Browsing by Author "Ali, M.S."

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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.
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 and continuous phase transition of a regular Hayward black hole in anti-de Sitter spacetime
(Physical Society of Japan, 2021) Naveena Kumara, A.; Ahmed Rizwan, C.L.; Hegde, K.; Ali, M.S.; Ajith, K.M.
In this article we study the thermodynamic phase transition of a regular Hayward-AdS black hole, by introducing a new order parameter, which is the potential conjugate to the magnetic charge arising from a 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 well-known interpretation of the phase transition of an AdS black hole as being a large and small black hole transition is re-interpreted as being a transition between a high-potential phase and a low-potential phase. The microstructure associated with this phase transition is studied using the Ruppeiner geometry. By investigating the behaviour of the Ruppeiner scalar curvature, we find that charged and uncharged (effective) molecules of the black hole have distinct microstructure, which is analogous to that of fermion and boson gas. © 2021 The Author(s) 2021. Published by Oxford University Press on behalf of the Physical Society of Japan.
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.
Null geodesics and thermodynamic phase transition of four-dimensional Gauss–Bonnet AdS black hole
(Academic Press Inc., 2021) Hegde, K.; Naveena Kumara, A.; Ahmed Rizwan, C.L.; Ali, M.S.; Ajith, K.M.
Modified gravity theories are of great interest in both observational and theoretical studies. In this article, we study the correlation between the null geodesics and the thermodynamic phase transition of a four-dimensional Gauss–Bonnet AdS black hole. Firstly, we study the phase structure of the black hole, using the coexistence and spinodal curves, to understand the phase transition in an extended phase space. The imprints of these phase transition features can be clearly observed from the functional dependence 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 parameters which characterise the critical behaviour. The correlation shows that thermodynamic phase transition can be studied by observing the effects of strong gravitational field on the photon orbit and vice versa. © 2021 Elsevier Inc.
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.
Rotating 4D Gauss–Bonnet black hole as a particle accelerator
(Academic Press Inc., 2021) Naveena Kumara, A.; Ahmed Rizwan, C.L.A.; Hegde, K.; Ali, M.S.; Ajith, K.M.
We demonstrate that the rotating four-dimensional Gauss– Bonnet black hole can act as a particle accelerator with arbitrarily high centre-of-mass (CM) energy, when collision of two general particles takes place near the event horizon. The particles are at rest initially at infinity, and by fine tuning their angular momenta within a finite range, they are released so that they follow the time-like geodesics in the black hole spacetime, and the collision taking place on the equatorial plane is observed. The Gauss–Bonnet coupling constant ?, provides a deviation in the results, from that observed in the Kerr black hole. The horizon structure, the range of allowed angular momentum and the critical angular momentum depend on the value of ?. Our results show that the CM energy depends on the coupling parameter ? in addition to the black hole spin a. For extremal cases, the CM energy diverges at the horizon, suggesting that Gauss–Bonnet black hole can also act as a particle accelerator similar to a Kerr black hole. For the non-extremal case, there exists a finite upper bound on the CM energy, the maximal value of which depends on the parameter ?. © 2021 Elsevier Inc.
Rotating black hole with an anisotropic matter field as a particle accelerator
(IOP Publishing Ltd, 2021) Ahmed Rizwan, C.L.; Naveena Kumara, A.; Hegde, K.; Ali, M.S.; Ajith, K.M.
Recently, a generalised solution for Einstein equations for a rotating black hole, surrounded by matter field was proposed, which is the Kerr-Newman spacetime with an anisotropic matter [Phys. Rev. D. 101 064067]. Due to the negative radial pressure of the anisotropic matter, the solution possesses an additional hair along with the conventional mass, charge and spin. In this article we show that, this new class of black holes can act as a cosmic 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 an arbitrary high value in the vicinity of the 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 obtained are interesting from an astrophysical perspective. © 2021 IOP Publishing Ltd.
Ruppeiner geometry, reentrant phase transition, and microstructure of Born-Infeld AdS black hole
(American Physical Society, 2021) Naveena Kumara, A.; Ahmed Rizwan, C.L.; Hegde, K.; Ajith, K.M.; Ali, M.S.
Born-Infeld AdS black hole exhibits a reentrant phase transition for certain values of the Born-Infeld parameter b. This behavior 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 transitions. 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 characterized 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. © 2021 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.