界能否被破坏，揭示短毛对黑洞事件视界的显著影响。
It has recently been revealed that spinning black holes of the photon-fluid model can support acoustic `clouds', stationary density fluctuations whose spatially regular radial eigenfunctions are determined by the $(2+1)$-dimensional Klein-Gordon equation of an effective massive scalar field. Motivated by this intriguing observation, we use {\it analytical} techniques in order to prove a no-short hair theorem for the composed acoustic-black-hole-scalar-clouds configurations. In particular, it is proved that the effective lengths of the stationary bound-state co-rotating acoustic scalar clouds a…
Can Rotating Black Holes Have Short Hairs?
本文考察无短毛定理在Konoplya-Rezzolla-Zhidenko-Stuchlík类和Johannsen类旋转黑洞中的有效性，发现Klein-Gordon可分离非真空广义相对论解仍满足无短毛性质，并给出推广到其他理论所需的最小附加判据。
Black Holes in Massive Dynamical Chern-Simons gravity: scalar hair and quasibound states at decoupling
研究超轻动力学标量场非最小耦合Chern-Simons项时旋转黑洞周围的大质量标量毛，数值模拟发现近黑洞偶极结构及远处质量诱导的振荡云，并给出特征频率谱分析。
The continuum-fitting and the iron line methods are leading techniques capable of probing the spacetime geometry around astrophysical black hole candidates and testing the no-hair theorem. In the present paper, we review the two approaches, from the astrophysical models and their assumptions, to the constraining power with present and future facilities.
Bondi-Hoyle-Lyttleton accretion onto a rotating black hole with ultralight scalar hair
数值研究带超轻标量毛的旋转黑洞上的相对论性BHL吸积，发现激波锥与驻点，拟合质量吸积率，并关联Sgr A*和微类星体的准周期振荡。
Short-Hair Black Holes and the Strong Cosmic Censorship Conjecture
本文在广义相对论框架下利用弱引力猜想和WKB近似方法研究短毛黑洞在标量场扰动下对强宇宙监督猜想的满足情况，发现电荷接近极端值时猜想被违反，而增大角动量量子数和度规方程阶数可推迟违反现象。
The maximal ADM mass for (mini-)boson stars (BSs) -- gravitating solitons of Einstein's gravity minimally coupled to a free, complex, mass $μ$, Klein-Gordon field -- is $M_{\rm ADM}^{\rm max}\sim M_{Pl}^2/μ$. Adding quartic self-interactions to the scalar field theory, described by the Lagrangian $\mathcal{L}_I=λ|Ψ|^4$, the maximal ADM mass becomes $M_{\rm ADM}^{\rm max}\sim \sqrtλM_{Pl}^3/μ^2$. Thus, for mini-BSs, astrophysically interesting masses require ultra-light scalar fields, whereas self-interacting BSs can reach such values for bosonic particles with Standard Model range masses. We i…
Strong Gravitational Lensing Effects of the Rotating Short-Haired Black Hole and Constraints from EHT Observations
本研究系统调查了旋转短毛黑洞的强引力透镜效应，发现短毛参数$Q_m$显著影响黑洞的事件视界半径、光子轨道半径及透镜系数，且在M87*和Sgr A*观测模拟中与其他模型存在显著的时间延迟差异，结合EHT观测数据有效约束了该模型的参数空间。
The successful observation of M87 supermassive black hole by the Black Hole Event Horizon Telescope(EHT) provides a very good opportunity to study the theory of gravity. In this work, we obtain the exact solution for the short hair black hole (BH) in the rotation situation, and calculate in detail how hairs affect the BH shadow. For the exact solution part, using the Newman-Janis algorithm, we generalize the spherically symmetric short-hair black hole metric to the rotation case (space-time lie element (2.25)). For the BH shadow part, we study two hairy BH models. In model 1, the properties of…
The elegant `no short hair' theorem states that, if a spherically-symmetric static black hole has hair, then this hair must extend beyond 3/2 the horizon radius. In the present paper we provide evidence for the failure of this theorem beyond the regime of spherically-symmetric static black holes. In particular, we show that rotating black holes can support extremely short-range stationary scalar configurations (linearized scalar `clouds') in their exterior regions. To that end, we solve analytically the Klein-Gordon-Kerr-Newman wave equation for a linearized massive scalar field in the regime …
Lower bound of black hole hair in pure Lovelock theory of gravity
本文在纯Lovelock引力中验证“无短发猜想”，证明静态球对称黑洞的毛发至少延伸至光子球，且该结论与维度数和Lovelock阶数无关。
We investigate the thermodynamics of hairy black holes in asymptotically Anti-de Sitter (AdS) space, including backreaction. Resorting to the Euclidean path integral approach, we show that matter conformally coupled to Einstein gravity in five dimensions may exhibit a phase transition whose endpoint turns out to be a hairy black hole in AdS5 space. The scalar field configuration happens to be regular everywhere outside and on the horizon, and behaves asymptotically in such a way that respects the AdS boundary conditions that are relevant for AdS/CFT. The theory presents other peculiar features…
In this work, we consider a very simple gravitational theory that contains a scalar field with its kinetic and potential terms minimally coupled to gravity, while the scalar field is assumed to have a coulombic form. In the context of this theory, we study an analytic, asymptotically flat, and regular (ultra-compact) black-hole solutions with non-trivial scalar hair of secondary type. At first, we examine the properties of the static and spherically symmetric black-hole solution -- firstly appeared in 1504.08209 [gr-qc] -- and we find that in the causal region of the spacetime the stress-energ…
Charged nutty black holes are hairy
研究揭示了带电纽结黑洞伴随的Misner弦中发现的电和磁单极子的物理本质，展示了这些弦携带非均匀的电和磁场流动，从而产生短程电磁毛发区。
It is by now well established that charged rotating Kerr-Newman black holes can support bound-state charged matter configurations which are made of minimally coupled massive scalar fields. We here prove that the externally supported stationary charged scalar configurations {\it cannot} be arbitrarily compact. In particular, for linearized charged massive scalar fields supported by charged rotating near-extremal Kerr-Newman black holes, we derive the remarkably compact lower bound $(r_{\text{field}}-r_+)/(r_+-r_-)>1/s^2$ on the effective lengths of the external charged scalar `clouds' [here $r_…
Using backwards ray tracing, we study the shadows of Kerr black holes with scalar hair (KBHsSH). KBHsSH interpolate continuously between Kerr BHs and boson stars (BSs), so we start by investigating the lensing of light due to BSs. Moving from the weak to the strong gravity region, BSs - which by themselves have no shadows - are classified, according to the lensing produced, as: $(i)$ non-compact, which yield no multiple images; $(ii)$ compact, which produce an increasing number of Einstein rings and multiple images of the whole celestial sphere; $(iii)$ ultra-compact, which possess light rings…
A note on no-hair properties of static black holes in four and higher dimensional spacetimes with cosmological constant
研究了带宇宙学常数的四维及更高维时空中静态黑洞的无毛性质，针对宇宙学常数为零、正、负三种情况分别证明或讨论了无毛定理及有毛黑洞存在的条件。
We investigate the photonsphere in the background of regular asymptotically flat compact stars. The analysis includes the general hairy compact star considering the matter fields' backreaction on the metric in various gravity theories. We prove that the photonsphere of the compact star has an upper bound expressed in terms of the ADM mass of the spacetime. In the case of negative isotropic trace, a stronger upper bound can be obtained.
Splitting the Gravitational Atom: Instabilities of Black Holes with Synchronized or Resonant Hair
研究同步或共振发的黑洞在引力原子形成中的不稳定性，揭示其动态分裂机制及对Kerr范式的偏离。
Cosmological implications for hairy black holes via spontaneous symmetry breaking: Are Hairy Black Holes Primordial?
本研究探讨ESGB理论中自发对称破缺产生的带毛黑洞是否与宇宙演化兼容，推导了耦合强度等的严格约束，发现仅质量约几克的超轻黑洞可形成标量毛，即带毛原初黑洞。
We investigate the possibility of having hairs on the cosmological horizon. The cosmological horizon shares similar properties of black hole horizons in the aspect of having hairs on the horizons. For those theories admitting haired black hole solutions, the nontrivial matter fields may reach and extend beyond the cosmological horizon. For Q-stars and boson stars, the matter fields cannot reach the cosmological horizon. The no short hair conjecture keeps valid, despite the asymptotic behavior (de Sitter or anti-de Sitter) of black hole solutions. We prove the no scalar hair theorem for anti-de…
Coherent quantum hairy black holes from gravitational decoupling: regularity, geodesics, and scalar ringdown
本文提出基于引力解耦的相干量子毛洞模型，通过高斯模糊引力势构造度规，展现无奇点几何和修改视界结构，并探讨强场修正对观测的约束。
Matter accretion onto the magnetically charged Euler-Heisenberg black hole with scalar hair
本文针对带标量毛的磁荷Euler-Heisenberg黑洞，利用哈密顿动力学方法研究多种理想流体的吸积过程，得到声点、解析解与吸积率规律，为理解黑洞吸积机制提供参考。
Motivated by the lack of a general parametrization for exotic compact objects, we construct a class of perturbative solutions valid for small (but otherwise generic) multipolar deviations from a Schwarzschild metric in general relativity. We introduce two classes of exotic compact objects, with "soft" and "hard" hair, for which the curvature at the surface is respectively comparable to or much larger than that at the corresponding black-hole horizon. We extend the Hartle-Thorne formalism to relax the assumption of equatorial symmetry and to include deformations induced by multipole moments hig…
Black holes with axion hair from cubic Riemann interactions
本研究构建了含三次黎曼耦合轴子场的有效场论黑洞解，证明轴子毛发为次级电荷且反作用仅出现在次领头阶，修正的霍金温度允许蒸发后留下黑洞残余。
Perturbations of massless external fields in Horndeski hairy black hole
本文研究了Horndeski毛黑洞中无质量外场的微扰传播，通过数值与解析方法求解主方程，发现该黑洞在微扰下保持稳定，且Horndeski毛发会显著影响Hawking辐射发射率及黑洞寿命。
In this short note, we consider the phases of gravity coupled to a $U(1)$ gauge field and charged scalar in 2+1 dimensions without a cosmological constant, but with box boundary conditions. This is an extension of the results in arXiv:1609.01208, but unlike in higher dimensions, here the physics has sharp differences from the corresponding AdS problem. This is because Einstein-Maxwell black holes cease to exist when the cosmological constant goes to zero. We show that hairy black holes also do not exist in the flat 2+1 dimensional box under some assumptions, but hairy boson stars do. There is …
Scalar-tensor theories are a compelling alternative to general relativity and one of the most accepted extensions of Einstein's theory. Black holes in these theories have no hair, but could grow "wigs" supported by time-dependent boundary conditions or spatial gradients. Time-dependent or spatially varying fields lead in general to nontrivial black hole dynamics, with potentially interesting experimental consequences. We carry out a numerical investigation of the dynamics of single and binary black holes in the presence of scalar fields. In particular we study gravitational and scalar radiatio…
Stable black hole solutions with cosmological hair
研究探讨了在动态暗能量理论中，黑洞解的稳定性与宇宙学毛发的影响，通过立方伽利略子模型，推导出稳定且规则的黑洞解，揭示了局部黑洞物理与宇宙学动态的关联。
We construct charged soliton solutions around spherical charged black holes with no angular momentum in asymptotically flat spacetime. These solutions are non-linear generalizations of charged scalar clouds, dubbed Q-ball hair or Q-clouds, and they do not contradict the non-existence theorem for free (linear) scalar clouds around charged black holes. These solutions are the first examples of O(3) solutions for Q-clouds around a non-extremal and non-rotating BH in the Abelian gauge theory. We show that a solution exists with an infinitely short hair in the limit of extremal black holes. We disc…
The simplicity of black holes, as characterized by no-hair theorems, is one of the most important mathematical results in the framework of general relativity. Are these theorems unique to black hole spacetimes, or do they also constrain the geometry around regions of spacetime with arbitrarily large (although finite) redshift? This paper presents a systematic study of this question and illustrates that no-hair theorems are not restricted to spacetimes with event horizons but are instead characteristic of spacetimes with deep enough gravitational wells, extending Israel's theorem to static spac…
Devoured by a Hairy Gargantua: Probing Massive Scalar Charges with Non-minimal Curvature Coupling with Extreme-Mass-Ratio Inspirals
该研究探讨与曲率非最小耦合的超轻标量场对黑洞及极端质量比旋近的影响，通过建模旋近过渡发现引力波相位偏移，还提出可利用 LISA 测量曲率耦合与标量质量的高精度。
The possibility of mining the rotational energy from black holes has far--reaching implications. Such energy extraction could occur even for isolated black holes, if hypothetical ultralight bosonic particles exist in Nature, leading to a new equilibrium state $-$ a black hole with synchronised bosonic hair $-$ whose lifetime could exceed the age of the Universe. A natural question is then: for an isolated black hole and at maximal efficiency, how large is the energy fraction $ε$ that can be extracted from a Kerr black hole by the superradiant growth of the dominant mode? In other words, how ha…
Self-regularized entropy: What does black hole entropy predict for tests of Kerr no-hair theorem?
本文研究了量子黑洞模型中无毛定理的检验，通过计算无毛熵，发现近视界态密度的变化使紫外发散自调节，从而消除了对人工截断的需要，并提出了观测目标。
Perturbed Kerr black holes emit gravitational radiation, which (for the practical purposes of gravitational-wave astronomy) consists of a superposition of damped sinusoids termed quasi-normal modes. The frequencies and time-constants of the modes depend only on the mass and spin of the black hole - a consequence of the no-hair theorem. It has been proposed that a measurement of two or more quasi-normal modes could be used to confirm that the source is a black hole and to test if general relativity continues to hold in ultra-strong gravitational fields. In this paper we propose a practical appr…
We investigate spherically symmetric and static gravitational fields representing black hole configurations in the framework of metric-affine gauge theories of gravity (MAG) in the presence of different matter fields. It is shown that in the triplet ansatz sector of MAG, black hole configurations in the presence of non-Abelian matter fields allow the existence of black hole hair. We analyze several cases of matter fields characterized by the presence of hair and for all of them we show the validity of the no short hair conjecture.
Quantum-Corrected and Hairy Black Holes via Analogue Gravity
本研究通过类比引力将带毛黑洞与 Calmet-Kuipers 度规映射到拉瓦尔喷管跨音速流动，利用 Mashhoon 方法分析微扰响应，揭示量子修正与毛状效应的不同作用机制，为黑洞研究提供运动学类比手段。
If a class of stars orbits the central black hole in our galaxy in short period (~ 0.1 year), high eccentricity (~ 0.9) orbits, they will experience precessions of their orbital planes induced by both relativistic frame-dragging and the quadrupolar gravity of the hole, at levels that could be as large as 10 microarcseconds per year, if the black hole is rotating faster than 1/2 of its maximum rotation rate. Astrometric observations of the orbits of at least two such stars can in principle lead to a determination of the angular momentum vector J of the black hole and its quadrupole moment Q_2. …
The idea that gravity can act as a regulator of ultraviolet divergences is almost a century old and has inspired several approaches to quantum gravity. In fact, a minimum Planckian length can be shown to emerge from the nonlinear dynamics of gravity in the effective field theory approach to gravitational scatterings at Planckian energies. A simple quantum description of the gravitational collapse of a ball of dust supports the conclusion that such a length scale is indeed dynamical and matter inside black holes forms extended cores of macroscopic size. The geometry of these quantum black holes…
Nonlinear Stability of Rotating Hairy Black Holes
本文研究了旋转毛黑洞的稳定性，发现当标量场质量次主导时，系统在非线性演化中保持稳定，而标量场质量主导时则在较短时间内出现不稳定性。
Exact black hole solutions in scalar-tensor theories
本文在标量-张量理论中检验无毛定理，通过对称性构造带毛黑洞，发现可规避无毛定理、消除中心奇点，并建立与高维引力的联系及向虫洞等新解的变换。
We study black holes in the recently proposed ghost-free theory with two gravitons, one of which is massive and another is massless. These black holes possess a regular event horizon which is common for both metrics and has the same values of the surface gravity and Hawking temperature with respect to each metric. The ratio of the event horizon radii measured by the two metrics is a free parameter that labels the solutions. We present a numerical evidence for their existence and find that they comprise several classes. Black holes within each class approach the same AdS-type asymptotic at infi…
We show that the observed angular diameter of the shadow of the ultra compact object Sgr A*, favours the existence of an extra spatial dimension. This holds irrespective of the nature of the ultra compact object, i.e., whether it is a wormhole or, a black hole mimicker, but with the common feature that both of them have an extra dimensional origin. This result holds true for the mass and the distance measurements of Sgr A* using both Keck and the Gravity collaborations and whether we use the observed image or, the observed shadow diameter. In particular, the central value of the observed shado…
Ultralight scalars have been predicted in a variety of scenarios, and advocated as a possible component of dark matter. These fields can form compact regular structures known as boson stars, or---in the presence of horizons---give rise to nontrivial time-dependent scalar hair and a stationary geometry. Because these fields can be coherent over large spatial extents, their interaction with "regular" matter can lead to very peculiar effects, most notably resonances. Here we study the motion of stars in a background describing black holes surrounded by non-axially symmetric scalar field profiles.…
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