We study the black-hole Meissner effect for extremal Kerr–Bertotti–Robinson (Kerr–BR) black holes, exact Einstein–Maxwell solutions describing a rotating black hole immersed in a uniform Bertotti–Robinson electromagnetic universe. We show that extremality alone is not sufficient for complete magnetic-flux expulsion in this family. For a purely magnetic external BR field, finite extremal holes with $$0<Ba<1$$ carry the exact northern-hemisphere flux $$\Phi _N=-2\pi Ba^2\sqrt{1-B^2a^2}$$ , which is nonzero and maximal in magnitude at $$Ba=1/\sqrt{2}$$ . Complete expulsion occurs only in the static near-horizon boundary limit $$Ba\rightarrow 1^-$$ , equivalently $$k\rightarrow 0$$ , where the $\textit{U}$(1) fibration of the near-horizon metric trivialises. The proof relies on two exact identities that hold at extremality for all values of the external field, $$\Omega _x|_{r_e}=0$$ and $$\Omega _r|_{r_e}=B^2a$$ , both consequences of the double-root structure of the horizon function $$\Delta $$ . Together they force the azimuthal potential $$A_\phi |_{r_e}$$ in the purely magnetic sector to become independent of the polar angle in the static boundary limit, so that the gauge-invariant horizon flux density $$F_{x\phi }|_{r_e}$$ and the flux through any smooth horizon cap vanish. The Kerr–BR result is contrasted with the Kerr–Melvin family, where the static limit occurs at a finite interior field strength, and with the Melvin–Kerr–Newman–Taub–NUT spacetime, where the NUT parameter is known to prevent expulsion. A complementary throat-geometry argument and qualitative implications for Blandford–Znajek jet suppression near the static boundary of the extremal branch are discussed.

Indonesia

Haryanto Siahaan

Meissner effect in Kerr–Bertotti–Robinson spacetime

Strong magnetic fields modify the thermodynamics of white dwarf (WD) matter by quantizing the transverse motion of degenerate electrons into Landau levels and splitting the pressure into components parallel and perpendicular to the magnetic field. We investigate how this microscopic pressure anisotropy affects ultra-dense WD sequences when Coulomb lattice contributions, electron capture thresholds, neutron drip limits, and nuclear composition are treated consistently in the equation of state (EoS). The stellar equilibrium structure is obtained by solving an anisotropic extension of the Tolman–Oppenheimer–Volkoff (TOV) equations, where $$P_r = P_\parallel $$ and $$P_t = P_\perp $$ denote the radial and tangential pressures, respectively. We compare the post-electron-capture branches with their corresponding pre-capture sequences for pure $$^{12}\textrm{C}$$ , $$^{16}\textrm{O}$$ , $$^{20}\textrm{Ne}$$ , and $$^{24}\textrm{Mg}$$ compositions, as well as for fixed 50/50 C/O, Ne/O, and Mg/Ne mixtures. The EoS predicts $$P_\perp < P_\parallel $$ in density ranges where only a few Landau levels are occupied. Consequently, pressure anisotropy shifts the stellar sequences toward smaller radii, with only minor changes in the maximum masses. We also find that post-capture configurations cannot be interpreted from the mass–radius plane alone, since the corresponding $$M(\rho _c)$$ curves reveal portions with $$dM/d\rho _c<0$$ that are flagged by the turning-point diagnostic after the maximum mass is reached. Our results show that strong magnetic fields significantly reduce the radii of low-mass WDs, placing the calculated branches in the same broad mass–radius domain as a selected observational sample, without constituting a specific observational identification or statistical validation, and suggesting that future high-precision observations of massive magnetic WDs could help assess the role of magnetic pressure anisotropy in determining their masses and radii.

Brazil

Edson Otoniel

Juan Pretel

Victor Alves

César Flores

Clésio Mota

Magnetic pressure anisotropy in ultra-dense white dwarfs: Landau quantization and neutronization effects

We develop a fully discrete and non-perturbative realization of two-dimensional Jackiw–Teitelboim (JT) gravity as an $${\textrm{sl}}(2,{\mathbb {R}})$$ BF theory. Using group-valued holonomies and Lie-algebra-valued dilatons, the bulk theory is shown to be purely topological, with all physical information encoded at the boundary. We analyze admissible discrete boundary conditions and derive the corresponding asymptotic symmetry algebras directly at the lattice level, including an affine Kac–Moody symmetry and its Brown–Henneaux reduction to a Virasoro algebra, together with the associated Virasoro–dilaton structure. A precise operator product expansion (OPE) dictionary is established by taking the controlled continuum limit of the discrete Poisson brackets. Beyond asymptotic symmetries, we provide an effective boundary description and a representation-theoretic quantization organized by monodromy sectors. Within this discrete framework, black hole entropy follows from gauge-invariant holonomy data and is expressed in terms of the dilaton Casimir, reproducing the Bekenstein–Hawking result within a framework that does not require an explicit Schwarzian action.

Türkiye

H. Özer

Aytül Filiz

Holonomies and boundary symmetries in discrete Jackiw–Teitelboim gravity

Black holes embedded in a dark matter background represent an important candidate model that extends the standard vacuum case, characterizing the strong gravitational environments at galactic centers and the gravitational influence of dark matter. This study focuses on dark matter halo-modified Kerr black holes and systematically investigates the observational characteristics of their surrounding thin accretion disks. Starting from static black hole solutions with dark matter halos, we employ the Newman–Janis algorithm to construct its rotating counterpart. The dark matter halo is modeled by a general double power-law density profile, whose integrated mass $$M_D(r)$$ modifies the radial mass function of Kerr metric to $$m(r) = M + M_D(r)$$ . Based on this spacetime, we analyze the observational properties of their thin accretion disk, including the radiative flux, temperature, differential luminosity, and spectral luminosity as a function of radius, with respect to the black hole spin and dark matter halo parameters. Furthermore, by combining ray-tracing methods, we simulate the bolometric image of the thin accretion disk under different black hole spins, dark matter halo parameters, and viewing inclinations. The dark matter halo enhances the gravitational potential, shifting the event horizon, innermost stable circular orbit, and photon shell outward, suppressing the peak radiative flux, and producing a fainter image, thereby generating signatures that are possibly observational distinguishable from standard vacuum Kerr black holes.

China

Jia-Ying Zhang

Wei-Lun Tong

Tian-Yun Ye

Yong-You Liu

Zhen Li

Observational signatures of thin accretion disks around rotating black holes embedded in dark matter halos

We derive analytic templates for the scalar and tensor primordial power spectra describing cosmologies that transition from kinetic dominance to slow-roll inflation in the presence of spatial curvature. Our results extend recent works in the literature, allowing us, in particular, to recover the scalar and tensor tilts analytically. We revisit the case of curvature-assisted single-exponential models in light of this framework. In the case of an open universe, the phase space of such models naturally includes cosmologies that start out in a kinetic-dominance regime followed by a parametrically controlled quasi-de Sitter phase. However, they do not fit in the framework of the templates, as their second Hubble slow-roll parameter remains of order one in the quasi-de Sitter regime.

France

Dimitrios Tsimpis

India

Govind Venugopal

Analytic results for slow-roll curved-space inflation and exponential potentials

We present a comprehensive study of the thermodynamic phase structure of Anti-de Sitter black holes in Einstein–Maxwell–Power–Yang–Mills gravity, reformulated through holographic duality as an ensemble problem in the dual conformal field theory (CFT). By deriving an extended first law in which the central charge $\textit{C}$ is treated as a thermodynamic variable, we systematically explore both the canonical and mixed ensembles. In the canonical ensemble with fixed charges, we identify a van der Waals-like phase transition between small and large black holes, characterized by a swallowtail structure and coexistence curves with a negative slope. In contrast, within the mixed ensemble at fixed electric potential, the system exhibits a Hawking–Page transition between confined and deconfined phases of the boundary CFT. Our key finding is the suppressive role of the non-Abelian Yang–Mills charge $$\tilde{q}$$ : increasing $$\tilde{q}$$ lowers both the minimum temperature and the Hawking–Page transition temperature, thereby significantly narrowing the stability window of the confined phase. Supported by detailed numerical analysis, these results reveal a rich, ensemble-dependent phase landscape and establish the nonlinear Yang–Mills sector as a critical controller of confinement physics in strongly coupled holographic systems.

Iran

Azerbaijan

Mohammad Alipour

Mohammad Afshar

Saeed Gashti

Behnam Pourhassan

Holographic CFT phase transitions and criticality for Einstein–Maxwell–Power–Yang–Mills AdS black holes

We study effective reheating in a scalar–Gauss–Bonnet inflationary model with a phase-space-dependent coupling $$\mu (\phi ,X),$$ in which a compact field-space feature is combined with a bounded kinetic gate. The modified inflationary background determines the pivot-scale quantities and the effective energy density at the end of inflation. These quantities are then used to derive the reheating duration $$N_{\textrm{re}}$$ and temperature $$T_{\textrm{re}}$$ through the thermal-history matching relation. We first perform two fixed-pivot reference scans by varying the overall Gauss–Bonnet strength $$\lambda _{\textrm{GB}}$$ and the kinetic-gate amplitude $$g_{_X}$$ separately. For the reference parameter choices, increasing either parameter increases $$N_{\textrm{re}}$$ and decreases $$T_{\textrm{re}}$$ for the selected reheating equations of state. Additional benchmark calculations clarify how these variations depend on the dynamical regime of the model. In the $$\lambda _{\textrm{GB}}$$ scan, the increase in $$N_{\textrm{re}}$$ and the decrease in $$T_{\textrm{re}}$$ persist, although both variations become strongly suppressed when the coupling is more localized or when the end of inflation is controlled more strongly by the E-model potential. In the $$g_{_X}$$ scan, stronger field-space localization and kinetic saturation can instead lead to a slight decrease in $$N_{\textrm{re}}$$ and an increase in $$T_{\textrm{re}}$$ as $$g_{_X}$$ is increased. When the bounded kinetic contribution is considered together with a weaker overall Gauss–Bonnet interaction, the resulting changes in the reheating quantities become nearly negligible. The fixed-pivot predictions of the representative and alternative benchmarks are compared with CMB constraints. These reheating constraints are then discussed for four representative values of the effective equation-of-state parameter, $$\overline{w}_{\textrm{re}}=-1/3,0,2/3,$$ and 1. Finally, the analysis is generalized to a time-dependent reheating equation of state $$w_{\textrm{re}}(N),$$ showing that the thermal-history matching relations depend only on its e-fold average $$\overline{w}_{\textrm{re}},$$ while the detailed evolution of $$w_{\textrm{re}}(N)$$ remains degenerate within the effective framework adopted here.

Ali Seidabadi

Sara Saghafi

Kourosh Nozari

Effective reheating in Gauss–Bonnet inflation with $$\mu (\phi ,X)$$ coupling

Within general relativity, we study a static, horizonless, asymptotically flat two throat geometry constructed as an inverse problem: the metric is prescribed and the anisotropic stress tensor required by the Einstein equations is reconstructed, without assuming a specific matter Lagrangian. A finite mass deformation of an earlier quartic embedding profile produces two symmetric throats and an intermediate equator. The deformation fixes the same finite geometrized mass at both spatial infinities, while the lapse uses the same mass parameter as an asymptotic consistency condition. The complete radial averaged null functional is related explicitly to the affinely parametrized ANEC integral, obtained exactly, and shown to be strictly negative. We classify the global null potential for all positive masses and nonnegative redshift deformation. Depending on the parameters, the unstable set consists of throat rings, four off throat rings, or exterior rings accompanied by a subcritical equatorial ring. In the four ring phase, all unstable rings share one critical impact scale but the inner and outer branches have different Lyapunov exponents. For nondegenerate critical rings, we derive the logarithmic strong deflection coefficients analytically; the cross throat coefficient is the sum of the local contributions from every global maximum traversed by the ray, as verified by direct numerical integration. The minimally coupled massless scalar exhibits phase dependent barriers and resonant transmission as a test field propagation diagnostic. The model therefore isolates finite mass effects on averaged energy conditions, global light rings, branch resolved strong lensing, and scalar transmission within a single prescribed geometry.

Anirudh Pradhan

K. Ghaderi

Saudi Arabia

M. Zeyauddin

K. Karimizadeh

Finite mass two throat wormholes: global light rings, branch resolved strong lensing, and scalar transmission

We investigate the extent to which pseudo-Newtonian gravitational potentials can reproduce classic tests of general relativity without resorting to full general relativistic formalisms. This is useful for the researchers seeking intuitive insight into relativistic gravity. Focusing on the perihelion precession of Mercury, gravitational redshift, and gravitational light bending, we derive analytical expressions for orbital precession and demonstrate that, with suitable physically acceptable parameters, pseudo-Newtonian approaches can accurately reproduce the observed perihelion advance and gravitational redshift. However, we confirm that no single potential consistently captures all relativistic effects. In particular, while certain parameters yield agreement with general relativity for planetary motion and redshift, they fail to reproduce gravitational lensing over a broad range of impact parameters. Our results highlight both the usefulness and limitations of pseudo-Newtonian methods in modeling gravitational phenomena. Although the pseudo-Newtonian approach cannot serve as universal substitutes for general relativity, especially in strong-field regimes, it provides valuable semi-analytical insight and pedagogical simplicity. Our results indicate the usefulness of the pseudo-Newtonian approach to uncover more complicated phenomena involved with strong field gravity in possibly modification to general relativity.

Naman Goyal

Banibrata Mukhopadhyay

Ashish Meena

Tests of general relativity in pseudo-Newtonian approach

Searches for new physics in low-background experiments infer a non-negative signal strength from few events and often report an upper limit. Nominal frequentist coverage requires both a valid interval construction and an adequate data model. We study how controlled model departures affect lower- and upper-endpoint coverage for six interval procedures in an exact Poisson counting experiment, dark-matter recoil spectra, and a neutrinoless-double-beta-decay peak search. We introduce the Poisson–Fisher degeneracy index $$\mathcal I_{\textrm{PF}}(\delta \nu ;\vartheta _0)=(\beta ,\gamma )$$ , which maps a specified expected-count deformation, after projection onto the complete fitted tangent space, to $$\beta $$ , the signed fitted signal shift in profiled standard-error units, and $$\gamma $$ , the Poisson–Fisher norm of the unabsorbed residual. Locally, the sign of $$\beta $$ identifies the threatened endpoint, while $$\gamma $$ provides a local indication of detectability by the saturated-Poisson goodness-of-fit test used here at a $$5\%$$ nominal type-I error rate. Across the studied deformations, positive signal-like bias degrades discovery-side coverage while making upper limits conservative; negative signal bias from overestimated signal efficiency can make the upper endpoint undercover. Calibration under the nominal simulator does not protect against misspecification of that simulator relative to the data-generating process. A plausible deformation with large $$|\beta |$$ and small $$\gamma $$ may therefore evade diagnosis and should be represented by a nuisance constrained with auxiliary information or included in a defensible envelope. In the exactly collinear constrained-nuisance benchmark, modelling the deformation restores coverage at the evaluated grid points, at a quantifiable cost in interval sensitivity.

Italy

Davide Pagano

Silent coverage failures in rare-event searches and a degeneracy index that predicts them