arXiv subjects

Kartheek Hegde

Publications and source records attributed to Kartheek Hegde.

15 recordsLinked to original sources

arXiv2026

Spectral Bifurcations in Quasinormal Modes of Regular BTZ Black Holes

We study the quasinormal spectrum of massless scalar fields propagating on a family of regular BTZ black holes arising from an infinite tower of dimensionally regularized Lovelock corrections. These geometries are asymptotically AdS, reduce to the standard BTZ solution in the limit $\ell \to 0$, and resolve the central singularity by introducing a smooth core controlled by the new length scale $\ell$. The scalar quasinormal modes are computed using both Leaver's continued-fraction method and the Horowitz-Hubeny power-series method; the two approaches agree to high accuracy across the parameter space. We find that the regularization preserves linear stability ($ω_I < 0$) while qualitatively reshaping the spectrum: as $\ell$ increases, BTZ-like complex branches collide with the imaginary axis and undergo a hierarchy of bifurcations into multiple purely imaginary branches, leading to mode switching and a nontrivial reordering of overtones as functions of $\ell$ and the harmonic index $m$. Our results place regular BTZ black holes within the emerging family of bifurcating quasinormal spectra known from nearly extremal and asymptotically AdS black holes, and highlight these $(2+1)$-dimensional geometries as a controlled arena for exploring geometric mechanisms behind spectral branching and late-time ringdown in regular black hole spacetimes.

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arXiv2026

Euclidean Thermodynamics and Lyapunov Exponents of Einstein-Power-Yang-Mills AdS Black Holes

We study the thermodynamics of Einstein-Power-Yang-Mills AdS black holes via the Euclidean path integral method, incorporating appropriate boundary and counterterms. By analyzing unstable timelike and null circular geodesics, we demonstrate that their Lyapunov exponents reflect the thermodynamic phase structure obtained from the Euclidean action. Specifically, the small-large black hole phase transition, analogous to a van der Waals fluid, is signaled by a discontinuity in the Lyapunov exponent. Treating this discontinuity as an order parameter, we observe a universal critical exponent of $1/2$, consistent with mean-field theory. These results extend previous insights from black hole spacetimes with Abelian charges to scenarios involving nonlinear, non-Abelian gauge fields, highlighting the interplay between black hole thermodynamics and chaotic dynamics.

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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

Spinning LQG black hole as a particle accelerator

We demonstrate that the spinning LQG black hole can act as a cosmic particle accelerator. The LQG solution is singularity-free and can possess spin greater than that of a Kerr black hole. The additional black hole hair, arising from quantum effects, significantly influences the particle dynamics around the black hole. Under suitable physical conditions, the center-of-mass energy can grow arbitrarily high during the collision of two generic particles in the spacetime of an extremal black hole. In the non-extremal case, there exists a finite upper bound on the center-of-mass energy, the maximum value of which depends on the LQG parameter. These results are particularly interesting from an astrophysical perspective, especially in the context of probing Planck-scale physics.

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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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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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arXiv2021

Dynamics and kinetics of phase transition for regular AdS black holes in general relativity coupled to non-linear electrodynamics

Employing the free energy landscape, we study the phase transition and its dynamics for a class of regular black holes in Anti-de Sitter spacetime governed by the coupling of non-linear electrodynamics, which reduces to Hayward and Bardeen solutions for particular values of spacetime parameters. The Fokker-Planck equation is solved numerically by imposing the reflecting boundary condition and a suitable initial condition, using which, we investigate the probabilistic evolution of regular AdS black holes. In this approach, the on-shell Gibbs free energy is treated as a function of the radius of the event horizon, which happens to be the order parameter of the phase transition. The numerical solution is also obtained for the absorbing boundary condition. The dynamics of switching between the coexistence small black hole phase and large black hole phase due to the thermal fluctuation is probed by calculating the first passage time. The effect of temperature on the dynamical process is also investigated.

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arXiv2020

Ruppeiner Geometry, Reentrant Phase transition and Microstructure of Born-Infeld AdS Black Hole

Born-Infeld AdS black hole exhibits a reentrant phase transition for certain values of the Born-Infeld parameter $b$. This behaviour is an additional feature compared to the van der Waals like phase transition observed in charged AdS black holes. Therefore, it is worth observing the underlying microscopic origin of this reentrant phase transition. Depending on the value of the parameter $b$, the black hole system has four different cases: no phase transition, a reentrant phase transition with two scenarios, or a van der Waals-like (standard) phase transition. In this article, by employing a novel Ruppeiner geometry method in the parameter space of temperature and volume, we investigate the microstructure of Born-Infeld AdS black hole via the phase transition study, which includes standard and reentrant phase transition. We find that the microstructures of the black hole that lead to standard and reentrant phase transitions are distinct in nature. The standard phase transition is characterised by the typical RN-AdS microstructure. In this case, the small black hole phase has a dominant repulsive interaction for the low temperature case. Interestingly, during the reentrant phase transition, displayed by the system in a range of pressures for specific $b$ values, the dominant attractive nature of interaction in the microstructure is preserved. Our results suggest that in the reentrant phase transition case, the intermediate black holes behave like a bosonic gas, and in the standard phase transition case the small black holes behave like a quantum anyon gas. In both cases, the large black hole phase displays an interaction similar to the bosonic gas. The critical phenomenon is observed from the curvature scalar, including the signature of the reentrant phase transition.

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arXiv2020

Coexistent Physics and Microstructure of the Regular Bardeen Black Hole in Anti-de Sitter Spacetime

We study the phase structure and the microscopic interactions in regular Bardeen AdS black hole. The stable and metastable phases in the black hole are analysed through coexistence and spinodal curves. The solutions are obtained numerically as the analytic solution to the coexistence curve is not feasible. The $P_r-T_r$ coexistence equation is obtained using a fitting formula. The coexistence and spinodal curves are plotted in $P_r-T_r$ and $T_r-V_r$ planes to explore the phase structure of the black hole. In the second part of our study, we were able to probe the microscopic interactions of regular Bardeen AdS black hole using the novel Ruppeiner geometry proposed by S.W. Wei \emph{et.al} Phys. Rev. Lett.123, 071103 (2019). It is found that the microscopic interactions are not same in the small black hole (SBH) and large black hole (LBH) phases. In the SBH phase, there exists a repulsive interaction in the microstructure in the low temperature regime. In contrast, the microstructure associated with the LBH phase has attractive interaction throughout the parameter space. We found that, along the coexistence temperature both the SBH and LBH branches diverge to negative infinity with a critical exponent equal to $1/2$.

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arXiv2020

Rotating Black Hole with an Anisotropic Matter Field as a Particle Accelerator

Recently, a generalised solution for Einstein equations of a rotating compact body, surrounded by matter field was proposed, which is the Kerr-Newman spacetime with an anisotropic matter. The solution possesses an additional hair, along with the conventional mass, charge and spin, which arises from the negative radial pressure of the anisotropic matter. In this article we show that, this new class of black holes can act as a particle accelerator during the collision of two generic particles in its gravitational field in the ergo-region. The centre of mass energy of the particles shoots to arbitrary high value in the vicinity of event horizon for the extremal black hole. The physical conditions for the collision to take place are obtained by studying the horizon structure and circular particle motion. The results are interesting from astrophysical perspective.

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arXiv2020

Null Geodesics and Thermodynamic Phase Transition of Four-Dimensional Gauss-Bonnet AdS Black Hole

Modified gravity theories are of great interest in both observational and theoretical studies. In this article we study the correlation between the null geodesics in the background of a four-dimensional Einstein-Maxwell-Gauss-Bonnet AdS black hole, a modification of Einstein gravity, and it's thermodynamic phase transition. We study the phase structure of the black hole, using the coexistence and spinodal curves, to understand the phase transition in a extended phase space. The imprints of this phase transition features are observed in the behaviour of photon orbit radius and minimum impact parameter with respect to the Hawking temperature and pressure. The change in these two quantities during the phase transition serve as order parameter which characterises the critical behaviour. The correlation shows that thermodynamic phase transition can be studied by observing the effects of strong gravitational field and vice versa.

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arXiv2020

Repulsive Interactions in the Microstructure of Regular Hayward Black Hole in Anti-de Sitter Spacetime

We study the interaction between the microstructures of Hayward-AdS black hole using Ruppeiner geometry. Our investigation shows that the dominant interaction between the black hole molecules is attractive in most part of the parametric space of temperature and volume, as in van der Waals system. However, in contrast to the van der Waals fluid, there exists a weak dominant repulsive interaction for small black hole phase in some parameter range. This result clearly distinguishes the interactions in a magnetically charged black hole from that of van der Waals fluid. However, these sort of interactions are characteristic for charged black holes since they do not dependent on magnetic charge or temperature.

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arXiv2020

Microstructure of five-dimensional neutral Gauss-Bonnet black hole in anti-de Sitter spacetime via $P-V$ criticality

In this article, we analytically investigate the microstructure of a five-dimensional neutral Gauss-Bonnet black hole, in the background of anti-de Sitter spacetime, using scalar curvature of the Ruppeiner geometry constructed via adiabatic compressibility. The microstructure details associated with the small-large black hole phase transition are probed in the parameter space of pressure and volume. The curvature scalar shows similar properties for both phases of the black hole, it diverges in the vicinity of critical point and approaches zero for extremal black holes. We show that the dominant interaction among black hole molecules is attractive. This study also affirms that the nature of the microstructure interaction remains unchanged during the small-large black hole phase transition, even though the microstructures are different for both phases.

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arXiv2020

Rotating 4D Gauss-Bonnet black hole as particle accelerator

We demonstrate that the four-dimensional Gauss-Bonnet black hole can act as a particle accelerator with arbitrarily high centre-of-mass energy, during the collision of two general particles near the event horizon. The Gauss-Bonnet coupling constant $α$, provides a deviation in the results from that of Kerr black hole. Our results show that the horizon structure, the range of allowed angular momentum and the critical angular momentum depend on the value of $α$. For extremal cases, the centre-of-mass energy diverges near the horizon, suggesting that Gauss-Bonnet black hole can also act as a particle accelerator like a Kerr black hole. This is interesting in the context of probing the Planck scale physics. For the non-extremal case there exists a finite upper bound on the centre-of-mass energy, the maximal value of which depends on the parameter $α$.

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arXiv2020

Microstructure and continuous phase transition of a regular Hayward black hole in anti-de Sitter spacetime

We study the phase transition of a regular Hayward-AdS black hole by introducing a new order parameter, the potential conjugate to the magnetic charge due to the non-linearly coupled electromagnetic field. We use Landau continuous phase transition theory to discuss the van der Waals like critical phenomena of the black hole. The popular interpretation of the AdS black hole phase transition as between a large and a small black hole is reinterpreted as the transition between a high potential phase and a low potential phase. The underlying microstructure for this phase transition is probed using the Ruppeiner geometry. By investigating the behaviour of the Ruppeiner scalar curvature, we find that the charged and uncharged (effective) molecules of the black hole have distinct microstructures analogous to fermion and boson gas.

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