arXiv subjects

Rong-Gen Cai

Publications and source records attributed to Rong-Gen Cai.

At least 19 recordsLinked to original sources

arXiv2026

Integrable mass of Kerr-Newman-Bertotti-Robinson black hole

The nontrivial asymptotic structure of rotating black holes in the Bertotti-Robinson universe complicates a direct determination of their conserved mass, leading previous studies to rely on a priori thermodynamic input. We address this long-standing problem using integrability methods, establish local analytic existence and uniqueness via the Cauchy-Kovalevskaja theorem, and obtain a closed expression via a rescaled recurrence whose termination is proved at all orders. The resulting Kerr-Newman-Bertotti-Robinson mass coincides with the expression previously derived using the canonical first law identity and reproduces the masses of both Kerr-Bertotti-Robinson subcases. With the mass thus fixed without invoking thermodynamic relations for the first time, we rigorously establish the first law of black-hole thermodynamics and the Smarr formula for a whole family of Kerr-Bertotti-Robinson black holes suitable for future applications.

gr-qc↗

arXiv2026

Emergent Scaling Symmetry and Universal Interior Criticality of Scalarized Black Holes

The interiors of spontaneously scalarized charged black holes exhibit a tower of universality classes, including the previously reported $1/2$ scaling. We show that their near-critical collapse is governed by an emergent scaling symmetry at the would-be Cauchy horizon. This symmetry renders the interior dynamics autonomous, model-independent, and exactly integrable, realizing first-kind self-similarity rather than Choptuik-type second-kind criticality. It also yields a ``seesaw amplification'': an arbitrarily small exterior hair produces an arbitrarily large interior response. The amplification obeys an amplitude identity relating three independently measurable quantities. The singularity exponent equals the critical exponent of the exterior scalar charge. These relations connect the exterior phase transition to the approach to the interior singularity through the scaling dynamics near the would-be Cauchy horizon.

hep-th↗

arXiv2026

Magnetic monopoles produced by first-order phase transition

We present the first 3D lattice simulations of magnetic monopole production from a first-order phase transition, using the Press-Ryden-Spergel method, capturing the subsequent annihilation stage and the entry into the scaling regime in an expanding universe. We establish the quantitative relationship between the initial bubble number and the residual monopole abundance, and calculate the magnetic field and gravitational wave power spectra sourced by monopole dynamics. Crucially, we find that the magnetic field spectrum displays a steep $k^{-5}$ decline beyond the monopole size $w_M$, whereas the peak of the gravitational wave spectrum aligns with the scale of the mean monopole separation $d_M$, governed by $k^3$ and $k^{-3}$ scalings on the low- and high-wavenumber sides of the peak, respectively. However, since the resulting magnetic field strength is excluded by blazar observations, magnetic monopoles cannot be the origin of primordial cosmic magnetic fields. Finally, we determine the allowed monopole mass under the constraints of the Parker bound ($m_M\lesssim2.18\times10^{11}\,\mathrm{GeV}$) and dark matter abundance ($m_M\lesssim1.58\times10^{12}\,\mathrm{GeV}$).

astro-ph.CO↗

arXiv2026

Primordial black hole formation in bulk-viscous cosmology

We investigate primordial black hole (PBH) formation in a cosmological background with bulk viscosity. Using numerical simulations, we determine the collapse threshold and the resulting PBH mass. We find that the critical threshold $μ_c$ retains a dependence on the equation-of-state parameter $w$ similar to that in the inviscid case, but is enhanced by an amount comparable to the bulk-viscosity strength $ε$. For fixed $w$, the increase in $μ_c$ is approximately linear in $ε$. By fitting the standard critical-scaling law for near-threshold collapse, we find that the bulk viscosity leads to an enhancement in the resulting PBH mass. These results indicate that bulk viscosity can systematically modify both the PBH threshold and PBH mass scaling law in the early Universe.

gr-qc↗

arXiv2026

Self-stimulated avalanche in superradiant axion clouds

Boson clouds formed via superradiance around spinning black holes offer a novel probe of ultralight particles. We show that such gravitational atoms can undergo a self-stimulated avalanche: a coherent quadrupolar transition is seeded by external gravitational waves and amplified by self-generated radiation feedback. We formulate an effective two-level description that captures the logistic population transfer and the resulting delayed gravitational-wave pulse with a characteristic envelope. This cooperative emission mechanism opens a new observational avenue into the ultralight dark sector.

gr-qc↗

arXiv2026

Evidence for orbital eccentricity supports a hierarchical origin for GW231123

GW231123 is the most massive binary black hole merger observed to date and one of the most intriguing hierarchical merger candidates in the pair-instability mass gap. Its exceptionally large inferred component spins and remarkably short in-band duration make the source highly sensitive to waveform physics near merger. Here we show that GW231123 is better described as an eccentric binary black-hole merger. We analyse data over 11--448 Hz using the eccentric, spin-precessing effective-one-body waveform model TEOBResumS--Dalí, accounting for detector calibration uncertainty. We measure $e_{5.5\,\mathrm{Hz}}=0.34^{+0.12}_{-0.23}$ at 90\% credibility and obtain a Bayes factor of 10.2 in favour of the eccentric model over its quasi-circular counterpart. GW231123-like injection--recovery studies further show that analyses starting at 20 Hz, as in previous studies, can obscure this eccentricity. Including eccentricity substantially changes the spin inference, weakening the preference for a near-extremal secondary spin and lowering its posterior median to $a_2\simeq0.7$, close to the characteristic spin expected for a black-hole merger remnant. These results provide a coherent dynamical interpretation of GW231123, in which residual eccentricity, a mass-gap black hole, and a remnant-like spin arise naturally from a hierarchical merger channel in a dense environment.

astro-ph.HE↗

arXiv2026

The pseudospectrum for gravitational perturbations in Kerr spacetimes

We study the pseudospectrum of the Kerr black hole for gravitational perturbations with spin weight $s=2$, the case of greatest interest in gravitational-wave astronomy. In the hyperboloidal framework with two-dimensional Chebyshev collocation, we formulate the quasinormal mode problem as a non-self-adjoint eigenvalue problem. For $s\neq0$ the fields $Ψ^{(s)}$ and $Ψ^{(-s)}$ are independent, so the conserved current method yields no single-field energy norm. We overcome this with the Teukolsky--Starobinsky identities, which relate $Ψ^{(-2)}$ to a fourth-order operator on $Ψ^{(2)}$ and yield an $H^2$ energy norm after symmetrization. With the $H^1$ norm known for $s=0$, this suggests the pattern $H^{|s|}$, verified for $s=0$ and $s=2$. All results use an $ω$-independent two-field hierarchy. We find that the pseudospectrum tightens by $1.1$ to $3.5$ decades at each step of $L^2\to H^1\to H^2$, yet stays large in every norm. The $s=+2$ and $s=-2$ operators are isospectral but not isopseudospectral: at twelve of fourteen probes the $ψ_0$ resolvent exceeds the $ψ_4$ one by $0.19$ to $0.82$ decades in the $H^2$ norm, while at the two probes furthest along the QNM line the ordering reverses by $0.18$ to $0.19$ decades. The gravitational pseudospectrum exceeds the scalar one by $0.5$ to $5.2$ decades in the window minimum and by $5.9$ decades in $L^2$ at $ωM=0.47-0.17\,i$, while along the damping direction the $s=2$ resolvent is non-monotonic and the $s=0$ resolvent smooth. QNM spectral instability is therefore a universal feature of Kerr, and is stronger for gravitational perturbations.

gr-qc↗

arXiv2026

Second-order multipole response of two nearby incoherent sources in wave-optical imaging of a Schwarzschild black hole

We study wave-optical imaging of two nearby, mutually incoherent point sources by a Schwarzschild black hole. Using the spherical-harmonic addition theorem, we construct partial-wave fields for arbitrary source directions and form images through a finite-aperture Fourier transform. To isolate the binary structure, we compare the binary image with that of a single source of equal total brightness placed at the brightness centroid. Expanding about the centroid removes the first-order term exactly, so the leading structural correction is controlled by the second central moment, $μ_2=4a^2q/(1+q)^2$. Numerical calculations confirm the expected dependence of the residual on source half-separation and brightness ratio. We then study its wavelength dependence. At long wavelengths, the imaging kernel varies little across the source separation, making the binary nearly indistinguishable from the centroid reference. As the wavelength decreases, finer fringes enhance the residual, which grows by about a factor of three over $3\leq Mω\leq12$ ($2.09\geqλ/M\geq0.52$), even though the pair remains unresolved in the Rayleigh sense. Meanwhile, the second-order approximation gradually breaks down: its relative error first reaches unity for $17<Mω<18$ ($0.349<λ/M<0.370$), indicating the need for higher-order source moments. These results demonstrate that the distinguishability of a nearby binary is intrinsically chromatic and identify the wavelength range in which the binary is both detectable through its residual and accurately described at second order.

gr-qc↗

arXiv2026

Poisson spot and wave scattering of scalar fields by conformal anomaly black holes: probing the near-horizon geometry

We investigate the scattering and diffraction of scalar plane waves by static conformal anomaly black holes. We determine the truncation requirements for the partial-wave series (PWS) method at finite distances and compute the full waveforms, which display a clear diffraction pattern: on the negative $z$-axis the field reduces to the incident plane wave, while on the positive $z$-axis a distorted plane wave coexists with a scattered spherical wave, and a bright Poisson spot appears at $θ=0$ surrounded by concentric diffraction rings. We obtain the on-axis intensity of the Poisson spot as a function of the radial coordinate. The intensity is most sensitive to the near-horizon geometry: close to the horizon it differs markedly from that of the RN black hole, whereas in the far field the two curves converge. We trace this to the metric functions, where different values of $f'(r_+)$ produce different phase increments that compress and shift the interference fringes, while the conformal anomaly correction falls off as $O(\tildeαM^2/r^4)$. By analyzing the potential barrier we study the absorption cross section, and use the series reduction method to accelerate the PWS convergence for the differential scattering cross section. The low- and high-frequency absorption cross sections are strongly correlated with the $\ell=0$ and $\ell$-dependent parts of the potential barrier, respectively. The differential cross section and glory scattering arise mainly from scattering in a finite region excluding a small neighborhood of the outer horizon. Charge and $\tildeα$ have opposite effects on the width of the glory peak but the same effect on its height.

gr-qc↗

arXiv2026

Relic gravitational waves from primordial gravitational collapses

A large primordial density perturbation of the Hubble scale will gravitationally collapse, generating an outgoing sound shell, whether or not a primordial black hole (PBH) is formed. In this Letter, we report a hybrid numerical analysis of the stochastic gravitational wave background induced by the collision of sound shells in the early Universe. The peak frequency and amplitude in the GW spectrum depend on the Hubble horizon and the abundance of sound shells. Abundant density perturbations would lead to GW backgrounds potentially detectable for future pulsar timing arrays and ground-based/space-borne detectors. For those perturbations that collapse into PBHs, future null detection of the corresponding high-frequency GW background could put new observational constraints on those PBHs that have already evaporated.

gr-qc↗

arXiv2026

Quasinormal modes of Schwarzschild--AdS black holes with a near-horizon reflective surface

We investigate the scalar quasinormal mode (QNM) spectrum of Schwarzschild-AdS black holes with a partially reflective surface placed near the event horizon. The corresponding QNM problem is solved numerically using Chebyshev spectral collocation. The resulting spectra exhibit a characteristic cavity structure at sufficiently large real part, where the modes form approximately equally spaced sequences whose spacing and damping are controlled mainly by the cavity length and the surface reflectivity. For large AdS radius, an additional weakly damped quasibound-state branch appears due to trapping between the centrifugal barrier and the AdS potential wall. We show that these spectral features can be understood within a finite radial cavity picture. A WKB analysis provides a quantitative asymptotic description of the spectra which have high real parts. We find that changes in the near-horizon boundary condition can lead to substantial changes in the QNM spectrum, indicating the strong sensitivity of Schwarzschild--AdS QNMs to the effect of horizon correction.

gr-qc↗

arXiv2026

On the Extended Kerr-Newman-Bertotti-Robinson Spacetime: Two Black Holes and a Naked Singularity in Bertotti-Robinson Universe

We introduce new coordinates that extend the Kerr--Newman--Bertotti--Robinson spacetime consistently with the previous reciprocal continuation. In these coordinates, the two points at $Ω=0$ are resolved into complete Bertotti--Robinson null infinities. The extended spacetime describes two black holes sharing a common exterior of nontrivial topology, together with a naked ring singularity. In the regular static limit, the new coordinates globally identify the spacetime with the balanced Alekseev--García geometry.

gr-qc↗

arXiv2026

Symmetry preservation in black hole quasinormal mode spectra

We establish a general relation between symmetries of gravitational theories and black hole (BH) quasinormal mode (QNM) spectra. We show that a symmetry implies isospectrality when it induces a bijection between the corresponding QNM boundary value problems. However, conformally related BHs have been reported to exhibit both conformal factor dependent and independent QNM spectra. To resolve this issue, we develop a reduction scheme for higher order perturbation equations. Applied to pure Weyl gravity, it yields the complete axial spectrum of Schwarzschild, including Regge-Wheeler and spin-1 branches. Our results show that these seemingly contradictory conclusions result from differences in the perturbation dynamics or in the boundary conditions.

gr-qc↗

arXiv2026

Global structure and stability of Kerr-Bertotti-Robinson spacetime

The surface $r=\infty$ of the Kerr-Bertotti-Robinson spacetime is not a genuine boundary. We construct its natural analytic extension and show that $r=+\infty$ of one KBR region is smoothly connected to $r=-\infty$ of a neighboring one. Repeated continuation produces an infinite chain of regions connected by wormhole-like bridges and exposes the neighboring ring singularity without an intervening horizon, which violates the weak cosmic censorship conjecture. We then study a test massless scalar field on a two-universe scattering segment to probe the stability of the spacetime. For axisymmetric perturbations, we analytically establish purely imaginary growing quasinormal modes for every $\ell$ and trace their origin to the chronology-violating region. In the $(\ell,m)=(2,2)$ sector, unstable branches occur for sufficiently large rotation and sufficiently small magnetic field. Their marginal real modes obey an exact horizon-flux balance, supporting a black-hole-bomb interpretation in which superradiant extraction is amplified by trapping within the double-barrier potential. The same cavity also supports families of weakly damped modes and may produce echo-like responses.

gr-qc↗

arXiv2026

Long-term 3+1 simulations of primordial black hole formation during radiation domination

We develop an efficient three-dimensional numerical-relativity framework for primordial black-hole (PBH) formation from superhorizon curvature perturbations in a radiation-dominated Universe. We implement flux-conservative relativistic hydrodynamics in the adaptive-mesh-refinement code \textsc{GRChombo} and introduce a cosmologically scaled Gamma-driver that allows the cosmic-time step to grow in proportion to the scale factor. For a representative long-term simulation, the scaled driver preserves the apparent-horizon mass evolution and constraint behavior while reducing the number of coarse-level advances by a factor of approximately $94$ relative to the standard driver. We also construct a conformal-time version of the moving-puncture gauge as an independent check. Applying the framework to a spherical Gaussian curvature profile, we find a collapse threshold $0.79578 < μ_c < 0.79580$ and a critical exponent $γ\simeq 0.3559$, consistent with previous spherically symmetric results. We further fit the late-time PBH mass growth to the Zel'dovich--Novikov accretion law, demonstrating that the code can follow both near-critical collapse and long-term post-formation evolution in three dimensions. The framework provides a foundation for future studies of PBH formation beyond spherical symmetry.

gr-qc↗

arXiv2026

Transgression Completion for Chern-Simons Black Hole Thermodynamics

The quantum statistical relation, which identifies the thermodynamic free energy with the on-shell Euclidean action, is a cornerstone of black hole thermodynamics. In this Letter, we demonstrate that it fails for charged magnetic black holes in five-dimensional Einstein-Maxwell-Chern-Simons theory unless the action is formulated in a gauge-invariant manner. In particular, the free energy computed via the quantum statistical relation violates both the first law of thermodynamics and hydrodynamics. We identify the missing finite contribution---a nonlocal radial integral---whose inclusion restores exact thermodynamic consistency. Remarkably, this term is not an ad hoc correction: combined with the local Chern-Simons representative, it forms the transgression form, yielding a strictly gauge-invariant action. The correct action is therefore not the local Chern-Simons term alone but its transgression completion---a physical requirement rather than a mere mathematical refinement. The same mechanism persists for helical, rotating, scalarized, higher-derivative, and higher-dimensional black holes. Our work establishes that topological interactions can fundamentally alter the relation between the Euclidean action and the thermodynamic potential and identifies the gauge-invariant transgression form as the correct framework for black hole thermodynamics in Chern-Simons-coupled theories.

gr-qc↗

arXiv2026

Notes on Kerr-Bertotti-Robinson Spacetime

The surface $r=\infty$ of the Kerr--Bertotti--Robinson (KBR) spacetime is not the collection of the endpoints of infinitely extended light rays, and the Coulomb type component of gravitational field strength represented by $Ψ_2$ remains nonvanishing there, indicating that this surface is not a real boundary. We construct a natural extension across this surface, which connects the exterior of one KBR region to the interior of a neighboring one and, upon iteration, produces an infinite chain of regions connected by wormhole-like bridges. The extension also exposes the neighboring ring singularity without an intervening horizon, challenging the weak cosmic censorship conjecture and raising the question of whether the extended geometry is stable under perturbations. We therefore study the quasinormal modes (QNMs) of a test massless scalar field on a two-universe scattering segment. For axisymmetric perturbations, the existence of purely imaginary unstable QNMs is analytically proved for every $\ell$, with their origin tied to the chronology-violating region. In the $(\ell,m)=(2,2)$ sector, unstable QNM branches driven by a black-hole-bomb mechanism are found. Finally, the wormhole geometry produces a double-barrier cavity and families of weakly damped QNMs, suggesting echo-like responses.

gr-qc↗

arXiv2026

Thermodynamics of Kerr-Bertotti-Robinson black hole

We investigate the thermodynamic properties of the Kerr-Bertotti-Robinson black hole, an exact Petrov type-D solution of Einstein-Maxwell theory describing a rotating black hole immersed in an external electromagnetic field. While the conserved angular momentum and electric charge can be computed straightforwardly, the conserved mass cannot be obtained through standard integrability methods due to the nontrivial asymptotically uniform external electromagnetic field. To overcome this difficulty, we adopt the Christodoulou-Ruffini mass relation as a thermodynamic definition of the conserved mass and identify the associated generator, thereby fixing the ambiguity in defining this conserved mass and constructing the thermodynamic potentials. These thermodynamic quantities naturally satisfy the first law of black-hole thermodynamics as well as the Smarr formula.

gr-qc↗