Papers

In this paper, we investigate the stability of canonical acoustic black hole under the perturbation of non-minimally coupling scalar field, electromagnetic field and Dirac field. The quasi-normal modes(QNMs) are computed using the three-order WKB approximation method. The results show that, for all these three kinds of perturbation, the values of the effective potential are positive, while the imaginary parts of QNMs are negative, the canonical acoustic black hole is a table. In details respectively, for scalar field perturbation, the effective potential and the QNMs depend on the angular quantum number $l$, the overtone number $n$, and the coupling parameter {\zeta}, the perturbation decays more rapidly for bigger {\zeta}. For electromagnetic field perturbation, the effective potential and QNMs depend on $l$ and $n$. For Dirac field perturbation, the effective potential and QNMs depend on $n$ and the total angular quantum number | k |.

The sudden removal of a potential barrier from a Bose-Einstein condensate (BEC) gives rise to a quantum version of a hydrodynamic dam break and leads to rich wave dynamics that have similarities to other localized defect problems such as domain wall dynamics in spin systems. Denoting $\Delta n$ as the initial difference in density between the upper $n_{1}$ and lower $n_{0}$ reservoirs on either side of the dam, we use the Gross-Pitaevskii equation to study the quasi-one dimensional case in both perturbative ($\Delta n \ll n_{0}$) and non-perturbative ($\Delta n \sim n_{1}$) regimes. In the perturbative regime a pair of outwardly propagating dispersive wavepackets forms which can be viewed as quantum versions of undular tidal bores that have analytic forms at long times in terms of the integrals of Airy functions with a wavelength that grows as $(\hbar^2 t)^{1/3}$. Airy functions are the universal wave functions that dress structurally stable fold caustics where pairs of rays coalesce, and, indeed, we show that the quantum dam break problem has the same ray structure as the naturally occurring caustic phenomenon of a double rainbow, including Alexander's dark band between the two bows where no light is scattered: we identify the intermediate density plateau in the dam break as an analogous `silent band' where only evanescent sound waves can exist. In the opposite regime of a non-perturbative dam break we show that a self-induced sonic horizon occurs when $\Delta n \geq (8/9)n_{1}$. A discussion of possible experimental schemes for amplification of quantum undulations is included as well as an alternative scheme for dispersive wave generation where a constant flow is imprinted on a BEC in box trap.

We establish a classification of the gravitational production of light particles across sharp transitions connecting different asymptotic forms of the scale factor. A family of smooth scale factors with a transition duration $\Delta\eta$ is characterized by its regularity in the sharp-transition limit $\Delta\eta\to 0$, leading to three qualitatively different universality classes: $C^0$, $C^1$, and $C^2$-or-smoother. We find that the $C^0$ class exhibits a quadratic enhancement in the energy density of produced particles, $\rho\propto\Delta\eta^{-2}$, while the $C^1$ class shows a logarithmic enhancement, $\rho\propto\ln(1/\Delta\eta)$. No sharp-transition enhancement occurs for the $C^2$-or-smoother class. Using a specific $C^\infty$ model of the scale factor, we derive the energy density analytically and confirm both the predicted quadratic scaling and its coefficient by numerically solving the mode equation. These results identify the regularity of the scale factor in the sharp limit as the key property controlling the sharp-transition enhancement.

We study the gravitational collapse of a spherically symmetric, inhomogeneous dust cloud in five-dimensional AdS Chern–Simons gravity. Working in the torsion-free sector and using comoving Lemaître–Tolman–Bondi coordinates, we derive the reduced field equations and obtain an exact collapsing interior solution. The areal radius evolves periodically in time, but the physical collapsing branch extends only up to shell focusing or an earlier loss of regularity, while the initial density determines a radial function whose square is proportional to the conserved quasilocal mass. This separates the roles of the initial velocity and density profiles in controlling shell focusing and trapping. We derive conditions for central regularity and for avoiding shell crossing, analyze the Kretschmann scalar, and show that shell focusing corresponds to a genuine curvature singularity, the weak and strong energy conditions are preserved throughout the regular collapsing phase. The homogeneous limit reduces consistently to a five-dimensional FLRW interior. The collapsing cloud is matched smoothly to the black-hole branch of the static AdS Chern–Simons exterior. The null expansions show that a finite-radius apparent horizon forms only when the quasilocal mass exceeds a critical threshold. Initially untrapped shells above this threshold become trapped before shell focusing, whereas below the total-mass threshold no finite-radius future-trapped symmetry sphere forms in the regular interior.

Torque balance between gravitational radiation reaction and magnetocentrifugal accretion is one reason why accreting neutron stars rotate slower than centrifugal break-up. Random processes in the accretion disk, such as Rayleigh-Taylor instabilities and flicker noise, drive the spin frequency $f_\ast$ and gravitational wave frequency $f_{\rm gw}\propto f_\ast$ to wander stochastically around their torque balance values. Here it is shown theoretically, in terms of an idealized Ornstein-Uhlenbeck model, that the standard deviation of $f_{\rm gw}$ is given by $\sigma_{f_{\rm gw}}(h_0) \propto f_{\rm gw} h_0 D \beta T^{1/2}$, or equivalently $\sigma_{f_{\rm gw}}(F_{\rm X}) \propto f_{\rm gw}^{1/2} F_{\rm X}^{1/2} D \beta T^{1/2}$, where $h_0$ is the characteristic wave strain, $F_{\rm X}$ is the X-ray flux, $D$ is the source distance, and $\beta$ and $T$ are the fractional amplitude and autocorrelation time-scale of the stochastic torque. One can use $\sigma_{f_{\rm gw}}(h_0)$ and $\sigma_{f_{\rm gw}}(F_{\rm X})$ to check the physical consistency of a detection candidate in a gravitational wave search by comparing with $\sigma_{f_{\rm gw}}$ observed. Spin wandering impacts search design, because it sets the maximum time interval $\max(T_{\rm coh})$, during which the signal may be treated as coherent. The Ornstein-Uhlenbeck calculation predicts $10^{-1} \lesssim \max(T_{\rm coh}) / (1\, {\rm day}) \lesssim 10^3$, with $3\times 10^{-2} \lesssim \beta T^{1/2} / (1 \, {\rm s^{1/2}} ) \lesssim 4$, for Rayleigh-Taylor instabilities and flicker noise. The predicted $\max(T_{\rm coh})$ is shorter typically than the year-long observation runs of audio-band interferometers operating today, emphasizing the utility of semi-coherent search algorithms, which divide the data into multiple coherent segments of duration $\max(T_{\rm coh})$.

We present and study a family of exact Taub-NUT-AdS solutions with scalar hair in four-dimensional ${\cal N}=2$ gauged supergravity. The solutions are supported solely by a nontrivial scalar field and its self-interaction potential, with all gauge fields switched off. We determine the conditions under which Killing horizons exist and demonstrate that the spacetime admits at most one Killing horizon, and whenever it exists it is necessarily nondegenerate. In particular, the NUT deformation can give rise to a horizon in a branch whose static limit is horizonless. We also establish that the solutions with nontrivial scalar hair admit no Killing spinors. As in ordinary Taub-NUT-AdS spacetime, the global structure is afflicted by closed timelike curves. We compute the conserved mass using the covariant phase space method and formulate a restricted first law with the NUT parameter held fixed. Finally, we study the Euclidean sector, including an exceptional family specific to Euclidean signature, and determine the regularity conditions for bolts and nuts.

The magnetic field geometry in the near-horizon region is very important in shaping polarimetric signatures of black hole images. With a hybrid magnetic field consisting of an axisymmetric split-monopole field and a tilted uniform external field, we investigate the effects of external magnetic field inclination on the polarimetric signatures of Sgr A* by employing a stationary semi-analytical radiatively inefficient accretion flow (RIAF) model and general relativistic polarized radiative transfer simulations. Our results show that the external magnetic field tilt significantly modifies the electric vector position angle (EVPA) pattern and brightness distribution. The induced non-axisymmetry from the tilted magnetic filed component renders polarization morphology dependent on the observer's azimuthal angle, which is a novel behavior absent in purely axisymmetric magnetic field configurations. In combination with Event Horizon Telescope (EHT) data, we analyse the possibility of such hybrid magnetic fields around Sgr A*.

One examines a codimension-one thick braneworld in the scalar representation of $f(Q,B)$ gravity, where $Q$ is the nonmetricity scalar, and $B$ is the boundary term satisfying $R=Q-B$. We write the gravitational sector in terms of two auxiliary scalar fields, $\varphi=f_Q$ and $\psi=f_B$, and an interaction potential $U(\varphi,\psi)$ obtained by a Legendre transformation from the $f(Q,B)$ function. Working in the coincident gauge and assuming a five-dimensional warped geometry with four-dimensional Poincaré symmetry, we derive the complete set of brane equations sourced by a canonical bulk scalar field. To accomplish our purpose, we address two models of smooth asymptotically AdS thick brane with localized scalar profiles. Furthermore, we also analyze tensor perturbations and demonstrate the localization and stability of the massless graviton.

We derive the axion Weak Gravity Conjecture (aWGC) from the nonperturbative fragmentation of Euclidean wormholes into charged defects. Requiring fragmentation to lower the Euclidean action fixes the axion action-to-charge bound and its order-one coefficient. Without additional scalars, this coefficient coincides with that of the Imaginary Distance Bound (IDB), while additional scalars strengthen the bound. The aWGC in the presence of multiple axio-dilaton pairs is discussed, leading to geometric representations. Axions satisfying the aWGC render the corresponding wormholes unstable, potentially suppressing their proliferation and resolving associated factorization problems.

We apply the canonical clock-sector framework developed in Ref.~\cite{Tavakoli:2026coc} to spatially flat FLRW cosmology in Brans–Dicke theory. The aim is to obtain an explicit cosmological realization of the frame-adapted relational dynamics and to determine the corresponding cosmological observables in closed form. Starting from the previously developed full ADM theory and restricting it to the homogeneous sector, we obtain the canonical clock momentum as $P_{\Phi}=p_{\phi}-ap_a/2\phi$. This identification allows the Jordan and Einstein-frame reduced systems to be represented on a common canonical phase space. We then consider a quadratic Jordan-frame potential for which the Einstein-frame potential is constant. The resulting relational FLRW system is exactly integrable. The complete trajectory can be expressed in terms of a single parameter $u$, yielding closed-form expressions for the scale factor, the cosmic-time Hubble rate, the Hubble slow-roll parameter, the number of e-folds, and the scalar equation-of-state parameter. In particular, the exact solution provides an explicit inflationary regime that begins at $u=\operatorname{arctanh}(1/\sqrt{3})$ and transitions toward an asymptotic de Sitter regime. We finally compare the exact solution with direct numerical integration in the adapted Jordan and Einstein descriptions, and with the naively reduced Jordan system based on the unshifted scalar momentum. The numerical results reproduce the exact trajectory and illustrate how the different reduced canonical representations are related by the clock-dependent transformation implied by the canonical clock-sector construction. The analysis provides an exactly solvable laboratory for relational frame equivalence in scalar–tensor cosmology and gives explicit cosmological observables that can be used in extensions to more general potentials and scalar–tensor models.

We study how well gravitational-wave phase deviations can be identified using a neural network with classification and regression heads. The classification head distinguishes general relativity (GR) from six parametrized post-Einsteinian phase families with exponents $b\in\{-7,-5,-3,-1,+1,+2\}$; the regression head predicts the logarithm of the coupling magnitude, $\log_{10}|\beta|$. The network input is a response function quantifying the sensitivity of waveform mismatch to waveform deformations. We use stationary Gaussian noise with the Advanced LIGO design spectrum for 262 sources and a measured Hanford spectrum from the third observing run (O3) for 132 sources, and also test injections into recorded Hanford strain. On the two Gaussian datasets, the network identifies the exponent of deviations clearly above the noise with accuracies of $0.427$ and $0.329$. To interpret these results, we compare the network with an approximate classifier built from predicted waveform residuals for the six phase families and same sources. The two classifiers often confuse the same families. Examining residuals left by different phase corrections, we link these errors to similarities between the underlying deviations. On selected recorded Hanford strain, networks trained on simulated or recorded noise achieve overall classification accuracies close to those on simulated Hanford noise. Of thirteen neural-network variants tested on the Advanced LIGO design dataset, the recurrent network has the highest mean classification accuracy for loud deviations, although all variants remain below the approximate classifier. The network nearly matches that classifier's accuracy, indicating that identification is limited by similarities between neighboring phase families rather than by the network, for known intrinsic source parameters.

In this thesis, we consider general relativity on a manifold with a timelike boundary of finite size, across all signs of the cosmological constant $\Lambda$. We impose conformal boundary conditions, whereby the conformal class of the induced metric and the trace of the extrinsic curvature $K$ are fixed at the boundary. In Lorentzian signature, we analyse the linearised Einstein equations about Minkowski, de Sitter (dS), and anti-de Sitter (AdS) space for a variety of boundaries, employing the Kodama-Ishibashi formalism adapted to manifolds with a boundary. In Euclidean signature, we study the thermodynamics of black hole and cosmological horizons enclosed by the boundary, computing the leading semiclassical approximation of the gravitational path integral, for which the boundary data define a conformal canonical ensemble of fixed conformal temperature and $K$.

Spin directions are promising observables for distinguishing between the formation channels of merging stellar-mass binary black holes with gravitational-wave observations. In this work, we challenge the standard expectation that binaries formed via dynamical interactions in stellar clusters have isotropically distributed spins and that binaries formed in isolation have spins aligned with the orbital angular momentum. For dynamically formed binaries, we account for observational findings suggesting that the spins of stars in clusters may exhibit a preferred degree of alignment and construct a simple geometrical description of the resulting black-hole spin directions. This results in an analytical joint distribution for the polar angles of first-generation black-hole binaries, whose key feature is a correlation between the two spin-orbit tilts. We show that, under reasonable assumptions and independently of the initial spin configuration, spins are fully randomized after a single black-hole merger, implying that binaries formed through hierarchical mergers have isotropically distributed spin orientations. For isolated binaries, we introduce a simple model for spin evolution as a function of the natal kick imparted during the first supernova explosion, while accounting for tidal interactions and mass-ratio reversal. The resulting distribution is also fully analytic. We investigate the impact of relativistic spin precession, as well as that of the finite escape speed of clusters, on our geometrically derived distributions. Our results provide a simple analytical framework for modeling the spin directions of merging black holes in gravitational-wave population fits.

Kerr black holes and electroweak sphalerons are remarkable solutions of two pillars of modern physics: General Relativity and the Standard Model of particle physics, respectively. In this work, we investigate their interplay by constructing nonlinear sphaleron clouds around Schwarzschild and Kerr black holes in the probe limit. We first revisit static and spinning (multi-)sphalerons in flat spacetime, using the measured electroweak parameters. We then explore the domain of existence and physical properties of the corresponding black-hole configurations, revealing a rich branch structure. Remarkably, the sphaleron clouds retain a finite mass gap and do not approach linear clouds within the families studied, distinguishing them from the familiar clouds associated with the onset of superradiant instabilities. We also obtain a closed-form leading-order Higgs solution on an exact Yang-Mills configuration in a Schwarzschild background.

In constrained Hamiltonian dynamics there are two views regarding how first-class constraints generate gauge transformations: individually or only in a certain combination, the Rosenfeld-Anderson-Bergmann-Castellani gauge generator. This gauge generator $G$ preserves Hamilton's equations and changes the canonical action at most by a boundary term; Hamiltonian's equations are the Euler-Lagrange equations for the canonical action. Hence the canonical formalism is equivalent to the Lagrangian formalism, and indeed subsumed within it (in important examples) with many canonical momenta serving as auxiliary fields. $G$ generates transformations basically equivalent to the usual 4-dimensional Lagrangian expressions, such as a 4-gradient in electromagnetism or a space-time coordinate transformation (on-shell) in General Relativity. It has been shown recently that separate first-class constraints lead to inequivalent observables between Proca non-gauge and Stueckelberg gauge massive electromagnetism. There is, however, widespread agreement that second-class constraints do not generate gauge transformations. Here it is shown that in such a sense as the first-class primary constraint in Maxwell's theory generates a gauge transformation, the second-class primary constraint in Proca's massive electromagnetism also generates a gauge transformation. Likewise the second-class primary constraints in various massive spin $2$ relatives of General Relativity generate as much of a gauge transformation as do the corresponding first-class primary constraints in GR. Hence the view that first-class constraints typically generate gauge transformations _individually_ faces a puzzle not faced by the gauge generator view.

We investigate the cosmological dynamics and observational viability of a logarithmic Herglotz-type $f(R,T)$ gravity model described by $f(R,T)=R+\beta\ln(-T/T_0)$. Assuming a spatially flat Friedmann–Lemaître–Robertson–Walker background and pressureless matter, the modified field equations are formulated as a coupled dynamical system involving the Herglotz field and the normalized matter density. A phenomenological closure relation is adopted for the effective Herglotz sector, allowing the background evolution to be determined numerically. The model is constrained using Cosmic Chronometer measurements, the DESI DR2 baryon acoustic oscillation compilation, and the Union3 supernova sample, considering both individual and combined datasets within a Bayesian Markov-chain Monte Carlo framework. The combined analysis provides a statistically consistent description of the background expansion. The reconstructed expansion history yields a present-day effective equation of state $w_{\rm eff,0}\simeq-0.702$ and exhibits the expected transition from accelerated expansion at late times toward decelerated expansion at higher redshifts. We further investigate the $Om(z)$ diagnostic and statefinder parameter. These results indicate that the logarithmic Herglotz model provides a consistent phenomenological description of the homogeneous and isotropic late-time expansion history, while allowing departures from the standard $\Lambda$CDM background.

In this work, we investigate the unstable circular orbits of spinning test particles and the associated classical orbital chaos bound in a hairy black hole spacetime admitting a double photon sphere structure. This background supports two distinct branches of unstable circular orbits, referred to as the inner and outer branches. We find that, within the parameter ranges considered, the inner branch always satisfies the chaos bound, whereas violations can occur on the outer branch. This behavior contrasts with many previous studies focusing primarily on near-horizon violations of the chaos bound and reveals a clear dependence of the bound on the orbital branch. Our analysis further shows that the total angular momentum of the particle has a pronounced effect on the local orbital instability and can enlarge the bound-violating region on the outer branch. Meanwhile, the deformation parameter of the background geometry regulates the validity of the bound by modifying both the orbital Lyapunov exponent and the black hole surface gravity. By contrast, the effect of the particle spin is comparatively weak and mainly shifts the existing unstable circular orbits and the corresponding critical boundaries through the spin-curvature coupling.

We present the implementation of a multidimensional general-relativistic Boltzmann solver for neutrino transport using the finite volume method. We extend the general-relativistic magnetohydrodynamics solver Gmunu to discretize the full $6$D phase space. We discretize the momentum space in spherical coordinates in the comoving frame, and the position space in the lab frame in Cartesian, cylindrical or spherical coordinates. As in the M1 scheme, we expand the interaction kernels up to first order in a Legendre series. We present a discretization scheme that ensures consistency between the number-conservative and non-conservative formulations in empty flat spacetime, multiple dimensions and coordinate systems, conserves energy to $\sim 1\%$ with $20$ energy bins in 1D tests. We discuss optimizations of the implicit solver for stiff source terms to minimize its computational cost. In particular, we introduce a method that leverages the Legendre expansion of the kernels to reduce the dimensionality of the problem. In a 1D core-collapse supernova snapshot, our method yields a speed-up factor of $300$ compared to a full-matrix LU method for $14$ angular bins when including energy- and species-coupling interactions. Finally, we validate our implementation on standard test cases and report quadratic convergence in space, energy and propagation angles. We compare our solver to the M1 scheme in simplified 1D configurations and find differences of about $10\%$ in the free-streaming luminosities in relaxation test cases, which we attribute primarily to the M1 closure relation. The average energies, on the other hand, agree between the two methods. We compare both methods on a core-collapse supernova test case and find an overall good agreement in the fluid variables' profiles at the time of core bounce for $20$ energy bins.

Gravitational-wave data is never analysed without some conditioning: filtering and tapering act multiplicatively on the noise, destroying the stationarity on which the Whittle likelihood — the backbone of nearly all gravitational-wave inference — rests. The problem is particularly evident for LISA, where gaps are expected to overlap almost all signals and the noise must be estimated jointly with those astrophysical signals. We provide an analytical and numerical framework to assess the impact of noise mis-modelling when signal and noise parameters are inferred jointly in the presence of gaps. From a Fisher-matrix formalism verified against Bayesian inference, we obtain closed forms for the model uncertainties, their maximum-likelihood estimates and the true scatter in the latter, for various approximations to the likelihood. The Fisher-based linear formalism is extended through the Godambe–White curvature to the non-linear regime that severe mis-modelling produces. We survey gap families from long, well-tapered interruptions to short, high-rate ones, injecting massive black-hole binaries. Except in the case of frequent gaps, we show that the windowed covariance is well approximated by its leading diagonal — the window convolved with the power spectral density — for the estimation of both signal and noise parameters. By omitting the convolution, the signal sector remains unbiased but the noise parameters, in particular the optical-metrology noise which is dominant at high frequency, is displaced by over an order of magnitude in power. At the highest glitch rates the diagonal approximation stays accurate yet loses an order of magnitude in precision. To remedy this, we propose an exact time-domain likelihood of the surviving samples which can be efficiently (and accurately solved) via preconditioned conjugate gradient methods.

We study first-order differential quantities built from solutions of second-order black-hole wave equations. These quantities can satisfy new differential equations that contain extra singular points, even when the original physical solutions remain regular. We show that an isolated zero of the transformation determinant can produce such an apparent singularity, but it does not by itself create a new resonance or quasinormal-mode frequency. We then study the corresponding Green functions and show that the physical transformed response has the same spectral poles as the original problem, provided that the transformation does not become globally degenerate. For the Regge–Wheeler and Zerilli equations, this means that ordinary turning points can appear as singular points of the derivative equation without changing the nonzero-frequency quasinormal-mode spectrum. The zero-frequency case is different and must be studied separately.

We study the gravitational action in the teleparallel equivalent of general relativity, where the spin connection contributes to the action through a surface term and, when chosen appropriately, regularizes the action by effectively playing the role of a counterterm. We resolve several apparent ambiguities in this procedure using the Schwarzschild spacetime as an example. We show that the apparent discrepancy between bulk and quasilocal actions originates from contributions associated with spacetime singularities, and that the quasilocal action agrees with the standard results obtained in general relativity. We further clarify the role of the tetrad choice in determining the corresponding spin connection, showing that the proper diagonal and canonical frames give the same Euclidean action, while only the proper diagonal frame reproduces the expected Lorentzian action on the Wheeler–DeWitt patch. Finally, we establish the uniqueness of the regularization scheme and show that imposing an appropriate fall-off behavior of the torsion tensor removes the remaining ambiguity and fixes the Euclidean action uniquely.

We investigate two late-time cosmological models generated by Kruglov's nonadditive entropy at the apparent horizon. Model I, originally derived by Kruglov, follows the Cai–Kim thermodynamic construction and leads to an implicit modified Friedmann equation, whereas Model II, proposed and derived in this work, is obtained from an entropy-integral prescription and contains an effective vacuum contribution. We formulate both models in terms of the dimensionless Hubble parameter, include radiation, impose exact present-day closure, and evolve the physical branch using analytic derivatives of the implicit Friedmann equations. We constrain the models with DESI DR2 baryon acoustic oscillations and a compressed CMB likelihood, with and without DES-Y5 Type-Ia supernovae. Model I is driven to a markedly lower Hubble constant, $H_0\simeq62.4,\mathrm{km,s^{-1},Mpc^{-1}}$ for the combined data, and remains non-phantom with $w_{\rm DE,0}\simeq-0.814$; it is strongly disfavoured relative to flat $\Lambda$CDM. Model II stays close to $\Lambda$CDM in the standard cosmological parameters and gives $w_{\rm DE,0}\simeq-1.003$, while its two entropy-sector parameters remain strongly degenerate. Its best-fit likelihood is comparable to $\Lambda$CDM, but the additional parameters are not supported by information criteria. These results show that the two thermodynamic realizations of the same nonadditive entropy have sharply different phenomenological consequences at the homogeneous-background level. In summary, the observational plausibility of entropy-based cosmology hinges critically on how the generalized horizon entropy is incorporated into the Friedmann dynamics.

We construct an exact static, magnetically charged solution of four-dimensional Einstein–Maxwell–dilaton theory that generalizes the centered Reissner–Nordström black hole in a Bertotti–Robinson magnetic background. Its pure-background limit is the magnetic EMD deformation of Bertotti–Robinson spacetime. Using the Emparan–Teo solution-generating technique, the charged construction reduces in canonical Weyl coordinates to replacing the Schwarzschild horizon rod by the Reissner–Nordström rod of half-length $\sigma=\sqrt{m^2-e^2}$ and integrating the resulting nonlinear background–rod interaction. This yields a completely explicit solution for arbitrary dilaton coupling $\alpha$, with the expected special limits and numerical verification of the full field equations. The horizon exhibits a coupled mechanical, electromagnetic and intrinsic response to the monopole–background interaction. The residual conical stress is tied to north–south magnetic polarization; increasing the background field produces the ordered reversal scales $B_{\rm pole}<B_{\rm crit}<B_{\rm eq}$, independent of $\alpha$ at fixed seed parameters $(m,e)$, while Gauss–Bonnet identifies the conical stress with the integrated distributional Gaussian-curvature contribution at the singular pole. Along the fixed-$(m,B,\alpha,C)$ near-extremal path considered here, every $\alpha>0$ gives vanishing horizon area and divergent curvature, whereas a correlated near-extremal strong-field limit yields a universal magnetic-reversal law.

In this paper, we discuss the dynamics of test particles in the space-time of spherically symmetric, charged boson stars and their frozen states. These latter objects consist of a de Sitter core and a black hole exterior with a shell of finite thickness interpolating between the two. We discuss charged and uncharged massive particles, find the ISCOs and determine whether collisions of massive particles close to the core of (frozen) boson stars can lead to large center of mass energies. We also discuss massless test particles and find that photons rings as well as light points are possible in the space-time of frozen boson stars. We compare our results with those in the corresponding Reissner-Nordstroem space-time and point out differences.

This study derives charged black string solutions in the presence of a Letelier–Alencar cloud of strings and quintessence within Einstein–Maxwell–$\Lambda$ theory. We study the curvature scalars in order to investigate the asymptotical behavior of these solutions. We calculate the relevant conserved and thermodynamic quantities, confirming their compliance with the first law of thermodynamics. System stability is rigorously analyzed through heat capacity and Gibbs free energy, identifying regimes of local and global equilibrium. Additionally, by interpreting the cosmological constant as thermodynamic pressure within an extended phase space, we establish the validity of the extended first law and derive the corresponding Smarr relation for these solutions. We also investigate the null geodesic structure, deriving the equation for the photon cylinder radius, which generalizes the known result for charged black strings by incorporating the effects of the Letelier–Alencar cloud of strings and quintessence. The numerical analysis of this equation determines the existence and location of photon cylinders as functions of the model parameters.

Black hole first-order phase transitions have been described by several seemingly independent frameworks, including local geometry, global topology, complex analysis, and thermodynamic geometry. While the first three have been unified, thermodynamic geometry has remained outside. We prove that the divergence points of the normalized Ruppeiner curvature scalar $R_N$ coincide exactly with the solutions of $T'(r_h)=0$, where $r_h$ is the horizon radius. These solutions include extremal points (spinodal points) and stationary inflection points (thermodynamic critical points). Thus, the divergence of $R_N$ is a necessary but not sufficient condition for a first-order phase transition. This clarifies the mathematical origin of curvature divergence and why thermodynamic geometry can reliably indicate but not alone confirm phase transitions. Using the local geometric framework as a central framework, we incorporate Ruppeiner geometry into this unified picture; a similar analysis also applies to Weinhold geometry. Consequently, the four frameworks are unified within a single structure based on the local folding of the temperature function. This advances our understanding of the mathematical structure of black hole first-order phase transitions and provides clues for possible extensions to other types of phase transitions.

Deriving emergent cosmology from the discrete geometry of Loop Quantum Gravity remains an open challenge. We solve the two-vertex model exactly, including general non-symmetric configurations, by introducing macroscopic global variables that form a closed Poisson subalgebra. This yields an intrinsic minimal area gap from the Casimir invariants of the subalgebra. In the semiclassical limit, a generalized Friedmann equation emerges incorporating graph anisotropies and quantum geometry corrections, which lead to a Big Bounce. This global formulation opens a coarse-graining route toward statistical descriptions of quantum spacetime.

Accurate and computationally efficient waveform models of compact binary coalescences are a fundamental requirement for gravitational wave data analysis. This work presents differentiable and hardware-accelerated implementations of the machine-learning surrogate models mlgw and mlgw-bns within the JAX ecosystem. The proposed framework enables JIT compilation, vector paralellization, native GPU acceleration, and automatic differentiation, while preserving the original surrogate training infrastructure and its applicability to arbitrary non-precessing waveform approximants. Benchmarks are performed in the binary black hole case with a newly trained surrogate of the SEOBNRv5HM approximant; on CPU, they show speed-ups in waveform evaluation time with respect to the original mlgw implementation that exceed one order of magnitude; further, when exploiting GPU acceleration and large-batch vectorization these reach approximately two orders of magnitude. The benchmarks performed in the case of binary neutron stars with a TEOBResumSPA surrogate model achieve similar gains on GPU. Our JAX-based surrogate modeling framework can be integrated into Bayesian parameter estimation pipelines through GWgpu-jax, an open-source package that connects JAX-compatible gravitational waveform generators to JAX-native sampling algorithms. We demonstrate this capability with a nested-sampling pipeline built upon BlackJax-NS, performing parameter estimation analyses of the GW150914 and GW170817 events with mlgw and mlgw-bns. The analyses require approximately twelve and seventeen minutes, respectively, on a single GPU and yield posterior distributions consistent with those reported by the LIGO-Virgo-KAGRA Collaboration. Beyond nested sampling, JAX automatic differentiation provides efficient gradient and Hessian evaluations of the waveform models, enabling integration with gradient-based Bayesian inference methods.

We constrain a scalar-vector-tensor (SVT) dark-energy model in which the vector sector drives the dark-energy equation of state below $-1$ at earlier times, while a canonical scalar field subsequently drives it across $-1$ toward a present-day value above $-1$. We confront the model with DES Year 5 supernovae, DESI DR2 baryon acoustic oscillations, and compressed cosmic microwave background information, and then include Gold-2017 redshift-space-distortion (RSD) measurements to probe structure growth. The background posterior assigns substantial weight to crossing histories and gives a better best fit than the flat $\Lambda$-cold-dark-matter ($\Lambda$CDM) model. The RSD analysis preserves this qualitative background evolution, constrains the clustering amplitude, and increases the posterior support for the directional crossing criterion. Since the growth analysis also imposes additional perturbative support conditions, this increase cannot be attributed to the RSD likelihood alone. Matched PolyChord calculations give Bayes factors larger than unity for SVT relative to $\Lambda$CDM under both prior-volume conventions considered here. The quantitative Bayes factor depends on the prior normalization assigned to the regular radiation-era branch and is larger in the RSD analysis under either convention. Overall, current expansion and structure-growth data are consistent with a stable dynamical crossing of the phantom divide in the SVT model.

We explore the energy extraction from a Kerr-like regular black hole coupled with a cloud of strings (CoS) via the Comisso-Asenjo mechanism. We study the effects of the black hole spin (a), black hole charge (g) and CoS parameter (b) on the ergo-region of the black hole and the magnetic reconnection process, which is governed by the Comisso-Asenjo mechanism. We also analyze the phase space involving negative energy density of the relativistic magnetized plasma at infinity. The rate of energy extraction in the magnetic reconnection process is also computed and compared with that of the Blandford-Znajek mechanism.