arXiv subjects

Md Sabir Ali

Publications and source records attributed to Md Sabir Ali.

At least 37 records · Page 2Linked to original sources

arXiv2022

Stationary black holes and stars in the Brans-Dicke theory with $Λ>0$ revisited

It was shown a few years back that for a stationary regular black hole or star solution in the Brans-Dicke theory with a positive cosmological constant $Λ$, endowed with a de Sitter or cosmological event horizon in the asymptotic region, not only there exists no non-trivial field configurations, but also the inverse Brans-Dicke parameter $ω^{-1}$ must be vanishing. This essentially reduces the theory to Einstein's General Relativity. The assumption of the existence of the cosmological horizon was crucial for this proof. However, since the Brans-Dicke field $ϕ$, couples directly to the $Λ$-term in the energy-momentum tensor as well as $Λ$ acts as a source in $ϕ$'s equation of motion, it seems reasonable to ask : can $ϕ$ become strong instead and screen the effect of $Λ$, at very large scales, so that the asymptotic de Sitter structure is replaced by some alternative, yet still acceptable boundary condition? In this work we analytically argue that no such alternative exists, as long as the spacetime is assumed to be free of any naked curvature singularity. We further support this result by providing explicit numerical computations. Thus we conclude that in the presence of a positive $Λ$, irrespective of whether the asymptotic de Sitter boundary condition is imposed or not, a regular stationary black hole or even a star solution in the Brans-Dicke theory always necessitates $ω^{-1}=0$, and thereby reducing the theory to General Relativity. The qualitative differences of this result with that of the standard no hair theorems are also pointed out.

gr-qc↗

arXiv2022

Rotating regular black holes in AdS spacetime and its shadow

The presence of a photon region around the black hole is an essential feature in receiving the emitted spectrum from the vicinity of the black hole by the distant observer. In this paper, we investigate the optical properties of rotating regular anti-de Sitter (AdS) black holes, which characterized by its mass $(M)$, spin parameter $(a)$, deviation parameter $(k)$ and the cosmological constant $(Λ)$ related to the curvature radius via $(Λ=-3/l^2)$. We derive the complete null geodesic equations of motion and study the unstable circular orbits for an observer at given Boyer-Lindquist coordinates ($r_O,\;\vartheta_O$) in the domain of outer communication and trace the photon rings. For the analytical part of our study, we investigate the observables, namely, shadow radius $(R_s)$ and distortion parameter $(δ_s)$. These rotating regular AdS black holes have smaller shadows when compare to the Kerr and regular spacetimes. We also estimate the energy emission rate of the black hole.

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arXiv2022

Rotating Lee-Wick Black Hole and Thermodynamics

We derive a singular solution for the rotating counterpart of Lee-Wick gravity having a point source in a higher-derivative theory. We critically analyze the thermodynamics of such a thermal system by evaluating mass parameters, angular velocity, and Hawking temperature. The system follows the first law of thermodynamics and leads to the expression of entropy. We further discuss the stability and phase transition of the theory by evaluating heat capacity and free energy. The phase transition occurs at the point of divergence and the temperature is maximum. Remarkably, the black hole is unstable for a small horizon radius and stable for a large horizon radius.

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arXiv2022

Thermodynamics and phase transition of rotating regular-de Sitter black holes

We analyze thermodynamic properties of the rotating regular black holes having mass ($M$), angular momentum ($a$), and a magnetic charge $(g)$, and encompass Kerr black hole ($g=0$). The mass $M$ has a minimum at the radius $r_+=r_+^{\star}$, where both the heat capacity and temperature vanish. The thermal phase transition is because of the divergence of heat capacity at a critical radius $r_{+}^C$ with stable (unstable) branches for $r_+ r_+^C$). We also generalize the rotating regular black holes in de Sitter (dS) background and analyzed its horizon structure to show that for each $g$, there are two critical values of the mass parameter $M_{\text{cr1}}$ and $M_{\text{cr2}}$ which correspond to the degenerate horizons. Thus, we have rotating regular-dS black holes with an additional cosmological horizon apart from the inner (Cauchy) and the outer (event) horizons. Next, we discuss the effective thermodynamic quantities of the rotating regular-dS black holes in the extended phase space where the cosmological constant ($Λ$) is considered as thermodynamic pressure. Combining the first laws at the two horizons, we calculate the heat capacity at constant pressure $C_P$, the volume expansion coefficient $α$, and the isothermal compressibility $κ_T$. At a critical point, the specific heat at constant pressure, the volume expansion coefficient, and the isothermal compressibility of the regular-dS black holes exhibit an infinite peak suggesting a second-order phase transition.

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arXiv2022

Vacuum polarisation of Dirac fermions in the cosmological de Sitter-global monopole spacetime

We study the vacuum polarisation effects of the Dirac fermionic field induced by a pointlike global monopole located in the cosmological de Sitter spacetime. First we derive the four orthonormal Dirac modes in this background in a closed form. Quantising the field using these modes, we then compute the fermionic condensate, $\langle 0| \overlineΨ Ψ| 0\rangle$, as well as the vacuum expectation value of the energy-momentum tensor for a massive Dirac field, regularised in a particular way. We have used the Abel-Plana summation formula in order to extract the global monopole contribution to these quantities and have investigated their variations numerically with respect to relevant parameters.

hep-th↗

arXiv2022

Gravitational lensing for stationary axisymmetric black holes in Eddington-inspired Born-Infeld gravity

The recent years witnessed a surge of interest of the lensing of the black holes arising from general as well as other modified theories of gravity due to the experimental data available from the EHT results. The EHT may open a new door indicating the possible existence of the rotating black hole solutions in modified theories of gravity in the strong field regime. With this motivation, we investigate in the present paper the equatorial lensing $(θ=π/2)$ by a recently obtained exact rotating black holes solution in EiBI theory in both the strong and weak field limits. Such black holes are the modification of Kerr-Newman black holes in general relativity, characterized by their mass ($M$), the charge ($Q$), and the rotation parameter ($a$). and an additional term $ε$ accounting for the correction to the Kerr-Newman solutions. We show numerically the variations of the impact parameter $u_m$, the light deflection coefficients $p$ and $q$, the total azimuthal bending angle $α_D$ and find a close dependence of these quantities on the charge parameter $r_q$, the correction term $ε$ and the spin $a$. We also calculate the angular position $θ_\infty$, and the angular separation $s$, and the magnification of the relativistic images. In addition, we also discuss the weak lensing of the black holes in EiBI theory using the Gauss-Bonnet theorem. We calculate the weak lensing parameter and find its variation with different values of the parameters $r_q$ and $ε$.

gr-qc↗

arXiv2021

Fermionic Bell violation in the presence of background electromagnetic fields in the cosmological de Sitter spacetime

The violation of the Bell inequality for Dirac fermions is investigated in the cosmological de Sitter spacetime, in the presence of background electromagnetic fields of constant strengths. The orthonormal Dirac mode functions are obtained and the relevant in-out squeezed state expansion in terms of the Bogoliubov coefficients are found. We focus on two scenarios here : strong electric field and heavy mass limits (with respect to the Hubble constant). Using the squeezed state expansion, we then demonstrate the Bell violations for the vacuum and some maximally entangled initial states. Even though a background magnetic field alone cannot create particles, in the presence of background electric field and or spacetime curvature, it can affect the particle creation rate. Our chief aim thus here is to investigate the role of the background magnetic field strength in the Bell violation. Qualitative differences in this regard for different maximally entangled initial states are shown. Further extension of these results to the so called $α$-vacua are also discussed.

hep-th↗

arXiv2021

Shadows of black hole surrounded by anisotropic fluid in Rastall theory

Due to the gravitational lensing effect, a black hole casts a shadow larger than its horizon over a bright background, and the shape and size can be calculated. The Event Horizon Telescope collaboration has produced the first direct image (shadow) of the black hole and it is in accordance with the shadow of a Kerr black hole of general relativity. However, deviations from the Kerr black hole arising from modified theories of gravity are not ruled out and they are important as they offer an arena to test these theories through astrophysical observation. This stimulates us to investigate rotating black holes surrounded by anisotropic fluid in Rastall theory namely a rotating Rastall black hole, which is characterized by mass $M$, spin $a$, field structure parameter $N_s$ and the Rastall parameter $ψ$. It encompasses, as special cases, Kerr ($N_s \to 0$) and Kerr-Newman ($s=0$ and $N_s = -Q^2 $) black holes. The rotating Rastall black hole is characterized by an additional cosmological-like horizon apart from Cauchy and event horizons. We derive an analytical formula for the shadow of a rotating Rastall black hole and go on to visualize the shadow of black holes for various values of the parameters for an observer at a given coordinate ($r_O, θ_O$) in the domain $[r_+,r_q]$.

gr-qc↗

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.

gr-qc↗

arXiv2020

Photon Orbits and Thermodynamic Phase Transition of Regular AdS Black Holes

We probe the phase structure of the regular AdS black holes using the null geodesics. The radius of photon orbit and minimum impact parameter shows a non-monotonous behaviour 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.

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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.

gr-qc↗

arXiv2020

Quasinormal modes, shadow and greybody factors of 5D electrically charged Bardeen black holes

We study quasinormal modes (QNMs) in 5D electrically charged Bardeen black holes spacetime by considering the scalar and electromagnetic field perturbations. The black holes spacetime is an exact solution of Einstein gravity coupled to nonlinear electrodynamics in five dimensions, which has nonsingular behavior. To calculate QNMs, we use the WKB approximation method up to sixth order. Due to the presence of electric charge $q_e > 0$, both the scalar and electromagnetic field perturbations decay more slowly when compared to the Schwarzschild-Tangherlini black holes. We discover that the scalar field perturbations oscillate more rapidly when compared to the electromagnetic field perturbations. In terms of damping, the scalar field perturbations damp more quickly. Graphically we show that the transmission (reflection) coefficients decrease (increase) with an increase in the magnitude of the electric charge $q_e$. The emission of gravitational waves allows spacetime to undergo damped oscillations due to the nonzero value of the imaginary part, which is always negative. The imaginary part of the QNMs frequencies is continuously decreasing with an increase in the magnitude of the electric charge $q_e$ for a given mode ($l,n$). A connection between the QNMs frequencies and the black hole shadow, as well as the geometric cross-section in the eikonal limit, is also described.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

arXiv2020

Rotating five-dimensional electrically charged Bardeen regular black holes

We derive a rotating counterpart of the five-dimensional electrically charged Bardeen regular black holes spacetime by employing the Giampieri algorithm on static one. The associated nonlinear electrodynamics source is computed in order to justify the rotating solution. We thoroughly discuss the energy conditions and the other properties of the rotating spacetime. The black hole thermodynamics of the rotating spacetime is also presented. In particular, the thermodynamic quantities such as the Hawking temperature and the heat capacity are calculated and plotted to see the thermal behavior. The Hawking temperature profile of the black hole implies that the regular black holes are thermally colder than its singular counterpart. On the other hand, we find that the heat capacity has two branches: the negative branch corresponds to the unstable phase and the positive branch corresponds to that of the stable phase for a suitable choice of the physical parameters characterizing the black holes.

gr-qc↗

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 $α$.

gr-qc↗

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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