arXiv subjects

Md Sabir Ali

Publications and source records attributed to Md Sabir Ali.

At least 19 recordsLinked to original sources

arXiv2026

Influence of Perfect Fluid Dark Matter on Shadow Observables of Yang-Mills modified charged black holes

We investigate the influence of perfect fluid dark matter (PFDM) on Yang--Mills--inspired charged black holes, with a particular focus on the resulting modifications to key black hole observables. By embedding a PFDM term into the spacetime geometry, we examine the alterations in shadow morphology, photon geodesics, and the associated energy emission spectra. Our analysis reveals that PFDM induces notable deviations in the shadow size, shape, and circularity, and significantly impacts the stability of circular orbits. Furthermore, the energy emission rate exhibits a strong dependence on both the Yang--Mills charge and the dark matter distribution. These results indicate that environmental effects arising from dark matter can imprint observable signatures on black hole shadows and radiation processes, offering a potential pathway to constrain dark matter models and probe non-Kerr geometries with forthcoming high-precision observations such as those from the Event Horizon Telescope and next-generation interferometers.

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arXiv2026

Topologically Charged Morris-Thorne-type Wormholes and the Energy Conditions

In this paper, we investigate topologically charged Morris-Thorne-type traversable wormholes by solving the Einstein field equations with an anisotropic fluid as the energy-momentum tensor and analysing the resulting solutions. In continuation to the earlier work (Eur. Phys. J C {\bf 84} (2024) 1037), we consider the shape functions such as: (i) $A(r)=r_0\,e^{r_0-r}$; (ii) $A(r)=r_0\,a^r/a^{r_0}$,\quad $0 < a<1$; (iii) $A(r)=r_0\,\left(\frac{\cosh r_0}{\cosh r}\right)^δ$,\quad $δ\geq 1$; (iv) $A(r)=\frac{1}{r}+\ln\!\frac{r}{r_0}$; (v) $A(r)=B\,r^n+(1-B)$; (vi) $A(r)=r_0\,\frac{\mbox{ln} (1+r)}{\mbox{ln} (1+r_0)}$, (vii) $A(r)=r_0+a\,r_0\,\left[\left(\frac{r}{r_0}\right)^β-1\right]$, where $β<1$ and $0 < a\,β<1$. We examine the energy conditions-namely, null, weak, strong, and dominant energy conditions and explore how topological charge influences or controls these conditions. Additionally, we calculate the anisotropy parameter to determine whether the wormhole geometry exhibits attractive or repulsive behavior. Our analysis demonstrates that the energy density of the anisotropic fluid is always positive. However, while some of the energy conditions are partially satisfied, others are violated.

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arXiv2026

Topological signatures in Kerr-Sen AdS black hole thermodynamics

Black hole thermodynamics and topology have emerged as a strong foundation for a coordinate-independent understanding of phase transitions. Using both Duan's topological current theory and a novel complex residue method, we perform a topological study of the Kerr-Sen AdS black hole arising in heterotic string theory. In turn, we find the zero points corresponding to on-shell black hole states and calculate their winding numbers to find the global topological charge by building the generalized off-shell free energy and examining the corresponding vector field in a parametric space. Our analysis reveals that the Kerr-Sen AdS black hole exhibits three distinct thermodynamic phases -- small, intermediate, and large black hole branches -- characterized by critical points with winding numbers $+1$, $-1$, and $+1$ respectively, culminating in a total topological charge $W = +1$. Significantly, this topological number remains invariant under variations of the dilaton charge parameter, indicating that the dilaton field does not alter the fundamental topological class established for Kerr-AdS and RN-AdS black holes. However, the rotation parameter proves crucial in determining the phase structure and the emergence of multiple critical points. We systematically examine three limiting configurations: the full Kerr-Sen AdS spacetime, the GMGHS AdS limit ($a = 0$), and the asymptotically flat Kerr-Sen case ($Λ= 0$). In addition, we propose a novel approach that analytically continues the thermodynamic characterisation into the complex plane. The characterized complex function, derived from the off-shell Gibbs free energy, possesses isolated singular points whose residues directly encode the winding numbers. Our results indicate that topology offers deep insights into black hole phase transitions, with potential implications to holographic dualities.

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arXiv2026

The extended phase space thermodynamics and Ehrenfest scheme for the Kerr-Sen AdS black holes

In the present work, we numerically investigate the horizon structure of the Kerr-Sen black holes in anti-de Sitter (AdS) spacetime. Further, we investigate the phase transitions and critical phenomena in Kerr-Sen-AdS black holes at the critical points. Such black holes are characterized by its mass ($M$), the dilaton charge ($Q$), and the negative cosmological constant, $Λ(<0)$. We define a dimensionless parameter $ε=\bar{J}/{\bar{Q}^2}$ and express the mass, temperature, volume, and Gibbs free energy in terms of $ε$ and its polynomials. Moreover, we numerically fit the data for the critical points and find that in the appropriate limit, the expressions for critical points would correspond to the respective critical points of the Kerr-AdS black hole thermodynamics. Such a study involves a systematic analysis of temperature, Gibbs free energy, and volume in the extended phase space. We provide an analytical verification of the nature of the phase transitions at the critical points by introducing the Ehrenfest equations. We also check that all three quantities, e.g., the specific heat at constant pressure, $C_P$, the volume expansion coefficient, $α$, and the isothermal compressibility, $κ_T$, diverge at the critical points. We find the $Prigogine$-$Defay$ ratio using the expressions of $C_P$, $α$, and $κ_T$, and find that it identically equals unity. Hence, the phase transition behavior of the Kerr-Sen-AdS black holes at their critical points is of second order. In addition, we propose investigating the energy extraction process via the Penrose process. Later, we calculate the speed of sound and adiabatic compressibility for the rotating Kerr-Sen-AdS black holes. Finally, on a specific note, we calculate the thermodynamic quantities of the boundary conformal field theory (CFT) dual to the extended phase space.

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arXiv2025

Thermodynamics, photon sphere and thermodynamic geometry of Ayón-Beato-García Spacetime

We study the thermodynamics of the Ayón-Beato-García black hole and the relationship between photon orbits and the thermodynamic phase transitions of the black hole in AdS spacetime. We then examine the interactions between the microstructures of the black hole using Ruppeiner geometry. The radius of the photon orbit and the minimum impact parameter behave non-monotonically below the critical point, mimicking the behaviour of Hawking temperature and pressure in extended thermodynamics. Their changes during the large black hole--small black hole phase transition serve as the order parameter, possessing a critical exponent of $1/2$. The results demonstrate that the gravity and thermodynamics of the Ayón-Beato-García black hole are closely related. Furthermore, we explore the thermodynamic geometry, which provides insight into the microstructure interactions of the black hole. We find that the large black hole phase is analogous to a bosonic gas with a dominant attractive interaction, while the small black hole phase behaves like an anyonic gas with both attractive and repulsive interactions.

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arXiv2025

Thermodynamics of ${P}$-${V}$ criticality in $d$-dimensional AdS black holes surrounded by a perfect fluid in Rastall theory

We study a $d$-dimensional Anti-de Sitter (AdS) black hole surrounded by a static anisotropic quintessence field within the framework of Rastall theory. The solution is characterized by several parameters including its mass ($M$), field structure parameter ($N_s$), Rastall coupling parameter ($ψ$), and the cosmological constant ($Λ$). Our objective here is to identify the $d$-dimensional black holes within the framework of Rastall theory, exploring special cases, e.g., a cosmological constant, dust, radiation, and quintessence fields. In addition to this, we derive the effective equation of state parameter, denoted as $ω_{eff}$. Next, we investigate the thermodynamics for $P$-$V$ criticality and phase transitions in the extended phase space of black hole thermodynamics. For a specific set of values for the Rastall coupling parameter, we numerically plot isothermal and isobaric curves in the reduced parameter space. We also compute the specific heat at constant pressure ($C_P$ ), volume expansion coefficient ($α$), and isothermal compressibility ($κ_T$) to deepen our understanding of the analogy between the thermodynamics of Rastall AdS black holes and that of a liquid-gas system. Our investigations indicate that the dimension of spacetime and the Rastall coupling parameter significantly influence the critical nature of phase transitions. By utilizing the expressions for $C_P$, $α$, and $κ_T$, we derive the Ehrenfest equations and perform an analytical investigation of phase transitions at their critical points. These results allow us to compare the thermodynamics of AdS black holes with liquid-gas systems, which closely mimics the behavior of van der Waals (vdWs) gases.

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arXiv2025

Thermodynamics and Shadows of Kerr black holes endowed with a global monopole charge

In this article, we present the thermodynamic and shadow properties of rotating black holes with global monopole charge. The angular diameter of Sgr A$^{*}$ black hole is 48.7 $\pm$ 7 $μas$, which is 8 $kps$ far away having a mass of $M = 4.0_{-0.6}^{+1.1} \times 10^6 M\odot$ as observed by Event Horizon Telescope and for the M87 black hole, the observed angular diameter is $θ_d = 42 \pm 3 μ$as, which is almost $16$ $Mpc$ far away with a mass of $M = (6.5 \pm 0.7) \times 10^9 M_\odot$. The global monopole charge parameter $α$ strongly affects the shape and size of the black hole shadow. We derived all the necessary equations to obtain the angular diameter of the rotating black hole shadow with the effect of the global monopole charge parameter $α$. For $α$ $\in$ (0, 0.08) with $a$ $\in$ $(0.7 M, 0.99 M)$, the angular diameter of M87 black hole shadow varies from $39$ $μas$ to $51$ $μas$. The angular diameter of Sgr A$^{*}$ black hole with global monopole charge parameter $α$ $\in$ (0, 0.04) and $a$ $\in$ $(0.7 M, 0.99 M)$, varies from $50$ $μas$ to $55$ $μas$. For bound values of $α$ and $a$, our results are consistent with the EHT observations.

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arXiv2025

Holographic thermodynamics of a five-dimensional neutral Gauss-Bonnet AdS black hole

Motivated by the recent progress on the holographic dual of the extended thermodynamics for black holes in anti-de-Sitter (AdS) space, we investiggate the hologrphic thermodynamics for the five-dimensional neutral Gauss-Bonnet AdS black hole in the context of the anti-de Sitter/conformal field theory (AdS/CFT) correspondence. Through the extended bulk thermodynamics for the five-dimensional Gauss-Bonnet AdS black hole, we derive the first law of the CFT thermodynamics which is obtained by directly translating the arbitrary conformal factors in the dual CFT. In addition to the newly defined chemical potential $μ$ conjugating to the central charge $C$, we obtain other pairs of thermodynamics for the CFT, such as the temperature $\tilde{T}$ and the entropy $S$, the Gauss-Bonnet coupling constant $\tildeα$ and its conjugate variable $ \tilde{\mathcal{A}}$, the pressure $\mathcal{P}$ and its conjugate volume $\mathcal{V}$. In the fixed $C$, $\mathcal{V}$ and $\tildeα$ canonical ensemble, we obtain the canonical description of the CFT thermodynamics and observe the standard swallowtail behavior in the Helmholtz free energy vs the temperature plot. The self-intersection point of the Helmholtz free energy indicates the phase transition between the high and low entropy states of the CFT. By using Maxwell's equal area law, we get the critical point and coexistence curve for the high and low entropy phases of the CFT. Besides, we get the critical exponents for the CFT, and find that the critical point and critical exponents associated with the $\tilde{T}-S$ criticality of the CFT are the same as those of the five-dimensional Gauss-Bonnet AdS black hole.

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arXiv2025

Greybody factor for an electrically charged regular-de Sitter black holes in $d$-dimensions

We investigate the propagation of scalar fields in the gravitational background of higher-dimensional, electrically charged, regular de Sitter black holes. Using an approximate analytical approach, we derive expressions for the greybody factor for both minimally and non-minimally coupled scalar fields. In the low-energy regime, we find that the greybody factor remains non-zero for minimal coupling but vanishes for non-minimal coupling, indicating a significant influence of curvature coupling on the emission profile. Examining the greybody factor alongside the effective potential, we explore how particle parameters (the angular momentum number and the non-minimal coupling constant) and spacetime parameters (the dimension, the cosmological constant, and the non-linear charge parameter) affect particle emission. While non-minimal coupling and higher angular momentum modes generally suppress the greybody factor, the non-linear charge parameter enhances it. We then compute the Hawking radiation spectra for these black holes and observe that, despite the non-linear charge enhancing the greybody factor, both non-minimal coupling and the non-linear charge ultimately reduce the total energy emission rate. These results provide insights into how modifications to classical black hole solutions in higher dimensions, through the inclusion of non-linear electrodynamics, impact their quantum emission properties.

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arXiv2025

Strong gravitational lensing of a five-dimensional charged, equally rotating black hole with a cosmological constant

We study the lensing phenomena of the strong gravity regime of five-dimensional charged, equally rotating black holes with a cosmological constant, familiarly known as the Cveti\v c-Lü-Pope black holes. These black holes are characterized by three observable parameters, the mass $M$, the charge $Q$ and the angular momentum $J$, in addition to the cosmological constant. We investigate the strong gravitational lensing observables, mainly the photon sphere radius, the minimum impact parameter, the deflection angle, the angular size, and the magnification of the relativistic images. We model the $M87$ and $SgrA^*$ for these observables. We also focus on the relativistic time delay effect in the strong-field regime of gravity and the impact of the observable on it. The analytical expressions for the observables of the relativistic images with vanishing angular momentum ($j=0$) are discussed in some detail. We shed a light on the gravitational time delay effect by incorporating the lensing observables. The gravitational time delay has a direct consequence on the photon sphere radius and hence on the quasinormal modes.

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arXiv2024

Thermodynamics of Einstein-Gauss-Bonnet Black Holes and Ensemble-averaged Theory

In this paper, using the ensemble-averaged theory, we define the thermodynamic free energy of Einstein-Gauss-Bonnet (EGB) black holes in anti-de Sitter (AdS) spacetime. This approach derives the gravitational partition function by incorporating non-saddle geometries besides the classical solutions. Unlike the sharp transition points seen in free energy calculated via saddle-point approximation, the ensemble-averaged free energy plotted against temperature shows a smoother behavior, suggesting that black hole phase transitions may be viewed as a small $G_N$ (Newton's gravitational constant) limit of the ensemble theory. This is similar to the behavior of black hole solutions in Einstein's gravity theory in AdS spacetime. We have obtained an expression for the quantum-corrected free energy for EGB-AdS black holes, and in the six-dimensional case, we observe a well-defined local minimum after the transition temperature which was absent in the earlier analysis of the classical free energy landscape. Furthermore, we expand the ensemble-averaged free energy in powers of $G_N$ to identify non-classical contributions. Our findings indicate that the similarities in the thermodynamic behavior between five-dimensional EGB-AdS and Reissner-Nordström-AdS (RN-AdS) black holes, as well as between six-dimensional EGB-AdS and Schwarzschild-AdS black holes, extend beyond the classical regime.

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arXiv2024

Thermodynamics, Phase Transition and Joule Thomson Expansion of 4-D Gauss-Bonnet AdS Black Hole

We explore the thermodynamic and phase transition properties of asymptotically AdS black holes within Einstein-Gauss-Bonnet gravity, focusing on Joule Thomson expansion. Thermodynamics is studied in the extended phase space, where the cosmological constant serves as thermodynamic pressure. We observe that the black hole undergoes a phase transition similar to that of a van der Waals system. We analyze charged and neutral cases separately to distinguish the effect of charge and Gauss Bonnet parameter on critical behavior and examine the phase structure. We find that the Gauss-Bonnet coupling parameter behaves similarly to black hole charge or spin, guiding the phase structure. To understand the underlying phase structure determined by the Gauss-Bonnet coefficient $α$, we introduce a new order parameter. We discover that the change in the conjugate variable to the Gauss-Bonnet parameter acts as an order parameter, demonstrating a critical exponent of $1/2$ in the vicinity of the critical point. Since the phase structure is analogous to that of a van der Waals fluid, we investigate the Joule-Thomson expansion of the black hole. We analytically study the Joule-Thomson expansion, focusing on three key characteristics: the Joule-Thomson coefficient, inversion curves, and isenthalpic curves. We obtain isenthalpic curves in the $T-P$ plane and illustrate the cooling-heating regions.

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arXiv2024

Topology of Born-Infeld-AdS Black Hole Phase Transitions: Bulk and CFT Sides

The thermodynamic criticality of the AdS black holes serves as an important structure during the thermal phase transition. This paper discusses about the critical points and their topology during thermal phase transitions of the Born-Infeld AdS black holes. We make such investigations using two different topological approaches, namely, using Duan's topological current $ϕ$-mapping theory, and the off-shell free energy. Within Duan's formalism, we observe that for a given value of the Born-Infeld parameter $b$, there exists an associated electric charge parameter $Q$, which is highly sensitive to the topological phase transitions. This way we examine the connections of the first-order phase transition and the topological nature of the critical points. We find that the topological nature has a possible breakdown in certain parametric ranges. In effect, we determine the unconventional and the conventional phase critical points as the creation (topologically vortex) and annihilation (topologically anti-vortex) points (pairs). As the second approach, we call the off-shell free energy to determine the topological classes: of which one corresponds to the AdS-Schwarzschild black hole phases, while the other provides a possible topological phase transition. Here we also reveal a novel phase transition between two unstable phases, namely, the unstable small black hole and the intermediate black holes. For a certain parametric values of the Born-Infeld parameter and the pressure, we also study the different topological descriptions that inevitably correspond to the AdS-Reissner-Nordstr$\Ddot{o}$m black hole phases. As a consistency check of the critical points during the topological phase transitions, we study the vortex/anti-vortex annihilation thermodynamics from local as well as global thermodynamic viewpoint.

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arXiv2024

Photon orbits and phase transition for Letelier AdS black holes immersed in perfect fluid dark matter

We obtain an exact solution of spherically symmetric Letelier AdS black holes immersed in perfect fluid dark matter (PFDM). Considering the cosmological constant as the positive pressure of the system and volume as its conjugate variable, we analyse the thermodynamics of our black holes in the extended phase space. Owing to the background clouds of strings parameter ($a$) and the parameter endowed with PFDM ($β$), we analyse the Hawking temperature, entropy and specific heat. We also investigate the relationship between the photon sphere radius and the phase transition for the Letelier AdS black holes immersed in PFDM. Through the analysis, we find with a particular condition, there are non-monotonic behaviours between the photon sphere radius, the impact parameter, the PFDM parameter, temperature, and pressure. We can regard both the changes of photon sphere radius and impact parameter before and after phase transition as the order parameter; their critical exponents near the critical point are equal to the same value 1/2, just like ordinary thermal systems. These indicate that a universal relation of gravity may exist near the critical point for a black hole thermodynamic system.

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arXiv2024

Lyapunov Exponents and Phase Structure of Lifshitz and Hyperscaling Violating Black Holes

We study the phase structure of hyperscaling violating black holes using Lyapunov exponents. For describing hyperscaling violating system, we chose a particular gravity model constructed from generalized Einstein-Maxwell-Dilaton action which includes the Lifshitz cases in appropriate limit. We study the relationship between Lyapunov exponents and black hole phase transitions considering both the timelike and null geodesics. We observe that, the black hole phase transiton properties are reflected in Lyapunov exponent where its multiple branches correspond to the distinct phases of the black hole. The discontinuos change of the Lyapunov exponent during the phase transition serve as an order parameter with critical exponent $1/2$ near the critical point. Our numerical study reveals that the correlation between the Lyapunov exponent and black hole thermodynamic properties can be generalised beyond the AdS spacetime. We find that it is independent of the hyperscaling violation parameter as well as the Lifshitz exponent.

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arXiv2023

Thermodynamics of Kerr-Sen-AdS black holes in the restricted phase space

We analyse the restricted phase space thermodynamics (RPST) of Kerr-Sen-AdS black holes with the central charge $C$ and its conjugate chemical potential $μ$ but exclude the familiar $PdV$ term in the first law of black hole thermodynamics. That gives rise to a new perspective on the thermodynamics of black holes. Using the scaling properties, we investigate the first law and the corresponding Euler formula. Such formalism has its beauty, to say, for example, the mass is considered to be a homogeneous function of the extensive variables in the first order. In contrast, the intensive variables are of zeroth order. Because of the complicated expressions of the metric, we numerically calculate the critical values of the thermodynamic quantities. We find the phase transition behaviour of the free energy and other thermodynamic conjugate variables that appear in the first law. The RPST of the Kerr-Sen-AdS black holes is like that of the Reissner-Nordstr$\ddot{o}$m-AdS and the Kerr-AdS black holes. Such notions of the phase transition behaviour show that there should be some underlying universality in the RPST formalism.

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arXiv2023

Born-Infeld-AdS black hole phase structure: Landau theory and free energy landscape approaches

We start with a brief overview of the basic thermodynamic properties of the Born-Infeld metric in AdS spacetime. Using the concept of the enthalpy characterizing the total mass of the black hole, in our present paper, we probe the thermal phase transition structure, the dynamic and kinetic behavior of the Born-Infeld-AdS black hole. The emergence of the triple point behavior and the possible ruling out the reentrant phase transition, for a certain parametric value of the charge on the free energy landscape, we scrutinize the stochastic dynamics and the kinetic processes. We describe such processes during the black hole phase transitions in terms of the Landau functional and equivalently by the Fokker-Planck equation in the context of black hole chemistry. Our analysis establishes a pertinent bridge between the thermal behavior among the different states of the Van-der-Waals-like fluids and the Born-Infeld-AdS black holes phases. To visualize the direct implications of the Landau functional of the usual Van-der-Waals-like fluids, we consistently employed the generic Landau formalism for the analysis of the black hole phase transitions of the Born-Infeld-AdS black holes. We find that such investigations are worthy of study in implementing the continuous phase transition behavior during the Hawking radiation. For more details, and in addition to the exploitation of the Landau functional, we introduce ...

hep-th↗

arXiv2023

Revisiting the Second Law and Weak Cosmic Censorship Conjecture in High-Dimensional Charged-AdS Black Hole: an Additional Assumption

The verification of the second law of black hole mechanics and the WCCC in the context of enthalpy as mass of the black hole and its related thermodynamic properties has not been tested through a vast number of literature in the recent past. Such studies are of great physical importance as they provide us with a large number of information regarding the thermodynamics and the dynamics of AdS black hole systems. We invest the prior limited surveys of such analysis to investigate the WCCC for the $D$- dimensional asymptotically AdS-charged black holes characterized by its mass ($M$), electric charge ($Q$), and AdS radius ($l$) under the absorption of scalar particles of charge $q$. We examine the WCCC by analyzing the energy-momentum condition of the electrically charged particles as absorbed by the black holes. We prove that the conjecture is well verified irrespective of whether the initial black hole configurations are extremal or non-extremal by changing its charge, the AdS radius, and their variations. We show that the first law and the WCCC are valid for all spacetime dimensions ($D$) independent of the choice of the parameters characterizing the black holes. But to verify the second law in the extremal and non-extremal configurations one has to be very cautious as it gets strongly affected by the choices of the values of the black hole parameters and their variations... In the context of the extended phase space, taking the grand canonical potential into account allow us to obtain the missing information about the variation of the cosmological constant necessary to construct the extended phase space, namely the notion of the black hole pressure, and which is absent in the previous literature so far.

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