Browsing by Author "Naveena Kumara, A.N."

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Coexistent physics and microstructure of the regular Bardeen black hole in anti-de Sitter spacetime
(Academic Press Inc. apjcs@harcourtbrace.com, 2020) Ahmed Rizwan, C.L.A.; Naveena Kumara, A.N.; Hegde, K.; Vaid, D.
We study the phase structure and the microscopic interactions of a 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 Pr?Tr coexistence equation is obtained using a fitting formula. The coexistence and spinodal curves are plotted in Pr?Tr and Tr?Vr 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 the regular Bardeen–AdS black hole using the novel Ruppeiner geometry proposed by Wei et al. (2019). It is found that the microscopic interactions are not the same in 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 an 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. © 2020 Elsevier Inc.
Dynamic phase transition of black holes in massive gravity
(Academic Press Inc., 2023) Safir, T.K.; Naveena Kumara, A.N.; Punacha, S.; Ahmed Rizwan, C.L.A.; Fairoos, C.; Vaid, D.
The dynamical properties of small-large black hole phase transition in dRGT non-linear massive gravity theory are studied based on the underlying free energy landscape. The free energy landscape is constructed by specifying the Gibbs free energy to every state, and the free energy profile is used to study the different black hole phases. The small-large black hole states are characterized by probability distribution functions and the kinetics of phase transition are described by the Fokker–Planck equation. Further, a detailed study of the first passage process is presented which describes the dynamics of phase transitions. Finally, we have investigated the effect of mass and topology on the dynamical properties of phase transitions of black holes in dRGT non-linear massive gravity theory. © 2023 Elsevier Inc.
Dynamics and kinetics of phase transition for regular AdS black holes in general relativity coupled to nonlinear electrodynamics
(World Scientific, 2023) Naveena Kumara, A.N.; Punacha, S.; Hegde, K.; Ahmed Rizwan, C.L.A.; Ajith, K.M.; Ali, M.S.
We study the stochastic dynamics and kinetics of the phase transition of the regular black holes in Anti-de Sitter spacetime by employing the free energy landscape. Our investigation focuses on two important classes of regular black hole solutions, namely the Hayward and Bardeen, which can be obtained from coupling of non-linear electrodynamics. The dynamics of the phase transition is described by the Fokker-Planck equation, using which, we investigate the probabilistic evolution of regular AdS black holes. We solve this equation numerically by imposing both reflecting and absorbing boundary conditions and appropriate initial conditions. In this context, the on-shell Gibbs free energy is treated as a function of the event horizon radius, where the difference of the event horizon radii in different phases serves as the order parameter for the phase transition. The study allows us to probe the dynamic process of transitioning between coexisting small and large black hole phases due to thermal fluctuations, as quantified by the calculation of the first passage time. Furthermore, we explore the influence of temperature on this dynamic process. This research contributes to a deeper understanding of the microstructures of regular AdS black holes. © 2023 World Scientific Publishing Company.
Euclidean thermodynamics and Lyapunov exponents of Einstein–Power–Yang–Mills AdS black holes
(Springer Nature, 2025) Karthik, R.; Dillirajan, D.; Ajith, K.M.; Hegde, K.; Punacha, S.; Naveena Kumara, A.N.
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. © The Author(s) 2025.
Greybody factor for an electrically charged regular-de Sitter black holes in d-dimensions
(Springer Nature, 2025) Ali, M.S.; Naveena Kumara, A.N.; Hegde, K.; Ahmed Rizwan, C.L.A.; Punacha, S.; Ajith, K.M.
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. © The Author(s) 2025.
Microstructure of five-dimensional neutral Gauss–Bonnet black hole in anti-de Sitter spacetime via P- V criticality
(Springer, 2023) Naveena Kumara, A.N.; Ahmed Rizwan, C.L.A.; Hegde, K.; Ali, M.S.; Ajith, K.M.
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, by using the 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 at the critical point with a critical exponent 2, and approaches zero for extremal black holes. We show that the dominant interaction among black hole molecules is attractive. This study also confirms 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. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
Perturbations of black holes surrounded by anisotropic matter field
(American Physical Society, 2025) Sagar J, C.; Karthik, R.; Hegde, K.; Ajith, K.M.; Punacha, S.; Naveena Kumara, A.N.
Our research aims to probe the anisotropic matter field around black holes using black hole perturbation theory. Black holes in the Universe are usually surrounded by matter or fields, and it is important to study the perturbation and the characteristic modes of a black hole that coexists with such a matter field. In this study, we focus on a family of black hole solutions to Einstein's equations that extend the Reissner-Nordström spacetime to include an anisotropic matter field. In addition to mass and charge, this type of black hole possesses additional hair due to the negative radial pressure of the anisotropic matter. We investigate the perturbations of the massless scalar and electromagnetic fields and calculate the quasinormal modes (QNMs). We also study the critical orbits around the black hole and their properties to investigate the connection between the eikonal QNMs, black hole shadow radius, and Lyapunov exponent. Additionally, we analyze the graybody factors and scattering coefficients using the perturbation results. Our findings indicate that the presence of anisotropic matter fields leads to a splitting in the QNM frequencies compared to the Schwarzschild case. This splitting feature is also reflected in the shadow radius, Lyapunov exponent, and graybody factors. © 2025 American Physical Society.
Phase transition and thermodynamic geometry of regular Bardeen black hole in higher dimensions
(American Institute of Physics Inc., 2020) Rajani, K.V.; Ahmed Rizwan, C.L.A.; Naveena Kumara, A.N.
In this work we study the thermodynamic properties and phase transition of higher dimensional regular Bardeen black hole in AdS spacetime. The black hole thermodynamics is studied using Hawking temperature vs Bekenstein entropy (T - S) plot, which shows a critical behaviour. This is followed by the investigation of the phase transition by observing the divergence of specific heat. The critical behaviour of the blackhole is further analysed in detail via Ruppeiner geometry. The divergence behaviour similar to specific heat is found in Ruppeiner curvature scalar confirms a second order phase transition. Â© 2020 Author(s).
Photon orbits and thermodynamic phase transition of regular AdS black holes
(American Physical Society subs@aip.org;revtex@aps.org;prx@aps.org;prxtex@aps.org;help@aps.org;prb@aps.org, 2020) Naveena Kumara, A.N.; Ahmed Rizwan, C.L.A.; Punacha, S.; Ajith, K.M.; Ali, M.S.
We probe the phase structure of the regular anti-de Sitter (AdS) black holes using the null geodesics. The radius of photon orbit and minimum impact parameter shows a nonmonotonous behavior below the critical values of the temperature and the pressure, corresponding to the phase transition in extended phase space. The respective differences of the radius of unstable circular orbit and the minimum impact parameter can be seen as the order parameter for the small-large black hole phase transition, with a critical exponent 1/2. Our study shows that there exists a close relationship between the gravity and thermodynamics for the regular AdS black holes. © 2020 American Physical Society.
Regular Bardeen AdS black hole as a heat engine
(Elsevier B.V., 2020) Rajani, K.V.; Ahmed Rizwan, C.L.A.; Naveena Kumara, A.N.; Vaid, D.; Ajith, K.M.
We investigate the thermodynamic phase transition and heat engine efficiency in regular Bardeen AdS black hole. The thermodynamics of the black hole is analyzed in extended phase space. A first order phase transition analogous to van der Waal system is evident from this study, which is affirmed by the specific heat divergence at the critical points. A conventional heat engine is constructed by considering the black hole as working substance. The efficiency is obtained via a thermodynamic cycle in the P?V plane, which receives and ejects heat. The heat engine efficiency is improved by adding a quintessence field. The analytical expression for heat engine efficiency is derived in terms of quintessence dark energy parameter. © 2020 The Author(s)
Repulsive interactions in the microstructure of regular Hayward black hole in anti-de Sitter spacetime
(Elsevier B.V., 2020) Naveena Kumara, A.N.; Ahmed Rizwan, C.L.A.; Hegde, K.; Ajith, K.M.
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. © 2020 The Author(s)
Theory of unpinning of spiral waves using circularly polarized electric fields in mathematical models of excitable media
(American Physical Society subs@aip.org;revtex@aps.org;prx@aps.org;prxtex@aps.org;help@aps.org;prb@aps.org, 2020) Punacha, S.; Naveena Kumara, A.N.; Shajahan, T.K.
Spiral waves of excitation are common in many physical, chemical, and biological systems. In physiological systems like the heart, such waves can lead to cardiac arrhythmias and need to be eliminated. Spiral waves anchor to heterogeneities in the excitable medium, and to eliminate them they need to be unpinned first. Several groups focused on developing strategies to unpin such pinned waves using electric shocks, pulsed electric fields, and recently, circularly polarized electric fields (CPEF). It was shown that in many situations, CPEF is more efficient at unpinning the wave compared to other existing methods. Here, we study how the circularly polarized field acts on the pinned spiral waves and unpins it. We show that the termination always happens within the first rotation of the electric field. For a given obstacle size, there exists a threshold time period of the CPEF below which the spiral can always be terminated. Our analytical formulation accurately predicts this threshold and explains the absence of the traditional unpinning window with the CPEF. We hope our theoretical work will stimulate further experimental studies about CPEF and low energy methods to eliminate spiral waves. © 2020 American Physical Society.
Thermodynamics, phase transition and Joule Thomson expansion of 4-D GaussBonnet AdS black hole
(World Scientific, 2024) Hegde, K.; Ahmed Rizwan, C.L.A.; Ajith, K.M.; Naveena Kumara, A.N.; Ali, M.S.; Punacha, S.
In this paper, 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, 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 °uid, 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. © 2024 World Scientific Publishing Company.
Thermodynamics, photon sphere and thermodynamic geometry of Ayón-Beato-Garcia space-time
(World Scientific, 2025) Hegde, K.; Naveena Kumara, A.N.; Ajith, K.M.; Ahmed Rizwan, C.L.A.; Ali, M.S.; Punacha, S.
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 space-time. 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 nonmonotonically below the critical point, mimicking the behavior 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. © 2025 World Scientific Publishing Company.