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Mubasher Jamil

Publications and source records attributed to Mubasher Jamil.

At least 73 records · Page 4Linked to original sources
arXiv2020

Traversable Wormholes Supported by GUP Corrected Casimir Energy

In this paper, we investigate the effect of the Generalized Uncertainty Principle (GUP) in the Casimir wormhole spacetime recently proposed by Garattini [Eur. Phys. J. C (2019) 79: 951]. In particular, we consider three types of the GUP relations, firstly the Kempf, Mangano and Mann (KMM) model, secondly the Detournay, Gabriel and Spindel (DGS) model, and finally the so called type II model for GUP principle. To this end, we consider three specific models of the redshift function along with two different EoS of state given by $\mathcal{P}_r(r)=ω_r(r) ρ(r)$ along with $\mathcal{P}_t(r)=ω_t (r)\mathcal{P}_r(r)$ and obtain a class of asymptotically flat wormhole solutions supported by Casimir energy under the effect of GUP. Furthermore we check the null, weak, and strong condition at the wormhole throat with a radius $r_0$, and show that in general the classical energy condition are violated by some arbitrary quantity at the wormhole throat. Importantly, we examine the wormhole geometry with semi-classical corrections via embedding diagrams. We also consider the ADM mass of the wormhole, the volume integral quantifier to calculate the amount of the exotic matter near the wormhole throat, and the deflection angle of light.

gr-qc↗
arXiv2020

Rotating regular black holes in conformal massive gravity

In this paper, we use a suitable conformal rescaling to construct static and rotating regular black holes in conformal massive gravity. The new metric is characterized by the mass $M$, the "scalar charge" $Q$, the angular momentum parameter $a$, the "hair parameter" $λ$, and the conformal scale factor encoded in the parameter $L$. We explore the shadow images and the deflection angles of relativistic massive particles in the spacetime geometry of a rotating regular black hole. For $λ\neq 0$ and $Q > 0$, the shadow is larger than the shadow of a Kerr black hole. In particular, if $λ< 0$, the shadow radius increases considerably. For $λ\neq 0$ and $Q < 0$, the shadow is smaller than the shadow of a Kerr black hole. Additionally we put observational constraints on the parameter $ Q $ using the latest Event Horizon Telescope (EHT) observation of the supermassive black hole M87*. Lastly, using the Gauss-Bonnet theorem, we show that the deflection angle of massive particles is strongly affected by the parameter $L$. The deflection angle might be used to distinguish rotating regular black holes from rotating singular black holes.

gr-qc↗
arXiv2020

Shadow Images of a Rotating Dyonic Black Hole with a Global Monopole Surrounded by Perfect Fluid

In this paper we revisit and extend the prior work of Filho and Bezerra [Phys. Rev D, 64, 084009 (2001)] to rotating dyonic global monopoles in presence of a perfect fluid. We then show that the surface topology at the event horizon, related to the metric computed, is a 2- sphere using the Gauss-Bonnet theorem. By choosing $ω=-1/3, 0, 1/3$ we investigate the effect of dark matter, dust, radiation on the silhouette of black hole. The presence of the global monopole parameter $γ$ and the perfect fluid parameters $\upsilon$, also deforms the shape of black hole's shadow, which has been depicted through graphical illustrations. In the end we analyse energy emission rate of rotating dyonic global monopole surrounded by perfect fluid with respect to parameters.

gr-qc↗
arXiv2019

Alternative approach to thermodynamic phase transitions

One of the major open problems in theoretical physics is a consistent quantum gravity theory.Recent developments in thermodynamic phase transitions ofblack holes and their van der Waals-like behavior may provide an interesting quantum interpretation of classical gravity. Studyingdifferent methods of investigating phase transitions can extend our insight into the nature of quantumgravity. In this paper, we present an alternative theoretical approach for finding thermodynamicphase transitions in the extended phase space. Unlike the standard methods based on the usualequation of state involving temperature, our approach usesa new quasi-equation constructed fromthe slope of temperature versus entropy. This approach addresses some of the shortcomings ofthe other methods, and provides a simple and powerful way of studying the critical behavior of athermodynamical system. Among the applications of this approach, we emphasize the analyticaldemonstration of possible phase transition points, and theidentification of the non-physical rangeof horizon radii for black holes.

gr-qc↗
arXiv2019

Black Hole Surrounded by a Dark Matter Halo in the M87 Galactic Center and its Identification with Shadow Images

In this paper we present a new black hole solution surrounded by dark matter halo in the galactic center using the mass model of M87 and that coming from the Universal Rotation Curve (URC) dark matter profile representing family of spiral galaxies. In both cases the DM halo density is cored with a size $r_0$ and a central density $ρ_0$: $ρ(r)= ρ_0/(1+r/r_0)(1+(r/r_0)^2)$. Since $r_0ρ_0=120 M_{\odot}$/pc$^2$ [Donato et al., MNRAS, 397, 1169, 2009], then by varying the central density one can reproduce the DM profile in any spiral. Using the Newman-Jains method we extend our solution to obtain a rotating black hole surrounded by dark matter halo. We find that, the apparent shape of the shadow beside the black hole spin $a$, it also depends on the central density of the surrounded dark matter $ρ_0$. As a specific example we consider the galaxy M87, with a central density $ρ_0=6.9\,\times 10^{6} M_{\odot}$/kpc$^3$ and a core radius $r_0=91.2$ kpc. In the case of M87, our analyses show that the effect of dark matter on the size of the black hole shadow is almost negligible compared to the shadow size of the Kerr vacuum solution hence the angular diameter $ 42$ $μ$as remains almost unaltered when the dark matter is considered. For a small totally dark matter dominated spiral such as UGC 7232, we find similar effect of dark matter on the shadow images compared to the M87. However, in specific conditions having a core radius comparable to the black hole mass and dark matter with very high density, we show that the shadow images decreases compared to the Kerr vacuum black hole. The effect of dark matter on the apparent shadow shape can shed some light in future observations as an indirect way to detect dark matter using the shadow images.

physics.gen-ph↗
arXiv2019

Shadows and deflection angle of charged and slowly rotating black holes in Einstein-Æther theory

In this paper, we study the shadow cast by two types of charged and slowly rotating black holes in Eisntein-aether theory of gravity. This two types of black holes are corresponding to two specific combinations of the coupling constants of the aether field, i.e., $c_{14} = 0$ but $c_{123}\neq 0$ for the first type and $c_{123} = 0$ for the second type, respectively. For both types of black holes, in addition to the mass and charge of the black holes, we show that the presence of the aether field can also affect the size of the shadow. For the first type black hole, it is shown that the shadow size increases with the parameter $c_{13}$, while for the second type black hole, the shadow size still increases with c13 but decreases with the parameter $c_{14}$. With these properties of these aether parameters, we also discuss the observational constraints on these parameters by using the data of the first black hole image by the Event Horizon Telescope. In addition, we also explore the effect of the aether field on the the deflection angle of light and the time delay by using the Gauss-Bonnet theorem. It is shown that, for a specific combination $c_{123} = 0$, the deflection angle/time delay is slightly affected by the aether parameter $c_{13}$ at the leading order.

gr-qc↗
arXiv2019

On the possibility of wormhole formation in the galactic halo due to dark matter Bose-Einstein condensates

It has been recently claimed that dark matter could be in the form of a Bose-Einstein condensate (BEC) in order to explain the dynamics at large distances from the galactic center [Boehmer, Harko, JCAP {\bf 0706}, 025 (2007)]. In this paper we explore the possibility of wormhole formation in galactic halos due to the dark matter BEC. In particular we have found a new wormhole solution supported by BEC dark matter using the expressions for the density profile of the BEC and rotation velocity along with the Einstein field equations to calculate the wormhole red shift function as well as the shape function. To this end, we show that for a specific choose of the central density of the condensates our wormhole solution satisfies the flare our condition. Furthermore we check the null, weak, and strong condition at the wormhole throat with a radius $r_0$, and shown that in general the energy condition are violated by some arbitrary quantity at the wormhole throat. Using the volume integral quantifier, and choosing reasonable values of parameters we have calculate the amount of BEC exotic matter near the wormhole throat, such that the wormhole extends form $r_0$ to a a cut off radius situated at $`a'$. Moreover we have introduced a Kerr-like metric for a rotating BEC wormhole to study the effect of BEC dark matter on the Lense-Thirring precession frequencies. Namely, we have shown that the obtained precession frequencies lie within a range of typical quasi-periodic oscillations (QPOs).

gr-qc↗
arXiv2019

String analog of Reissner-Nordström black holes cannot be overcharged

In this work we attempt to overcharge extremal and nearly extremal charged black holes in string theory, known as the Garfinkle-Horowitz-Strominger solution. We first show that extremal black holes cannot be overcharged analogous to the case of Reissner-Nordström black holes. Contrary to their analogues in general relativity, nearly extremal black holes can neither be overcharged beyond extremality, nor can they be driven to extremality by the interaction with test particles. Therefore the analysis in this work also imply that the third law of black hole thermodynamics holds for the relevant charged black holes in string theory perturbed by test particles. This can be interpreted as a stronger version of the third law since one can drop out the continuity proviso for the relevant process.

gr-qc↗
arXiv2019

Distinguishing a Kerr-like black hole and a naked singularity in perfect fluid dark matter via precession frequencies

We study a Kerr-like black hole and naked singularity in perfect fluid dark matter (PFDM). The critical value of spin parameter $a_c$ is presented to differentiate the black hole from naked singularity. It is seen that for any fixed value of dark matter parameter $α$ the rotating object is black hole if $a\leq a_c$ and naked singularity if $a>a_c$. Also for $-2\leqα<2/3$ the size of the black hole horizons decrease whereas for $2/3<α$ it increases. We also study spin precession frequency of a test gyroscope attached to stationary observer to differentiate a black hole from naked singularity in perfect fluid dark matter. For the black hole, spin precession frequency blows up as the observer reaches the central object while for naked singularity it remains finite except at the ring singularity. Moreover, we study Lense-Thirring precession for a Kerr-like black hole and geodetic precession for Schwarzschild black hole in perfect fluid dark matter. To this end, we have calculated the Kepler frequency (KF), the vertical epicyclic frequency (VEF), and the nodal plane precession frequency (NPPF). Our results show that, the PFDM parameter $α$ significantly affects those frequencies. This difference can be used by astrophysical observations in the near future to shed some light on the nature of dark matter.

gr-qc↗
arXiv2019

Shadow and Deflection Angle of Rotating Black Holes in Perfect Fluid Dark Matter with a Cosmological Constant

The presence of dark matter around a black hole remarkably affects its spacetime. We consider the effects of dark matter on the shadow of a new solution to the Einstein equations that describes a rotating black hole in the background of perfect dark matter fluid (PFDM), along with its extension to nonzero cosmological constant $Λ$. Working in Boyer-Lindquist coordinates, we consider the effects of the PFDM parameter $α$ on the shadow cast by a black hole with respect to an observer at position $(r_o,θ_o)$. By applying the Gauss-Bonnet theorem to the optical geometry we find that notable distortions from a Kerr black hole can occur. We describe their dependence on $α$ and $Λ$.

gr-qc↗
arXiv2018

Black Holes in the Turbulent Phase of Viscous Rip Cosmology

We study the phantom fluid in the late universe, thus assuming the equation of state parameter $w$ to be less than $-1$. The fluid is assumed to consist of two components, one laminar component $ρ$ and one turbulent component $ρ_T$, the latter set proportional to $ρ$ as well as to the Hubble parameter, $ρ_T =3τHρ$ with $τ$ a positive constant associated with the turbulence. The effective energy density is taken to be $ρ_e= ρ+ ρ_T$, and the corresponding effective pressure is $p_e=w ρ_e$, with $w$ constant. These basic assumptions lead to a Big Rip universe; the physical quantities diverging during a finite rip time $t_s$. We then consider the mass accretion of a black hole in such a universe. The most natural assumption of setting the rate $dM/dt$ proportional to $M^2$ times the sum $ρ_e+p_e$, leads to a negative mass accretion, where $M(t)$ goes to zero linearly in $(t_s-t)$ near the singularity. The Hubble parameter diverges as $(t_s-t)^{-1}$, whereas $ρ_e$ and $p_e$ diverge as $(t_s-t)^{-2}$. We also discuss other options and include, for the sake of comparison, some essential properties of mass accretion in the early (inflationary) universe.

gr-qc↗
arXiv2018

Spherical accretion by normal and phantom Einstein--Maxwell--dilaton black holes

We investigate the spherical accretion process for general static spherically symmetric fluids. We analyze this process by using the general metric ansatz for spherically symmetric black holes. We specialize to the case of normal and phantom isothermal fluids and investigate their accretion process onto normal and phantom Einstein-Maxwell-dilaton black holes. Backreaction effects are discussed.

gr-qc↗
arXiv2018

Distinguishing rotating Kiselev black hole from naked singularity using spin precession of test gyroscope

We study the critical values of the quintessential and spin parameters, to distinguish a rotating Kiselev black hole (RKBH) from a naked singularity. For any value of the dimensionless quintessential parameter $ω_{q} \in (-1, -1/3)$, when increasing the value of quintessential parameter $α$, the size of the event horizon increases, whereas the size of the outer horizon decreases. We then study the spin precession of a test gyroscope attached to a stationary observer in this spacetime. Using the spin precessions we differentiate black holes from naked singularities. If the precession frequency becomes large, as approaching to the central object in the quintessential field along any direction, then the spacetime is a black hole. A spacetime will contain a naked singularity if the precession frequency remains finite everywhere except at the singularity itself. Finally, we study the Lense-Thirring precession frequency for rotating Kiseleb black hole and the geodetic precession for Kiselev black hole.

gr-qc↗
arXiv2018

The Effects of Running Gravitational Coupling On Rotating Black Holes

In this work we investigate the consequences of running gravitational coupling on the properties of rotating black holes. Apart from the changes induced in the space-time structure of such black holes, we also study the implications to Penrose process and geodetic precession. We are motivated by the functional form of gravitational coupling previously investigated in the context of infra-red limit of asymptotic safe gravity theory. In this approach, the involvement of a new parameter $\tildeξ$ in this solution makes it different from Schwarzschild black hole. The Killing horizon, event horizon and singularity of the computed metric is then discussed. It is noticed that the ergosphere is increased as $\tildeξ$ increases. Considering the black hole solution in equatorial plane, the geodesics of particles, both null and time like cases, are explored. The effective potential is computed and graphically analyzed for different values of parameter $\tildeξ$. The energy extraction from black hole is investigated via Penrose process. For the same values of spin parameter, the numerical results suggest that the efficiency of Penrose process is greater in quantum corrected gravity than in Kerr Black Hole. At the end, a brief discussion on Lense-Thirring frequency is also done.

gr-qc↗
arXiv2017

Generalized Second Law of Thermodynamic in Modified Teleparallel Theory

This study is conducted to examine the validity of generalized second law of thermodynamics (GSLT) in flat FRW for modified teleparallel gravity involving coupling between a scalar field with the torsion scalar $T$ and the boundary term $B=2\nabla_μT^μ$. This theory is very useful since it can reproduce other important well-known scalar field theories in suitable limits. The validity of the first and second law of thermodynamics at the apparent horizon is discussed for any coupling. As examples, we have also explored the validity of those thermodynamics laws in some new cosmological solutions under the theory. Additionally, we have also considered the logarithmic entropy corrected relation and discuss the GSLT at the apparent horizon.

gr-qc↗
arXiv2017

Strong Gravitational Lensing by a Charged Kiselev Black Hole

We study the gravitational lensing scenario where the lens is a spherically symmetric charged black hole (BH) surrounded by quintessence matter. The null geodesic equations in the curved background of the black hole are derived. The resulting trajectory equation is solved analytically via perturbation and series methods for special choice of parameters and the distance of the closest approach to black hole is calculated. We also derive the lens equation giving the bending angle of light in the curved background. In the strong field approximation, the solution of the lens equation is also obtained for all values of the quintessence parameter $w_q$. For all $w_q$, we show that there are no stable closed null orbits and that corrections to the deflection angle for the Reissner-Nordström black hole when the observer and the source are at large, but finite, distances from the lens do not depend on the charge up to the inverse of the distances squared. A part of the present work, analyzed however with a different approach, is the extension of {\it Phys. Rev. D \textbf{92}, 084042 (2015)} where the uncharged case has been treated

gr-qc↗
arXiv2017

Dynamics and center of mass energy of colliding particles around black hole in f(R) gravity

We have investigated the dynamics of particles in the vicinity of a static spherically symmetric black hole in f(R) gravity. Using the Euler-Lagrange method the dynamical equations of a neutral particle are obtained. Assuming that the particle is initially moving in the innermost stable circular orbit, we have calculated its escape velocity, after a collision with some other particle. The conditions for the escape of colliding particles are discussed. The effective potential and the trajectories of the escaping particles are studied graphically.

gr-qc↗