arXiv subjects

Ratchaphat Nakarachinda

Publications and source records attributed to Ratchaphat Nakarachinda.

At least 19 recordsLinked to original sources

arXiv2026

Perturbations and stability of black holes with static scalar hair in general GLPV theories

We derive the background equations and complete odd- and even-parity quadratic actions for static, spherically symmetric black holes with radial scalar hair in general quartic-quintic Gleyzes-Langlois-Piazza-Vernizzi (GLPV) theories, including Horndeski. On regular, nondegenerate branches, the formulation applies across Killing horizons and yields local no-ghost conditions and radial and angular eikonal characteristics. Tensor modes in both parity sectors share the same squared radial speed. When the radial coordinate is timelike, generic antisymmetric mixing induces nonstandard large-multipole scaling for both even-parity angular branches. Requiring both branches to have finite, nonzero eikonal phase speeds selects the Horndeski-related compatibility condition between the quartic and quintic beyond-Horndeski functions. This condition aligns the covariant degeneracy directions and permits a local disformal map to Horndeski when regular and invertible. For a finite, nonzero scalar kinetic term at the horizon, every nontrivial branch of the exact quadratic GLPV black hole is locally unstable or has a degenerate tensor cone near a simple outer horizon. Apart from identified exceptions, this obstruction extends to general shift- and reflection-symmetric quadratic GLPV theories and the regular Horndeski-related branch of the quartic-quintic class. Finally, for scalar-Gauss-Bonnet black holes with a vanishing horizon scalar kinetic term, we construct analytic power-law quartic beyond-Horndeski deformations whose associated quintic function is fixed by the same condition. At sufficiently small coupling, all local no-ghost and high-frequency gradient-stability conditions hold throughout the exterior. For a linear Gauss-Bonnet coupling, we estimate an interior scale below which the background and stability expansions lose perturbative control; this does not imply a physical instability.

gr-qc↗

arXiv2026

Extended Thermodynamics and Renyi Entropy Beyond Fixed Central Charge

An outstanding problem in the framework of conformal thermodynamics concerns the interpretation of variations in the central charge $C$. In this paper, we construct a novel central-charge Rényi entropy via the Casini-Huerta-Myers (CHM) map by considering thermal CFTs on a hyperbolic cylinder within a fixed charge, field theory volume and central charge potential $(\tilde{Q},\mathcal{V},μ_C)$ grand canonical ensemble. We demonstrate that the resulting entropy satisfies all four fundamental Rényi entropy inequalities throughout the admissible range of $μ_C$, establishing its consistency as a genuine Rényi measure. Physically, this novel measure extends conventional Rényi entropy by capturing the degree of entanglement across a statistical ensemble of holographic CFTs with fluctuating degrees of freedom. Furthermore, our conformal thermodynamic analysis of near-extremal configurations reveals that residual entropy arises from the central charge sector rather than thermal excitations. The mass gap that separates the extremal state and the first thermal excitation introduces a characteristic temperature scale $\tilde{T}_*$, which translates via the CHM map into a distinguished characteristic Rényi index $n_*$. Crucially, we propose that $n_*$ separates the theory space into two qualitatively distinct statistical regimes: a dominant-theory regime ($n > n_*$) governed by the most probable CFT realizations, and a multi-theory regime ($n < n_*$) where a broader spectrum of fluctuating theories and higher-energy modular excitations becomes increasingly relevant.

hep-th↗

arXiv2026

Black Hole Thermodynamics via Tsallis Statistical Mechanics and Phase Transitions Probed by Optical Characteristics

We develop a non-extensive thermodynamic framework for Reissner--Nordström black holes based on a near-horizon photon-gas model within Tsallis statistics. We derive the generalized Bekenstein--Hawking entropy based on such an approach, consistent with the Bekenstein--Hawking area law in the extensive limit, $q \rightarrow 1$. The induced deformation gives rise to a rich thermodynamic structure consisting of small, intermediate, and large black-hole branches, exhibiting Van der Waals-like phase transitions characterized by mean-field critical exponents. We further establish an optical--thermodynamic analogy by relating photon-sphere observables, including orbital periods and Lyapunov exponents, to thermodynamic variables. These optical signatures qualitatively track the thermodynamic critical behavior and phase structure, suggesting their potential relevance as observational probes in future high-resolution measurements. These results may shed light on a conceptual connection between non-extensive entropy, black-hole critical phenomena, and strong-gravity optics.

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arXiv2025

Schwarzschild-de Sitter black hole as a correlated qubit system via entropic identification

The thermodynamic behaviours of multi-horizon black holes such as a Schwarzschild-de Sitter black hole have been one of the long-standing mysteries in gravitational physics since they involve quantum natures in gravitational systems and that the search for quantum gravity has not reached its conclusion. In this work, we seeked for a possibility of realising the Schwarzschild-de Sitter black hole as a correlated qubit system, where each of the event horizon is treated as a qubit and both of them are correlated in a way that two qubits could be. By identifying the entropies of subsystems to those of qubits, we successfully constructed the reduced density matrices of the two subsystems as well as the density matrix for the Schwarzschild-de Sitter black hole, modelled as 2-correlating qubits. Moreover, our results suggested that when the gravitational effect has its role in the qubit systems, supposedly like black holes, the correlation between qubits are constrained with a lower bound more stringent than the so-called Araki-Lieb triangle inequality.

gr-qc↗

arXiv2025

Greybody Factor for massive scalar field in charged black hole

The greybody factor of a massive scalar field in the Reissner-Nordström black hole is investigated using the Wentzel-Kramers-Brillouin (WKB) approximation and rigorous bound methods. We found that the transmission probability and behavior of the potential are directly related in such a way that the higher the potential, the lower the greybody factor. Both methods achieve a similar conclusion, which states that the graybody factor and the mass of the scalar field have an inverse relationship. This can be interpreted in a similar way in quantum mechanics, namely the scalar field with the higher mass will encounter a stronger interaction from the potential and then it is more difficult to penetrate through the potential barrier. The rigorous bound has the advantage of not only being possible to calculate analytically, but also being applicable to a wider range of parameter values compared to the standard WKB approximation.

gr-qc↗

arXiv2025

Greybody factors of charged black holes with axion hair

We study the greybody factor of charged hairy black holes (BHs) that arise due to the presence of an axion coupled to the electromagnetic field. Specifically, we consider spin-0 and spin-1 test particles propagating in the background of BHs with axion hair, where the spacetime geometry is modified compared to that of the Reissner-Nordstrom (RN) BH. In contrast to the RN solution, with a given total BH charge, the effective potential for test particles depends on the ratio of electric to magnetic charges. In other words, charged BHs with axion hair breaks the electric-magnetic duality present in the RN solution. We compute the transmission coefficient of test particles plunging into the charged hairy BH and find that the deviation from the RN solution is particularly evident for higher multipole moments. Precise measurements of greybody factors can thus serve as probes for the possible existence of axions coupled to the electromagnetic field, as well as potential signatures of magnetic monopoles.

gr-qc↗

arXiv2025

Black Hole Thermodynamics via Tsallis Statistical Mechanics

An investigation of black hole thermodynamics based on Tsallis statistical mechanics is explored through the study of the thermodynamics of a gas system located near the horizon of a black hole. In spite of the difficulty in exploring black hole thermodynamics through statistical mechanics, the entropy of the nearby gas system is found to be proportional to the black hole's horizon area using Gibbs-Boltzmann statistical mechanics. This allows us to study black hole thermodynamics by using statistical mechanics through the thermodynamic behaviors of the gas system. Since the entropy of the black hole is proportional to the horizon area, it is more suitable to use non-extensive statistical mechanics instead of the usual Gibbs-Boltzmann ones. In this work, the black hole entropy is derived based on Tsallis statistical mechanics, one of well-known non-extensive statistical mechanics. It is found that the black hole entropy gets a modification due to non-extensivity. By using such an entropy, the black hole can be stabilized due to the non-extensivity, and the bound on the non-extensive parameter is also determined.

gr-qc↗

arXiv2025

$q$-Equilibrium of Gas in Spacetime of Multi-horizon Black Holes

We investigate the possibility of describing the thermal system with different temperatures for a black hole with multiple horizons. The black hole with two horizons such as Schwarzschild-de Sitter black hole corresponds to two thermal systems with generically different temperatures. Then, it is not suitable to describe these systems with equilibrium thermodynamics corresponding to Gibbs-Boltzmann kinetic theory. In the present work, we investigate such thermal systems by using hydrostatic equilibrium thermodynamics. Assuming that the gas between the horizons obeys the Tsallis statistical mechanics, we found that it is possible to obtain the temperature gradient for the classical gas. Interestingly, the gas behaves as classical gas near the horizon and behaves like quantum gas around flat spacetime with constant temperature. As a result, the multi-horizon black holes in hydrostatic equilibrium can be in a stable configuration with the aspect of the $q$-kinetic theory.

gr-qc↗

arXiv2025

Thermodynamics of Black Holes with Rényi Entropy from Classical Gravity

The nonextensive nature of black holes is one of the most intriguing discoveries. In fact, the black hole entropy is a nonextensive quantity that scales by its surface area at the event horizon. In our work, we extend the thermodynamic phase space of black holes by treating the nonextensive parameter of the Rényi entropy as the thermodynamic variable. Using Euler's theorem for a homogeneous function of the black holes' mass, the compatible Smarr formula and the first law of black hole thermodynamics can be obtained. It is also demonstrated that, by keeping the same form of the black hole mass, the Rényi temperature is straightforwardly defined as proposed in the literature. Since many different types of black holes can indeed be successfully treated with such a procedure, our consideration is fairly general. It is worthwhile to argue that the black hole thermodynamics in Rényi statistics is rooted from the relation among geometric quantities in the same way as the standard approach corresponding to the Gibbs-Boltzmann statistics. Even though our results are based on classical gravity, they may pave the way to derive the Rényi temperature using the notion of quantum field in curved spacetime.

gr-qc↗

arXiv2025

Thermodynamic Stability of Schwarzschild-de Sitter Black holes with Rényi entropy

Even though, classically, a black holes is gravitational object, it can be treated as a thermodynamic object when quantum effect is taken into account. It is found that by using Gibbs-Boltzmann entropy, thermodynamic system associated Schwarzschild-de Sitter black hole is unstable while it is stable under description of Rényi entropy. According to Rényi entropy, the thermodynamic phase space needed to be extended. Specifically, the non-extensive parameter must be treated as a thermodynamic variable. In this work, we investigate the thermodynamic stability of Schwarzschild-de Sitter black hole with Rényi entropy by treating non-extensive parameter as chemical potential. We found that it is possible to obtain the stability of the black hole under the process with fixing pressure, temperature and number of particles. We also found that there exist the phase transitions from the hot gas to the stable black hole in such the process. Therefore, if such a black hole is possibly observed, the non-extensive effect of Rényi statistics may provide a physical insight beyond the standard approach to black hole thermodynamics.

gr-qc↗

arXiv2024

Rényi Holographic Dark Energy

In this work, the holographic dark energy model is constructed by using the non-extensive nature of the Schwarzschild black hole via the Rényi entropy. Due to the non-extensivity, the black hole can be stable under the process of fixing the non-extensive parameter. A change undergoing such a process would then motivate us to define the energy density of the Rényi holographic dark energy (RHDE). As a result, the RHDE with choosing the characteristic length scale as the Hubble radius provides the late-time expansion without the issue of causality. Remarkably, the proposed dark energy model contains the non-extensive length scale parameter additional to the standard $Λ$CDM model. The cosmic evolution can be characterized by comparing the size of the Universe to this length scale. Moreover, the preferable value of the non-extensive length scale is determined by fitting the model to recent observations. The results of this work would shed light on the interplay between the thermodynamic description of the black hole with non-extensivity and the classical gravity description of the evolution of the Universe.

gr-qc↗

arXiv2023

Cosmology in theories with spontaneous scalarization of neutron stars

In a model of spontaneous scalarization of neutron stars proposed by Damour and Esposite-Farese, a general relativistic branch becomes unstable to trigger tachyonic growth of a scalar field $ϕ$ toward a scalarized branch. Applying this scenario to cosmology, there is fatal tachyonic instability of $ϕ$ during inflation and matter dominance being incompatible with solar-system constraints on today's field value $ϕ_0$. In the presence of a four-point coupling $g^2 ϕ^2 χ^2/2$ between $ϕ$ and an inflaton field $χ$, it was argued by Anson et al. that a positive mass squared heavier than the square of a Hubble expansion rate leads to the exponential suppression of $ϕ$ during inflation and that $ϕ_0$ can remain small even with the growth of $ϕ$ after the radiation-dominated epoch. For several inflaton potentials approximated as $V(χ)=m^2 χ^2/2$ about the potential minimum, we study the dynamics of $ϕ$ during reheating as well as other cosmological epochs in detail. For certain ranges of the coupling $g$, the homogeneous field $ϕ$ can be amplified by parametric resonance during a coherent oscillation of the inflaton. Incorporating the backreaction of created particles under a Hartree approximation, the maximum values of $ϕ$ reached during preheating are significantly smaller than those obtained without the backreaction. We also find that the minimum values of $g$ consistent with solar system bounds on $ϕ$ at the end of reheating are of order $10^{-5}$ and hence there is a wide range of acceptable values of $g$. Thus, the scenario proposed by Anson et al. naturally leads to the viable cosmological evolution of $ϕ$ consistent with local gravity constraints, without modifying the property of scalarized neutron stars.

gr-qc↗

arXiv2022

Holographic Dark Energy from the Anti-de Sitter Black Hole

The anti-de Sitter (AdS) black hole plays an important role in the holographic principle. In this study, the upper bound in energy corresponding to the mass of the Schwarzschild black hole is modified to be that of the AdS black hole. Via the correspondence between the ultraviolet (UV) and infrared (IR) cutoffs, the constant term in the energy density of the holographic dark energy (HDE) can be obtained from the negative cosmological constant from the black hole. Interestingly, the proposed dark energy model could drive the late-time expansion of the Universe without the causality violation. The cosmic evolution is investigated by choosing the Hubble and particle horizons as the IR length scales. It is found that the accelerated expansion at late time can be obtained for both cases. This result may shed light on the connection between the AdS black hole and the de Sitter (dS) spacetime in the context of cosmology.

gr-qc↗

arXiv2022

Thermodynamics of Asymptotically de-Sitter Black Hole in dRGT Massive Gravity from Rényi entropy

The thermodynamic properties of the de Rham-Gabadadze-Tolley (dRGT) black hole in the asymptotically de Sitter (dS) spacetime are investigated by using Rényi entropy. It has been found that the black hole with asymptotically dS spacetime described by the standard Gibbs-Boltzmann statistics cannot be thermodynamically stable. Moreover, there generically exist two horizons corresponding to two thermodynamic systems with different temperatures, leading to a nonequilibrium state. Therefore, in order to obtain the stable dRGT black hole, we use the alternative Rényi statistics to analyze the thermodynamics properties in both the separated system approach and the effective system approach. Interestingly, we found that it is possible concurrently obtain positive pressure and volume for the dRGT black hole while it is not for the Schwarzschild-de Sitter (Sch-dS) black hole. Furthermore, the bounds on the nonextensive parameter for which the black hole being thermodynamically stable are determined. In addition, the key differences between the systems described by different approaches, e.g., temperature profiles and types of the Hawking-Page phase transition are pointed out.

gr-qc↗

arXiv2022

Emergent Phase, Thermodynamic Geometry and Criticality of Charged Black Holes from Rényi Statistics

Recently, a novel emergent phase can occur from thermodynamic consideration of the asymptotically flat Reissner-Nordström black hole (RN-AF) using Rényi statistics. We present an analysis of the thermodynamical and mechanical stabilities of the RN-AF in both the Gibbs-Boltzmann (GB) and the alternative Rényi statistics when charge $q$ and electrostatic potential $ϕ$ are treated as pressure and volume, respectively. Interestingly, the emergent phase of the RN-AF can be both thermodynamically and mechanically stable in some range of parameters in the framework of Rényi thermodynamics. With the construction of the Maxwell equal area law in $q-ϕ$ plane, the coexistence line between the near-extremal black hole phase and the emergent phase can be found in some values of charge which can be associated as the vapor pressure at which the liquid and gas phases coexist. In the aspect of thermodynamic geometry, the microscopic interaction between the black hole microstructures can be repulsive in the Rényi description. This implies that a novel correlation between the microstates of a self-gravitating system could be emerged via the nonextensive nature of long-range interaction systems. Finally, we also investigate the critical phenomena of the RN-AF in Rényi statistics compared to that of the van der Waals (vdW) fluid and find that the critical exponents of the relevant physical quantities of both systems are identical. This implies that both systems are in the same universality class of the phase transition.

hep-th↗

arXiv2022

Thermodynamics of Black String from Rényi entropy in de Rham-Gabadadze-Tolley Massive Gravity Theory

The de Rham-Gabadadze-Tolley (dRGT) black string solution is a cylindrically symmetric and static solution of the Einstein field equation with graviton mass term. For the asymptotically de Sitter (dS) solution, it is possible to obtain the black string with two event horizons corresponding to two thermodynamic systems. The Rényi entropy is one of the entropic forms which is suitable to deal with nonextensive properties of the black string. In this work, we investigated the possibility to obtain a stable black string by using the Rényi entropy in both separated and effective approaches. We found that the nonextensivity provides the thermodynamically stable black string with moderate size in both approaches. The transition from the hot gas phase to the moderate-sized stable black string in the separated/effective description is a first-order/zeroth-order phase transition. The significant ways to distinguish the black string from both approaches are discussed.

gr-qc↗

arXiv2021

Effective thermodynamical system of Schwarzschild-de Sitter black holes from Rényi statistics

It has been known that the Schwarzschild-de Sitter (Sch-dS) black hole may not be in thermal equilibrium and also be found to be thermodynamically unstable in the standard black hole thermodynamics. In the present work, we investigate the possibility to realize the thermodynamical stability of the Sch-dS black hole as an effective system by using the Rényi statistics, which includes the non-extensive nature of black holes. Our results indicate that the non-extensivity allows the black hole to be thermodynamically stable which gives rise to the lower bound on the non-extensive parameter. By comparing the results to ones in the separated system approach, we find that the effective temperature is always smaller than the black hole horizon temperature and the thermodynamically stable black hole in effective approach is always larger than one in separated approach at a certain temperature. There exists only the zeroth-order phase transition from the the hot gas phase to the black hole phase for the effective system while it is possible to have the transition of both the zeroth order and the first order for the separated system.

gr-qc↗

arXiv2020

Quasinormal modes of a massless Dirac field in de Rham-Gabadadze-Tolley massive gravity

The quasinormal modes of a massless Dirac field in the de Rham-Gabadadze-Tolley (dRGT) massive gravity theory with asymptotically de Sitter spacetime are investigated using the Wentzel- Kramers-Brillouin (WKB) approximation. The effective potential for the massless Dirac field due to the dRGT black hole is derived. It is found that the shape of the potential depends crucially on the structure of the graviton mass and the behavior of the quasinormal modes is controlled by the graviton mass parameters. Higher potentials give stronger damping of the quasinormal modes. We compare our results to the Schwarzschild-de Sitter case. Our numerical calculations are checked using Pad$\acute{e}$ approximation and found that the quasinormal mode frequencies converge to ones with reasonable accuracy.

gr-qc↗