arXiv subjects

İzzet Sakallı

Publications and source records attributed to İzzet Sakallı.

At least 19 recordsLinked to original sources

arXiv2026

Simpson-Visser regularization of the quantum Oppenheimer-Snyder-Datt black hole

We study the spherical geometry from the Simpson-Visser prescription $r\to\sqrt{r^2+a^2}$ to the quantum Oppenheimer-Snyder-Datt (qOSD) black hole of loop quantum cosmology, whose lapse function $A=1-2M/R+αM^2/R^4$ carries the correction $α$. This family interpolates between the qOSD black hole ($a\to0$), the Simpson-Visser black bounce ($α\to0$), and Schwarzschild. The geometry is regular for every $a>0$, the Ricci scalar at the throat is blind to $α$ while the Kretschmann scalar is not, and the null energy condition is violated at the throat by $-1/(4πa^2)$. The $(α,a)$ plane splits into a two-horizon regular black hole, a single-horizon black hole with a hidden bounce, no Cauchy horizon, and a traversable wormhole; the middle region removes the mass-inflation channel of regular black holes without fine tuning. Because circular orbits are governed by the areal radius, photon sphere, shadow, ISCO and azimuthal epicyclic frequency depend on $α$ only, whereas the Lyapunov exponent and radial epicyclic frequency carry the factor $\sqrt{1-a^2/R^2}$: shadow constrains only $α$, while ringdown damping and the twin-peak quasiperiodic-oscillation ratio are sensitive to $a$. We derive horizon thermodynamics with a first-law entropy that departs from the area law, show that the bounce changes the thermodynamic topological charge from $W=-1$ to $W=0$ at every $α$, and compute scalar, electromagnetic and Dirac quasinormal spectra using third-order WKB, Chebyshev spectral collocation and time-domain integration, which agree with one another and with eikonal scaling. Greybody bounds, Hawking emission, cascade sparsity and ringdown quality factor complete the analysis, and in the wormhole region the double-peaked potential produces echoes.

gr-qc↗

arXiv2026

Modified Black Hole Physics in Quantum Gravity with Quintessence and Topological Defects

We study a static, spherically symmetric black hole (BH) of the quantum Oppenheimer-Snyder (QOS) type surrounded by a quintessence field (QF) and a cloud of strings (CS). Because the line element has the mass-function form, the Einstein tensor is linear in the mass function, and the three contributions enter the effective field equations additively with no cross terms. For the state parameter $w=-2/3$ adopted throughout, the geometry is not asymptotically flat. The horizon condition is a quintic in $r$ whose real positive roots are an inner horizon, the BH horizon, and an outer quintessence horizon, and the static region is bounded by the last two. Working inside that region, we obtain the photon sphere, the critical impact parameter, and the shadow radius seen by a static observer at finite radius, together with null and timelike geodesics, the Lyapunov exponent of circular null orbits, the orbital speed, and the geodetic precession frequency. For scalar, electromagnetic (EM), and Dirac test fields we derive the master equations and establish mode stability, using an S-deformation for the scalar sector and the supersymmetric factorization of the Dirac pair. The associated quasinormal mode (QNM) frequencies are computed at third-order WKB order and benchmarked against Schwarzschild. Damping weakens as the CS parameter and the QF normalization grow, whereas the loop quantum gravity (LQG) deformation acts only weakly. On the thermodynamic side we fix the surface-gravity prescription appropriate to a spacetime with two Killing horizons. The Hawking temperature stays non-negative over the whole admissible branch and vanishes at the degenerate configuration in which the BH horizon merges with the quintessence horizon, so no negative-temperature phase arises. Specific heat and Gibbs free energy are re-examined on that branch.

gr-qc↗

arXiv2026

Quantum-Corrected Black Holes in Higher Dimensions: From Gravitational Collapse to Remnants, WGC-Like Behavior and Accretion Signatures

We study the gravitational collapse of a homogeneous dust sphere in higher-dimensional spacetime, with the loop quantum gravity correction entered through a modified Friedmann equation. Its most striking feature is an endpoint: at a finite horizon radius the temperature vanishes and evaporation stops, leaving a remnant. Reading $α$ as an effective charge, we find behavior reminiscent of the Weak Gravity Conjecture (WGC), although no $U(1)$ gauge field is present in the model. Solving the degeneracy conditions with the untruncated metric function, rather than with a mass expansion that is not uniform near the endpoint, we obtain closed-form expressions for the remnant radius and mass in $D=4,5,6,7$. The unscaled ratio $α/M_{\text{remnant}}^2$ is constant only in four dimensions and grows as $α^{4-D}$ elsewhere, but it carries dimension $L^{8-2D}$, so that growth is an artifact of the units. The dimensionless combination $\tilde{\mathcal{R}} = (α/M^2) r_0^{2D-8}$ settles instead on a finite $α$-independent number in every dimension, decreasing from $27/64$ in four dimensions to $13824/(15625π^4)$ in seven. We also deform the quantum parameter, $α\to α+ \varepsilon$, and evaluate $-T(\partial S/\partial \varepsilon)_M$ in the remnant limit. Because $(\partial M/\partial r)_α$ vanishes there, the fixed-entropy derivative reduces exactly to a fixed-radius one, and the resulting combination $\mathcal{U}$ is finite in each dimension but depends on $\varepsilon$ except for $D=5$. What is invariant is the product $\mathcal{R}\,\mathcal{U}\,M_{\text{remnant}} = (D-3)/2$, which we verify numerically. Lastly, we study Bondi accretion onto the quantum-corrected black hole and obtain the Eddington luminosity for each dimension.

hep-th↗

arXiv2026

Stationary scalar clouds, quasinormal ringing, and observational signatures of (near-)extremal rotating Kalb-Ramond black holes: from the superradiant threshold to EHT bounds

We study a massive scalar field on the rotating Kalb-Ramond black hole of Kumar, Ghosh and Wang, in the (near-)extremal corner of its three-parameter family $(a,γ,λ)$. We verify by direct substitution that the Klein-Gordon-Fock equation separates on this background, with radial and angular separation constants related by $Λ_{rad}=Λ_{ang}+a^2ω^2-2amω$. At exact extremality the horizon polynomial and the function $K(r)$ share a zero, rendering $K^2/Δ$ regular there and reducing the radial problem to Whittaker form on the Kerr line $(γ=0)$ and the Kerr-Newman line $(λ=1)$. The stationary resonances saturate $ω=mΩ_H$ and satisfy the bound-state condition $μ>mΩ_H$, with overtones accumulating at the threshold from above; in the genuinely Kalb-Ramond interior we solve the exact radial equation as a two-point boundary value problem. Moving off extremality, the clouds continue onto the co-rotating zero-damped branch $ω_n=mΩ_H-iκ(n+1/2+β)$, governed by the same near-horizon exponent $β$; the photon-sphere family is a distinct observable. Greybody factors and superradiant amplification are obtained by direct integration, recovering the classical scalar amplification on Kerr. Strong-field lensing is treated in the Bozza-Tsukamoto limit with rotating photon orbits of both senses, where the prograde logarithmic coefficient departs strongly from its Schwarzschild value. The shadow contour follows from the Carter-Hamilton-Jacobi separation. Crossing the Event Horizon Telescope window of Zubair, Raza and Maqsood with the cloud-existence region yields a narrower bound on the Lorentz-violating parameters $(γ,λ)$ than either constraint alone.

gr-qc↗

arXiv2026

Quantum-Corrected Thermodynamics, Dirac Perturbations, Geodesic Structure, and Topological Phases of Black Holes with Non-Minimal Logarithmic Coupling

We study the thermodynamic and dynamical properties of static, spherically symmetric BHs in Einstein-Maxwell theory modified by a non-minimal $\ln(R)F^{2}$ coupling. The Hawking temperature follows from the Hamilton-Jacobi form of the fermionic tunnelling method for spin-$\tfrac12$ particles, and it carries scale-dependent logarithmic corrections. We then analyze the propagation of massless Dirac fields, compute the quasinormal-mode (QNM) spectrum with the third-order WKB approximation, and read off a quality factor whose balance between oscillation and damping depends on the logarithmic coupling in a mode-dependent way. Moving outward from the horizon, we work out the transmission of the fermionic field and its Hawking emission, and we solve the null and timelike geodesic problems to obtain the photon sphere, the shadow radius, the innermost stable circular orbit (ISCO), the associated zoom-whirl bound orbits, and the orbital and epicyclic frequencies that set the twin-peak quasiperiodic-oscillation (QPO) ratio. A photon-sphere reading of the eikonal QNM frequencies ties the geodesic sector back to the field perturbations. On the thermodynamic side, we build the phase space with quantum-geometric corrections through the Barrow entropy, and we characterize the global phase structure with the topological method, where the winding numbers of the off-shell free energy are governed by the interplay of the fractal Barrow deformation, the electric charge, and the logarithmic coupling. We find that the effective pressure vanishes exactly on the topological defect line, which links the pressure sign to the local stability of each branch.

gr-qc↗

arXiv2026

Systematic Light Propagation Bias from the Heliosphere and Its Impact on the Hubble Tension

The Hubble tension (HT) represents one of the most significant discrepancies in modern cosmology, with local distance measurements yielding $H_0 = 73.5 \pm 1.4$ km/s/Mpc while cosmic microwave background (CMB) observations predict $H_0 = 67.4 \pm 0.5$ km/s/Mpc. We propose that this tension arises from systematic effects introduced by our Solar System's heliospheric (HS) environment on local distance measurements. The HS creates a complex medium of heated plasma and energetic neutral atoms (ENAs) beyond the heliopause (HP), where interstellar medium (ISM) temperatures rise significantly. This thermal gradient and particle environment may systematically affect observations of Cepheid variables and Type Ia supernovae (SNe~Ia) used in the local cosmic distance ladder (CDL), biasing distance measurements and artificially inflating the measured Hubble constant. We present theoretical calculations showing how HS effects could account for up to $\sim 8\%$ of the observed 8--9\% discrepancy, with the realistic contribution lying in the $\sim 3$--$8\%$ range once anisotropy, partial calibration cancellation, and chromatic suppression are included, and discuss observational tests to validate this hypothesis.

gr-qc↗

arXiv2026

Where Thermodynamics Meets Geometry: Critical-Radius Coincidences in Confining-NED Black Holes with Barrow Entropy

We study a static, spherically symmetric black hole obtained from Einstein gravity coupled to a nonlinear electrodynamics model with a quark--antiquark confinement interaction. The metric extends Reissner--Nordström by a logarithmic correction controlled by $ζ$, modifying both horizon structure and the near-singularity regime. The Hamilton--Jacobi tunneling method for Dirac fermions yields the Hawking temperature; the $ζ$-dependent terms suppress the small-horizon divergence and signal a remnant. Quantum-gravitational fluctuations are incorporated through Barrow entropy with deformation index $Δ$. Within the extended phase space we compute the internal energy, free energy, pressure, heat capacity, isothermal compressibility, and Joule--Thomson coefficient. The heat capacity locates $Δ$-dependent stability regions; the compressibility stays negative across the domain analysed here, marking a mechanically rigid phase with no van der Waals criticality in this branch. The central result is a quadruple coincidence: the peak Hawking temperature, the heat-capacity divergence, the Joule--Thomson inversion, and the zero of the radial tidal force all sit at one radius $r_\star$ defined by $A''(r_\star)=0$, while the extremal horizon and the angular tidal-force zero coincide via $A'(r_h)=0$. These reduce the full critical-point analysis to two scalar equations on $A(r)$. Geometric tidal accelerations are mapped against the thermodynamic critical curves. Event Horizon Telescope observations of Sgr~A* translate into a constraint $ζ\lesssim 0.7$ at $Q/M=0.5$, leaving a finite window open. The confinement term induces observable corrections to geodesic deviation.

gr-qc↗

arXiv2026

CFT Constraints on the Weak Gravity Conjecture

The Weak Gravity Conjecture (WGC) is a swampland criterion of long standing: any consistent theory of quantum gravity must contain a charged particle whose charge-to-mass ratio exceeds that of an extremal black hole, so that gravity remains the weakest force. The AdS/CFT correspondence offers a calculable boundary handle on bulk gravity, and the imaginary parts of bulk quasinormal modes are read off the boundary as poles of a retarded Green's function. We show that the WGC follows from this boundary calculation in two settings that fall outside the Reissner--Nordström idealisation: static spherically symmetric black holes in dRGT massive gravity, and dyonic black holes in Einstein--ModMax non-linear electrodynamics. The chain runs from the metric and gauge field, through the charged Klein--Gordon equation, into a near-horizon scaling limit whose radial equation reduces to Whittaker form; the conformal weight $ν_0$ then enters a damping-time inequality. For the dRGT black hole every massive-gravity parameter ($α,β,m_g,h$) cancels out, leaving the universal saturation $q/(m r_+) \geq 1/\sqrt{2} \approx 0.707$. For the Einstein--ModMax black hole the duality-symmetric non-linearity parameter $γ$ survives, and yields $q/(m r_+) \geq e^{-γ/2}$, which reduces to the Reissner--Nordström bound $q/(m r_+) \geq 1$ in the Maxwell limit $γ\to 0$. Either result is of order unity, and the second weakens monotonically as the non-linearity grows. We then relax three of the simplifying assumptions of the dRGT derivation, namely exact extremality, minimal coupling, and the absence of higher-curvature terms. The cancellation breaks. Each correction reintroduces $m_g,α,β$ into the bound through a controlled functional dependence, and we tabulate and plot the relaxed forms across parameter space.

hep-th↗

arXiv2026

Renormalization-group improved Schwarzschild black hole: shadow, ringdown, and strong cosmic censorship

A renormalization-group (RG) improved Schwarzschild-like black hole (BH) is studied here, with a lapse that interpolates between a classical Schwarzschild exterior and a quantum-smoothed interior set by a cutoff scale $ξ$ and an interpolation parameter $γ$. We work out the horizon structure together with the photon sphere and shadow radius $R_{\mathrm{sh}}$, set up the scalar, electromagnetic, and Dirac Regge-Wheeler-Zerilli problems in a single treatment, and compute the fundamental and overtone quasinormal modes by sixth-order WKB, cross-checked against time-domain ringdown. For $ξ>0$ and $γ>0$ the geometry is regular, with a de Sitter core. Strong Cosmic Censorship (SCC) is examined at the inner Cauchy horizon, which the improved geometry generates without charge or rotation. The quasinormal spectral gap $β=|\mathrm{Im}\,ω|/κ_-$ stays multipole-independent at the $6\%$ level and follows $β\simeqλ_{L}/(2κ_{-})$. It remains below the de Sitter Christodoulou bound across the parameter range, and the asymptotically flat late-time tail places the geometry in the SCC-respecting class. A thermodynamic analysis identifies a Davies-type phase transition of the outer horizon, with the Schwarzschild $T_{H}\propto 1/r_{+}$ decay replaced by a bell-curve profile peaking at $T_{H}^{\max}\simeq 0.062$. A scan of the $(ξ,γ)$ plane gathers the joint behavior of the shadow, the scalar barrier, the SCC ratio, and $T_H$. Set against Bardeen, Hayward, and Bonanno-Reuter BHs at matched perturbation scale, the improved Schwarzschild BH is the most Schwarzschild-like of the regular-BH family, its static shadow radius degenerate with Hayward and Bonanno-Reuter at the percent level. The closing analysis takes up the sparsity of the Hawking flux and the energy-emission rate, both tied to the outer-horizon surface gravity through a single auxiliary function.

gr-qc↗

arXiv2026

MCMC Constraints on Dyonic Kalb-Ramond Black Holes with a Cloud of Strings from Twin-Peak QPOs and EHT Shadows

We study a dyonic black hole in a Lorentz-violating gravity that carries a background Kalb--Ramond field and is pierced by a cloud of strings. The resulting metric reduces to the recent Lin--Liu--Liu solution when the string density~$ξ$ is switched off, and to the Duan and Yang solutions in further degenerate limits. We work out the timelike circular geodesics and read off the quasi-periodic oscillation (QPO) frequencies $ν_ϕ$, $ν_r$ and $ν_θ$ within both the relativistic-precession and epicyclic-resonance models. We then map these frequencies onto the observed twin-peak signals of XTE~J1550$-$564, GRO~J1655$-$40 and GRS~1915$+$105, and place constraints on $(\ell, ξ)$ from a Markov chain Monte Carlo (MCMC) fit. We extract the full thermodynamic dictionary, first law and Smarr relation included, and follow the heat capacity, free energy and sparsity of Hawking radiation through their dependence on the four parameters $(M, Q, p, \ell, ξ)$. Finally, we compute the spectral energy emission rate and look at the photon-sphere and shadow radii in the presence of the cosmic string. The Lorentz-violating coupling $\ell$, the magnetic charge $p$, and the string density $ξ$ all leave distinct fingerprints on the dynamical, thermodynamic and radiative observables, with $ξ$ exerting the strongest pull on the ISCO, the shadow size and the sparsity of Hawking emission

gr-qc↗

arXiv2026

Are Petrov type-N and D spacetimes admitting CTCs valid in $f(R,\mathcal{L}_m,Φ,X)$ gravity?

We ask whether two classical time-machine geometries, the Ori (2005) compact-vacuum-core metric and the Ahmed (2018) four-dimensional generalisation of Misner space, remain admissible exact solutions when the gravitational sector is enlarged to the recently proposed $f(R,\mathcal{L}_{m},Φ,X)$ class, an extension of $f(R,\mathcal{L}_{m})$ that couples curvature, the matter Lagrangian density, a scalar field $Φ$, and its kinetic invariant $X = g^{μν}\nabla_μΦ\nabla_νΦ$. Working with the explicit model $f = R + \mathcal{L}_{m} + (λ/2)\,X$ and a vanishing scalar potential, we compute the curvature invariants, the modified field equations, and the effective stress-energy components produced by the harmonic scalar profile $Φ(x,y) = a(x^{2}-y^{2})/2$ in both backgrounds. The Ricci scalar vanishes for the Ori metric and obeys $R = e^{f}(f_{,xx}+f_{,yy})$ for the Ahmed metric; the kinetic invariant takes the explicit forms $X = a^{2}(x^{2}+y^{2})$ and $X = a^{2}e^{f}(x^{2}+y^{2})$, respectively. Both metrics solve the field equations of the modified theory with anisotropic matter sources, and the chronology-violating regions $g_{zz}<0$ (Ori) and $g_{ψψ}<0$ (Ahmed) survive the modification. Energy-density profiles measured by a closed-timelike-curve observer match those measured by a static observer outside the chronology horizon, so the additional scalar degree of freedom in $f(R,\mathcal{L}_{m},Φ,X)$ gravity does not enforce a chronology-protection mechanism in either background. The conclusion mirrors the parallel result for the Li time-machine and supplies a consistency test for scalar-extended modified gravity in non-globally-hyperbolic settings.

gr-qc↗

arXiv2026

Greybody Factor, Resonant Frequencies, and Entropy Quantization of Charged Scalar Fields in the Kerr-EMDA Black Hole

We study charged massive scalar field perturbations on the rotating black hole (BH) background of Einstein-Maxwell-Dilaton-Axion (EMDA) theory, known as the Kerr-EMDA BH. Starting from the gauge-covariant Klein-Gordon equation (KGE), we perform a full separation of variables and obtain exact analytical solutions for both the angular and radial parts in terms of confluent Heun functions (CHFs). Unlike the earlier neutral scalar treatment by Senjaya and Ponglertsakul [Eur. Phys. J. C \textbf{85}, 352 (2025)], the electromagnetic coupling $q$ fundamentally alters the structure of the Heun parameters and produces qualitatively new physics. Applying the CHF polynomial condition, we derive the resonant frequency spectrum whose imaginary parts are equispaced with $|Δω_I| = 1/(2M)$, a universal spacing determined solely by the BH mass. Via the Maggiore prescription and the first law of BH thermodynamics, this yields a parameter-dependent entropy quantum $δS_{\text{BH}} = 4πr_+/(r_+ - r_-)$, which reduces to $4π$ for Schwarzschild but diverges at extremality -- {\color{black}in contrast to the universal $2π$ obtained for the rotating linear dilaton BH (RLDBH).} We construct the effective potential governing scalar wave scattering and analyze its dependence on the dilaton parameter $D$, rotation $a$, and scalar charge $q$. In the massless uncharged limit, the CHF reduces to the Gauss hypergeometric function, {\color{black}enabling us to compute the first analytical greybody factor (GF) for the Kerr-EMDA geometry; we show that this reduction extends to massless charged scalars, yielding a closed-form GF that captures superradiant amplification.} We examine how the dilaton deformation distinguishes the Kerr-EMDA spectrum from the standard Kerr and Kerr-Newman cases.

gr-qc↗

arXiv2026

Particle Dynamics, Shadow and Hawking Sparsity of a Kalb-Ramond Black Hole Coupled to Nonlinear Electrodynamics

We study the timelike and null geodesic structure of a static, spherically symmetric black hole sourced by a Kalb--Ramond (KR) field coupled to nonlinear electrodynamics (NED). The geometry is characterized by the mass $M$, the magnetic monopole charge $q$, and the Lorentz-violating parameters $(γ,λ)$. Closed-form expressions are derived for the effective potential, as well as the specific energy and angular momentum of massive particles on circular orbits. We further analyze the photon sphere, black hole shadow, and the Lyapunov exponent associated with unstable null circular geodesics. The latter determines the eikonal quasinormal-mode frequencies through $ω_{\rm eik}=(\ell+1/2)\,Ω_c-i(n+1/2)\,|λ_L|$. The shadow radius is compared with the Event Horizon Telescope (EHT) observations of M87$^\ast$ and Sgr~A$^\ast$, allowing us to identify the viable region in the $(q,γ)$ parameter space. Finally, we compute the Hawking temperature, horizon area, and the Gray--Visser sparsity parameter. We demonstrate that the combined effects of the KR field and magnetic monopole charge increase the sparsity parameter from the Schwarzschild value $16π^3 \simeq 496$ to nearly $1.7\times10^3$. This indicates a significantly sparser Hawking cascade compared to the Schwarzschild case, while the photon ring remains consistent with the EHT $1σ$ observational bounds across most of the physically allowed parameter range.

gr-qc↗

arXiv2026

Quantum-Corrected Thermodynamics of Conformal Weyl Gravity Black Holes: GUP Effects and Phase Transitions

We investigate the thermodynamic properties of black holes in Conformal Weyl Gravity (CWG) using the Mannheim-Kazanas solution, with particular emphasis on quantum corrections that become significant near the Planck scale. Our analysis employs the Hamilton-Jacobi tunneling formalism to derive the Hawking temperature, revealing explicit contributions from the conformal parameters $β$, $γ$, and $k$ that lead to substantial deviations from the behavior of a Schwarzschild black hole. We incorporate quantum gravitational effects through the Generalized Uncertainty Principle, demonstrating systematic suppression of thermal radiation in the near-Planckian regime. Using an exponentially corrected entropy model, we compute the complete spectrum of QC thermodynamic potentials, including internal energy, pressure, heat capacity, and free energies. Our heat capacity analysis shows divergence behavior that separates stable and unstable regions, indicating possible thermodynamic transitions controlled by the scale-dependent parameter $γ$. The Joule-Thomson expansion analysis shows distinct cooling and heating regimes with inversion points that shift systematically with CWG parameters, capturing QC phase transitions absent in general relativity. We also examine gravitational redshift in CWG geometry, finding complex radial dependence that highlights modifications compared to the Schwarzschild case, although redshift alone cannot observationally distinguish CWG from Einstein's theory. Our results demonstrate that CWG offers a consistent framework for studying black hole thermodynamics beyond general relativity, with quantum corrections modifying phase structures in the near-Planckian regime, though these effects are not expected to yield direct observational consequences.

gr-qc↗

arXiv2026

Thermal and Optical Signatures of Einstein-Dyonic ModMax Black Holes with GUP and Plasma Modifications

We explore the thermodynamic and optical properties of Einstein-Dyonic-ModMax (EDM) black holes (BHs), incorporating quantum gravity corrections and plasma effects. The ModMax theory promotes the classical Maxwell theory to a non-linear electrodynamics with a larger symmetry structure (electromagnetic duality plus conformal invariance), and provides dyonic BH solutions characterized by both electric and magnetic charges modulated by the nonlinearity parameter $γ$. Using the Hamilton-Jacobi tunneling formalism, we derive the Hawking radiation spectrum and demonstrate how the Generalized Uncertainty Principle (GUP) modifies the thermal emission, potentially leading to stable remnants. Our analysis of gravitational lensing employs the Gauss-Bonnet theorem to compute light deflection angles in both vacuum and plasma environments, revealing strong dependencies on the ModMax parameter and plasma density. We extend this to axion-plasmon environments, uncovering frequency-dependent modifications that could serve as dark matter signatures. The photon motion analysis in plasma media shows how the exponential damping term $e^{-γ}$ affects electromagnetic backreaction on spacetime geometry. We compute quantum-corrected thermodynamic quantities, including internal energy, Helmholtz free energy, pressure, and heat capacity, using exponentially modified entropy models. The heat capacity exhibits second-order phase transitions with critical points shifting as functions of $γ$, indicating rich thermodynamic phase structures. The energy condition analysis shows that classical ModMax electrodynamics satisfies the null and weak energy conditions, while the observed near-horizon violations arise only after incorporating quantum-corrected entropy effects.

gr-qc↗

arXiv2026

Astrophysical Signatures of Einstein-Skyrme Anti-de Sitter Black Holes: Epicyclic Frequencies and QPO Constraints

We study the geodesic motion and epicyclic oscillations of massive test particles around a static, spherically symmetric black hole (BH) solution of the Einstein--Skyrme (ES) theory in Anti-de Sitter (AdS) spacetime. The lapse function of this BH depends on the Skyrme coupling $η$, a charge-like parameter $Q$ inherited from the Skyrme term, and the cosmological constant $Λ<0$. We first map out the horizon structure and identify three regimes-non-extremal BH (NEBH), extremal BH (EBH), and naked BH (NBH)-showing that the NEBH $\to$ EBH $\to$ NBH transition is governed by $Q$ rather than $η$, which enters $f(r)$ only as a constant shift. We then derive the effective potential (EP), locate the innermost stable circular orbit (ISCO), and compute the radiative efficiency, finding that $\mathcal{E}_{\rm ISCO}>1$ in AdS renders the standard Novikov-Thorne formula negative. The corrected radial epicyclic frequency $Ω_r$ reveals a distinctive AdS signature: $ν_r$ grows at large $r$ and overtakes the orbital frequency $ν_ϕ$, causing the periastron precession frequency $ν_p = ν_φ- ν_r$ to change sign-a feature absent in asymptotically flat geometries. Adopting the relativistic precession (RP) model for quasi-periodic oscillations (QPOs), we perform a Markov chain Monte Carlo (MCMC) analysis using twin-peak QPO data from XTE~J1550-564, GRO~J1655-40, Sgr~A$^*$, and M82~X-1. The posteriors converge to $Q\approx 0.6$ across all sources, with orbital radii near $r\approx 4.2\,M$ and masses consistent with independent estimates, demonstrating that the ES-AdS BH accommodates the observed frequency pairs within physically motivated parameter ranges.

gr-qc↗

arXiv2026

Spin-($0$, $1$, $\frac{1}{2}$) Field Perturbations, Quasinormal Modes, Overtones, Greybody Factors and Strong Cosmic Censorship of Einstein-Skyrme Black Holes

We carry out a multi-spin perturbation-theory study of the four-dimensional Einstein-Skyrme (ES) anti-de Sitter (AdS) black hole (BH), whose lapse $f(r)=1-8πK-2M/r+4πKλ/r^{2}$ inherits two couplings from the hadronic model -- the pion combination $K=F_π^{2}/4$ and the Skyrme coupling $e$ -- with $Kλ=1/e^{2}$ pinned by the theory rather than being a free integration constant. After deriving the Klein-Gordon, Maxwell and Dirac effective potentials on this background, we compute the quasinormal modes (QNMs) with the sixth-order WKB formula and cross-check them against the thirteenth-order Padé-improved expansion and the eikonal limit set by the unstable photon sphere. The first overtone $(n=1)$ of the scalar and electromagnetic channels reveals a mild Konoplya-Zhidenko anomaly: the ratio $|\mathrm{Im}\,ω_{1}|/|\mathrm{Im}\,ω_{0}|$ drifts monotonically from $2.42$ to $2.54$, sitting noticeably below the Schwarzschild value near $3$. The dominant scalar mode is independently reproduced to better than $0.2\%$ by a time-domain Prony fit. Greybody factors for all three spins follow the ordering $T_{\rm EM}<T_{\rm scalar}<T_{\rm Dirac}$. Testing strong cosmic censorship at the Cauchy horizon, we find the Christodoulou parameter $β\lesssim 4\times 10^{-3}$ across the admissible $(K,e)$ window -- more than two orders of magnitude below the threshold $1/2$ -- with the margin protected by the theory itself.

gr-qc↗

arXiv2026

Shadow, Sparsity of Radiation and Energy Emission Rate in Skyrmion Black Holes

We examine several observable optical properties of a Skyrmion black hole (BH), focusing on the photon sphere, BH shadow, and photon trajectories. The Skyrme term, along with other geometric parameters of the spacetime, determines the photon sphere location and shapes the resulting BH shadow. Parameter variations produce observable departures from standard BH geometries, offering potential signatures of nonlinear field effects. We also analyze the sparsity of Hawking radiation and the associated energy emission spectra, showing how these quantities respond to the Skyrme coupling and background parameters. Our findings illuminate the connection between nonlinear field contributions and BH optics, with implications for observational and theoretical studies of modified gravity scenarios.

gr-qc↗