arXiv subjects

Piyabut Burikham

Publications and source records attributed to Piyabut Burikham.

At least 19 recordsLinked to original sources

arXiv2026

New Universal Relations for Magnetized Neutron Stars

Unlike the neutron-star mass--radius relation, which depends sensitively on the internal stellar structure through the equation of state of dense nuclear matter, certain neutron-star properties obey approximately universal relations that exhibit only weak dependence on the equation of state. In this paper, we explore new universal relations for magnetized neutron stars. We focus on a purely poloidal dipolar magnetic-field configuration and treat the effects of the magnetic field perturbatively to second order in the field strength. After appropriately normalizing the relevant quantities by the stellar mass and the magnetic-field strength at the pole, we first identify an exact universal relation between the magnetic dipole moment and stellar compactness. We then find approximate universal relations among the magnetic dipole moment, magnetically-induced stellar quadrupole moment, and ellipticity, with fractional variations due to the equation of state at the level of O(10%). We support these numerical findings with analytic estimates for Newtonian polytropes, which reproduce the observed behavior. Among the newly discovered relations, the one connecting the magnetic dipole moment and stellar quadrupole moment may be particularly useful for the analysis of gravitational-wave signals from binaries containing magnetized neutron stars.

gr-qc↗

arXiv2026

Holographic Thermodynamics of Dyonic Dilaton AdS Black Holes

We investigate the holographic dual of the extended bulk thermodynamics of dyonic dilaton AdS black holes by allowing both the cosmological constant and Newton's gravitational constant to vary. In the dual CFT thermodynamics, the central charge $C$ and chemical potential $\mu$ as its conjugate enter the thermodynamic relations, in addition to $(\tilde{T},\tilde{S})$, $(\tilde{Q},\tilde{\Phi})$, $(\tilde{P},\tilde{\Psi})$, $(\tilde{\mathcal{P}},\tilde{V})$. We consider sixteen different ensembles related to those five pairs of thermodynamic variables which we separate into fixed volume and pressure ensembles. Due to the symmetry on ($\tilde{Q},\tilde{P}$), we only need to analyze five ensembles each in fixed volume and pressure ensembles. In the fixed volume ensembles, it is found that the fixed ($\tilde{Q},\tilde{P},\tilde{V},\mu$) and ($\tilde{\Phi},\tilde{P},\tilde{V},\mu$) exhibit a zeroth-order phase transition. Interestingly, the fixed ($\tilde{\Phi},\tilde{\Psi},\tilde{V},C$) ensemble shows the zeroth- and first-order phase transitions at above and below critical potential $\tilde{\Upsilon}_c$, respectively. In the fixed pressure ensembles, it is found that richer structures appear where the sign of $\mu$ heavily influences phase space for several ensembles. For fixed ($\tilde{Q},\tilde{P},\tilde{\mathcal{P}},\mu$) and ($\tilde{\Phi},\tilde{P},\tilde{\mathcal{P}},\mu$), they show the zeroth-, first-, and second-order phase transitions. For the fixed ($\tilde{\Phi},\tilde{\Psi},\tilde{\mathcal{P}},\mu$) ensemble, there is only a zeroth-order phase transition occurs. The other ensembles which we do not mention eventually do not show any phase transition. These findings provide deeper insight into how bulk gravitational variations, particularly regarding the cosmological and gravitational constants, translate to exact critical phenomena and richer phase structures within CFT.

hep-th↗

arXiv2026

Entropy Quantization and Quasi-normal Modes of Dyonic Kerr-Sen Black Holes

We explore the properties of inner and outer horizon thermodynamics of dyonic Kerr-Sen black hole (DKSBH). It is observed that the entropy (or area) product is universal, depending only on the angular momentum of the BH. We then proceed to study the dual conformal field theory~(CFT) in the Kerr/CFT correspondence using thermodynamic relations and compute the central charges from 2D CFT. The central charges are found to be universal with only angular momentum dependence. By comparing to Kerr-Newman BH, it is found that the essential difference is in the right-moving sector of the CFT. Interestingly, we can then explicitly produce the non-vanishing central charges related to its static solution, the dyonic dilaton BH, using the thermodynamic method. Moreover, from the CFT relations to multi-horizon thermodynamics, we find the analytical expression of the quasi-normal modes~(QNM) spectra in terms of BH parameters. This QNM computation supports the Bekenstein-Hod conjecture on the BH's entropy quantization.

hep-th↗

arXiv2026

Quasi-bound States of Scalar field inside the Dyonic Kerr-Sen Black Hole

We found sets of exact analytic quasi-stationary states of a massive scalar field in a dyonic Kerr-Sen black hole~(DKSBH) background in the maximally extended spacetime region. A central novelty is the use of horizon-regular ingoing Eddington-Finkelstein coordinates, which enables a direct and unambiguous imposition of the ingoing boundary condition at the horizon. The exact radial solutions are in the form of confluent Heun functions. Imposing regularity at spatial infinity enforces a series truncation condition, yielding an exact quantization of the quasi-stationary frequencies. The spectrum exhibits a rich multi-branch structure, which we show splits into two distinct classes: modes that are insensitive to the black hole spin and charges and modes that explicitly depend on them. We uncover a clear asymmetry between co-rotating and counter-rotating configurations, driven by the spin-angular momentum coupling, as well as a systematic shift of the spectrum induced by electric and magnetic charges. The physical branches exhibit a universal behavior: modes with positive real frequency possess positive imaginary parts and therefore grow exponentially in time, whereas modes with negative real frequency are damped and decay. This suggests that positive-energy excitations in the region behind the outer horizon including the inner region of the inner horizon which contains the closed-timelike-curve, exponentially destabilize the background spacetime, supporting Hawking's chronology protection conjecture. In addition, the purely imaginary modes contain no oscillatory component and hence do not propagate through the spacetime, preventing traveling excitations along closed timelike curves and remaining consistent with the conjecture.

hep-th↗

arXiv2026

Thermodynamics and Geometrical Optics of Reissner Nordstrom de Sitter Black Holes in Noncommutative Geometry

We investigate the thermodynamic, optical, and dynamical properties of Reissner-Nordstrom-de Sitter black holes in a noncommutative spacetime with a minimal length scale Theta. Within a two-horizon framework, we formulate an effective first law of thermodynamics and introduce an entropy capturing correlations between the event and cosmological horizons. Imposing the lukewarm condition, where both horizons share a common temperature, uniquely determines the entropy correction and yields closed-form expressions for thermodynamic quantities. The analysis reveals a noncommutativity-induced second-order phase transition, emphasizing the role of short-distance structure. On the optical side, we study photon motion and weak gravitational lensing, showing that noncommutativity modifies the effective potential and critical impact parameter. Using the Gauss-Bonnet method, we derive the weak deflection angle and analyze the effects of charge and cosmological constant. We further connect geometry and dynamics through the Lyapunov exponent and quasinormal modes, showing systematic impacts on orbital instability and damping.

hep-th↗

arXiv2026

Theory space and stability analysis of General Relativistic cosmological solutions in modified gravity

Some aspects of two General Relativistic cosmological solutions, an exact $Λ$CDM-like cosmological solution $j=1$ ($j$ is cosmographic jerk parameter), and a specifically designed toy cosmological solution $j=1+3\varepsilon(q-1/2)$ ($q$ is cosmographic deceleration parameter, $0<|\varepsilon|<1$) that is capable of accommodating a phantom crossing scenario as suggested by DESI DR2, are studied within the context of $f(R)$ gravity, by portraying them as a \emph{flow} in the 2-dimensional \emph{theory space} spanned by the quantities $r=\frac{R f'}{f}, m=\frac{R f''}{f'}$. For the $f(R)$ theories exactly reproducing a background $Λ$CDM-like expansion history $j=1$, it is shown by means of a \emph{cosmographic} reconstruction approach that the curvature degree of freedom need not necessarily behave like an effective cosmological constant, and that cosmologies under different possible such theories lead to different possible values of $Ω_{m0}$. With the theory space analysis, it is also shown that $Λ$CDM-mimicking $f(R)$ cosmologies that asymptote to General Relativistic $Λ$CDM in the limit $q\to1/2$, are prone to instability under small homogeneous and isotropic perturbation, casting a doubt on achieving an exact $Λ$CDM-like cosmological solution $j=1$ within $f(R)$ gravity. Regarding the toy cosmological solution $j=1+3\varepsilon(q-1/2)$ that is capable of accommodating a phantom crossing scenario, it is shown that possible underlying $f(R)$ theories that admit it as a solution are inevitably plagued by tachyonic instability ($f''(R)<0$). All the above physically interesting conclusions are derived without explicitly reconstructing, even numerically, the functional form of the underlying $f(R)$, which demonstrates the edge of the $r$-$m$ theory space analysis over the traditional explicit reconstruction approach.

gr-qc↗

arXiv2025

Slowly Rotating and Tidal Deformation of Nonlocal Modified Tolman VII Star

We investigate the moment of inertia, quadrupole deformation, and tidal deformation within the framework of nonlocal gravity, utilizing the exact modified Tolman-VII (NEMTVII) density model with an isotropic perfect fluid. The Love number~$(k_{2})$ is derived using standard even-parity perturbation theory. Additionally, we explore the observational implications by analyzing the tidal deformability parameter~$( λ_{\textrm{tid}} )$ in comparison with the constraints from GW170817, GW190425, PSR J0348+0432, and PSR J0740+6620. We found that the results are consistent with the tidal constraint when $α\gtrsim 1.6$ with the small $β$. For slowly rotating object, the dimensionless moment of inertia~$( \bar{I} )$, rotational Love parameter~$( \barλ_{\textrm{rot}} )$, and quadrupole moment~$( \bar{Q} )$ are fully determined by the perturbed metric. Our findings reveal that the nonlocal parameter~$( β)$ significantly affects the star radius. For a fixed $β$ and varying $α$, the $I$-Love-$Q$ relations are found to be universal. For varying $β$, the $I$-Love-$Q$ relations become non-universal.

gr-qc↗

arXiv2025

The effective phase space and $e$-folding of the Starobinsky and extended Starobinsky model of inflation

For zero spatial curvature, cosmological phase space of Starobinsky and extended Starobinsky inflationary model show three apparent attractors; the fixed angle attractor in the large field limit, the final attractor representing reheating phase in the small field region, and the apparent attractor corresponding to the slow-roll condition connecting between the large-field and small-field region. To consider the total $e$-folding likelihood of the model, Remmen-Carroll conserved measure is constructed and normalized. Using the measure, the total e-folding number $N$ and its expectation value $\left\langle N \right\rangle$ are calculated. Our results show that most classical slow-roll trajectories which intersect the Planck surface have $N<60$, and $ϕ_{\rm UV}>5.5 M^{*}_{\rm Pl}$ is required for $N>60$. It is found that for $ϕ_{\rm UV}\in [5.22,5.50]M^{*}_{\rm Pl}$ which satisfies the constraint on the spectral index, $n_s = 0.9658 \pm 0.0040\,\,\,(68\%\,\,{\rm CL})$, the expectation value $\langle N \rangle \simeq 3.5 - 4$ for trajectories intersecting the Planck surface in the Starobinsky model. For extended Starobinsky model with additional $R^3$ term parametrized by a coupling parameter $α$, the expectation value when inflation starts from the top of the potential shifts to $\left\langle N \right\rangle = 4.025,4.336$ for $α= 10^{-4},6.5\times 10^{-5}$ respectively. In the Starobinsky model even at very large inflaton cutoff $ϕ_{\rm UV}$ where the field value is super-Planckian, the energy density from the (saturating) inflaton potential and the Hubble parameter are still sub-Planckian and therefore the inflation occurs within the semi-classical regime.

gr-qc↗

arXiv2025

Scalar Instabilities Inside The Extremal Dyonic Kerr-Sen Black Hole: Novel Exact Solutions and Chronology Protection Conjecture

We investigate the stability of test scalar fields in the region inside the extremal Dyonic Kerr-Sen black hole (DKSBH) horizon, where closed timelike curves exist. We successfully find and present the novel exact solutions to the Klein-Gordon equation in the extremal DKSBH spacetime in terms of the Double Confluent Heun functions. The spacetime stability is explored by investigating the scalar's quasiresonance~(QS) frequencies obtained from polynomial condition of the Double Confluent Heun function. We found that both massive and massless scalar quasiresonances are double branched, having purely positive and negative imaginary frequencies, therefore, do not propagate, prohibiting time travel and suggesting no violation of Hawking's Chronology Protection Conjecture (CPC). However, only the positive branch with $0\leqΩ_0<2(n+1)$ and negative branch with $Ω_0>2(n+1)$ that grow exponentially has the ability to destroy spacetime. Remarkably, a new mass scale $M_{p}^{2}/M$, where $M$ is the black hole mass, is found to play a crucial role. The QS zeroth modes flip sign between purely damping and purely growing when the scalar mass is at this mass scale.

gr-qc↗

arXiv2025

Revisiting Chronology Protection Conjecture in The Dyonic Kerr-Sen Black Hole Spacetime

The Chronology Protection Conjecture (CPC) was first introduced by Hawking after his semi-classical investigation to the behaviour of a spacetime with closed timelike curves (CTCs) in response to scalar perturbation. It is argued that there would be instabilities leading to amplification of the perturbation and finally causing collapse of the region with CTCs. In this work, we investigate the CPC by exactly solve the Klein-Gordon equation in the region inside the inner horizon of the non-extremal Dyonic Kerr-Sen~(KS) black hole, where closed timelike curves exist. Successfully find the exact radial solution, we apply the polynomial condition that turns into rule of the energy quantization. The quasinormal modes~(QNMs) of the scalar fields in the region inside the inner horizon of the rotating black hole with nonzero energy have only positive imaginary part describing states that grow in time. The exponentially growing modes will backreact and deform the spacetime region where CTC exists. The CPC is proven to be valid in the Dyonic Kerr-Sen black hole spacetime. Moreover, since the Dyonic Kerr-Sen black hole is the most general axisymmetric black hole solution of the string inspired Einstein-Maxwell-dilaton-axion (EMDA) theory, the semiclassical proof in this work is also valid for all simpler rotating black holes of the EMDA theory. The structure of the Dyonic KS spacetime distinctive from the Kerr-Newman counterpart is also explored.

gr-qc↗

arXiv2024

Observational Constraints on Extended Starobinsky and Weyl Gravity Model of Inflation

We present constraints on the extended Starobinsky and Weyl gravity model of inflation using updated available observational data. The data includes cosmic microwave background (CMB) anisotropy measurements from Planck and BICEP/Keck 2018 (BK18), as well as large-scale structure data encompassing cosmic shear and galaxy autocorrelation and cross-correlation functions measurements from Dark Energy Survey (DES), baryonic acoustic oscillation (BAO) measurements from 6dF, MGS and BOSS, and distance measurements from supernovae type Ia from Pantheon+ samples. By introducing a single additional parameter, each model extends the Starobinsky model to encompass larger region of parameter space while remaining consistent with all observational data. Our findings demonstrate that the inclusion of higher-order terms loosen the constraint on the upper bound of $e$-folding number $N_{\rm e}$ due to the presence of small additional parameter. The maximum limit on $N_{\rm e}$ could be refined by considering the reheating process to $N_{\rm e}<55-59$ for $k_{*}=0.002, 0.05$ Mpc$^{-1}$. These models extend viable range of tensor-to-scalar ratio~($r$) to very small value $r<0.002$ in contrast to the original $R^2$ Starobinsky model. In addition, our results continue to emphasize the tension in $H_0$ and $S_8$ between early-time CMB measurements and late-time large-scale structure observations.

astro-ph.CO↗

arXiv2024

The Exact Relativistic Scalar Quasibound States of The Dyonic Kerr-Sen Black Hole: Quantized Energy, and Hawking Radiation

We consider Klein-Gordon equation in the Dyonic Kerr-Sen black hole background, which is the charged rotating axially symmetric solution of the Einstein-Maxwell-Dilaton-Axion theory of gravity. The black hole incorporates electric, magnetic, dilatonic and axionic charges and is constructed in 3+1 dimensional spacetime. We begin our investigations with the construction of the scalar field's governing equation, i.e., the covariant Klein-Gordon equation. With the help of the ansatz of separation of variables, we successfully separate the polar part, and find the exact solution in terms of Spheroidal Harmonics, while the radial exact solution is obtained in terms of the Confluent Heun function. The quantization of the quasibound state is done by applying the polynomial condition of the Confluent Heun function that gives rise to discrete complex-valued energy levels for massive scalar fields. The real part is the scalar field relativistic quantized energy, while the imaginary part represents the quasibound states's decay. We present all of the sixteen possible exact energy solutions for both massive and massless scalars. We also present the investigation the Hawking radiation of the Dyonic Kerr-Sen black hole's apparent horizon, via the Sigurd-Sannan method by making use of the obtained exact scalar wave functions. The radiation distribution function, and the Hawking temperature are successfully obtained.

gr-qc↗

arXiv2024

Dual CFT on Nariai limit for Kerr-Sen-dS black holes

In this work, we study the Kerr-Sen-de Sitter black hole~(BH) in the Nariai limit where the event and cosmological horizon coincide. We show that the near-horizon Kerr-Sen-de Sitter black hole in Nariai limit is a fiber over AdS$_2$ with an appropriate coordinate transformation, instead of fiber over dS$_2$. Hence, we can compute the associated central charge and CFT temperature by using the Kerr/CFT method. It is remarkably exhibited that through Cardy's growth of states, the Bekenstein-Hawking entropy on cosmological horizon is reproduced. Moreover, we show that the radial equation of the quantum scalar field in $J$- and $Q$-pictures on this charged rotating background in Nariai limit can be portrayed in quadratic Casimir operator form with $SL(2,R)\times SL(2,R)$ isometry. We also compute the corresponding thermodynamic quantities from CFT to find the absorption cross-section and real-time correlator in $J$-picture. In $Q$-picture, we do not find a well-defined CFT description. We then extend the study of quantum scalar field in Nariai limit for Kerr-Newman-dS black hole solution and show that the hidden conformal symmetry on this black hole's background in in $J$- and $Q$-pictures is well-defined.

hep-th↗

arXiv2024

Thermodynamics of the Weyl Geometric Gravity Black Holes

We consider the thermodynamic properties of an exact black hole solution obtained in Weyl geometric gravity theory, by considering the simplest conformally invariant action, constructed from the square of the Weyl scalar, and the strength of the Weyl vector only. The action is linearized in the Weyl scalar by introducing an auxiliary scalar field, and thus it can be reformulated as a scalar-vector-tensor theory in a Riemann space, in the presence of a nonminimal coupling between the Ricci scalar and the scalar field. In static spherical symmetry, this theory admits an exact black hole solution, which generalizes the standard Schwarzschild-de Sitter solution through the presence of two new terms in the metric, having a linear and a quadratic dependence on the radial coordinate, respectively. The solution is obtained by assuming that the Weyl vector has only a radial component. After studying the locations of the event and cosmological horizons of the Weyl geometric black hole, we investigate in detail the thermodynamical (quantum properties) of this type of black holes, by considering the Hawking temperature, the entropy, specific heat and the Helmholtz free energy functions on both the event and the cosmological horizons. The Weyl geometric black holes have thermodynamic properties that clearly differentiate them from similar solutions of other modified gravity theories. The obtained results may lead to the possibility of a better understanding of the properties of the black holes in alternative gravity, and of the relevance of the thermodynamic aspects in black hole physics.

gr-qc↗

arXiv2023

Gravitational Perturbation in Nonlocal Modified Tolman VII Model

In comparison to the original Tolman VII model, Exact Modified Tolman VII (EMTVII) with one additional parameter can increase the compactness of compact object. When the compactness is in the ultracompact regime, the quasinormal modes~(QNMs) of the trapped mode as well as the gravitational echoes become more viable. Starting with the EMTVII model, we introduce nonlocality into the matter sector and analyze the effective potential, the QNMs, and the gravitational echoes of the compact and ultracompact object in the nonlocal model. The nonlocal gravity version of EMTVII~(NEMTVII) is parametrized by the nonlocal parameter~($ β$), modified Tolman VII parameter ($ α$), and the compactness ($ \mathcal{C}$). It is found that the nonlocal profile produces the smeared surface and consequently reduce the compactness. The maximum compactness $\mathcal{C}_{max}=0.4$ occurs when $α=0=β$, i.e., EMTVII with no smearing. For relatively small value of $β= 0.01$ and the compactness $ \mathcal{C} \lesssim 0.2667$~(with $M=2.14$ solar masses, $R=11.835$ km at $α=1.4$), the causality condition and the dominant energy condition~(DEC) are satisfied. The quasinormal modes of the gravitational perturbation are calculated using Bohr-Sommerfeld (BS) fitting and we find that the nonlocality produces less trapped modes than the original (EMTVII) counterpart. At high compactness, gravitational echoes are simulated numerically. Echoes are found to exist in the parameter space where the dominant energy condition and the causality condition are violated.

gr-qc↗

arXiv2023

Reply to Comment on "Dark matter as a Weyl geometric effect"

In a recent Comment on the paper "Dark matter as a Weyl geometric effect", by Burikham et al., Phys. Rev. D 107, 064008 (2023), posted on arxiv. org as eprint arXiv:2306.11926, it was claimed that the exact solution found in the above mentioned paper by Burikham et al. "is wrong". In this Reply to the Comment we present, in a clear and comprehensive way, a step by step derivation of the exact solution of the vacuum static spherically symmetric field equations of the Weyl geometric gravity theory, and we show that, contrary to the claims in arXiv:2306.11926, the obtained solution is correct, and it satisfies all the equations of motion of the basic theory. Hence, it can be considered as a viable alternative model for the explanation of the behavior of the galactic rotation curves, without invoking the presence of dark matter.

gr-qc↗

arXiv2023

Dark matter as a Weyl geometric effect

We investigate the possibility that the observed behavior of test particles outside galaxies, which is usually explained by assuming the existence of dark matter, is the result of the dynamical evolution of particles in a Weyl type geometry, and its associated conformally invariant Weyl geometric quadratic gravity. As a first step in our investigations we write down the simplest possible conformally invariant gravitational action, constructed in Weyl geometry, and containing the Weyl scalar, and the strength of the Weyl vector only. By introducing an auxiliary scalar field, the theoretical model can be reformulated in the Riemann geometry as scalar-vector-tensor theory, containing a scalar field, and the Weyl vector, respectively. The field equations of the theory are derived in the metric formalism, in the absence of matter. A specific static, spherically symmetric model, in which the Weyl vector has only a radial component, is considered. In this case, an exact analytic solution of the gravitational field equations can be obtained. The behavior of the galactic rotation curves is also considered in detail, and it is shown that an effective geometric mass term, with an associated density profile, can also be introduced. Three particular cases, corresponding to some specific functional forms of the Weyl vector, are also investigated. A comparison of the model with a selected sample of galactic rotation curves is also performed when an explicit breaking of conformal invariance is introduced, which allows the fix of the numerical values of the free parameters of the model. Our results show that Weyl geometric models can be considered as a viable theoretical alternative to the dark matter paradigm.

gr-qc↗

arXiv2023

Normal and Quasinormal Modes of Holographic Multiquark Star

The quadrupole normal-mode oscillation frequency $f_{n}$ of multiquark star are computed for $n=1-5$. At the transition from low to high density multiquark in the core region, the first 2 modes jump to larger values, a distinctive signature of the presence of the high-density core. When the star oscillation couples with spacetime, gravitational waves~(GW) will be generated and the star will undergo damped oscillation. The quasinormal modes~(QNMs) of the oscillation are computed using two methods, direct scan and WKB, for QNMs with small and large imaginary parts respectively. The small imaginary QNMs have frequencies $1.5-2.6$ kHz and damping times $0.19-1.7$ secs for multiquark star with mass $M=0.6-2.1 M_{\odot}$~(solar mass). The WKB QNMs with large imaginary parts have frequencies $5.98-9.81$ kHz and damping times $0.13-0.46$ ms for $M\simeq 0.3-2.1 M_{\odot}$. They are found to be the fluid $f-$modes and spacetime curvature $w-$modes respectively.

gr-qc↗