arXiv subjects

Kimet Jusufi

Publications and source records attributed to Kimet Jusufi.

At least 55 records · Page 3Linked to original sources

arXiv2023

Topological classes of thermodynamics of black holes in perfect fluid dark matter background

In this paper we explore the topological classes of thermodynamics of a family of black holes. In particular we investigate the influence of distinct fields, including the electric field, non-linear magnetic field, along with the perfect fluid matter background that can mimic dark matter in large distances. In light of these considerations, we shall henceforth denote this fluid as perfect fluid dark matter (PFDM). Our analysis reveals that the winding and topological numbers for the Schwarzschild and Kerr black holes in PFDM background are the same as the Schwarzschild and Kerr black holes, however for the Kerr-AdS background in PFDM we obtain a different topological number compared to the Kerr black hole in PFDM. Furthermore, we explore in details the interplay of electric charge and nonlinear magnetic charge, impacting the topological classes of thermodynamics both in the absence and presence of PFDM. Interestingly, it is demonstrated that the topological numbers associated with the static Hayward black holes, both in the absence and presence of PFDM, deviate from those of the Schwarzschild black hole. This observation shows that the presence of a magnetic charge introduces an additional role and can alter the topological numbers. Finally, our study culminates with the comprehensive analysis of the topological numbers pertaining to the Hayward black hole, considering the combined effects of PFDM and rotation.

gr-qc↗

arXiv2023

Observational constraints on Yukawa cosmology and connection with black hole shadows

We confront Yukawa modified cosmology, proposed in arXiv:2304.11492 [Jusufi et al. arXiv:2304.11492], with data from Supernovae Type Ia (SNe Ia) and Hubble parameter (OHD) observations. Yukawa cosmology is obtained from a Yukawa-like gravitational potential, with coupling parameter $α$ and wavelength parameter $λ$, which gives rise to modified Friedmann equations. We show that the agreement with observations is very efficient, and within $1σ$ confidence level we find the best-fit parameters $λ=\left(2693_{-1262}^{+1191}\right)\, \rm Mpc$, $α=0.416_{-0.326}^{+1.137}$, and a graviton mass of $m_{g}=\left(2.374_{-0.728}^{+2.095}\right)\times 10^{-42}\, \text{GeV}$. Additionally, we establish a connection between the effective dark matter and dark energy density parameters and the angular radius of the black hole shadow of the SgrA and M87 black holes in the low-redshift limit, consistent with the Event Horizon Telescope findings.

astro-ph.CO↗

arXiv2023

Apparent dark matter as a non-local manifestation of emergent gravity

We disclose a close correspondence between Verlinde's Emergent Gravity (VEG) theory and the non-local gravity theories. Such non-local effects can play crucial role at small distances as well as in large scale structures. In particular, we argue that the emergent gravity effectively is a manifestation of the entanglement entropy and can modify Newton's law of gravity as well as address the flat rotation curves of spiral galaxies. In the cosmological setup, we have considered three different models for the apparent dark matter density. In the first model, we have found that Friedmann equations get modified due to the presence of the apparent dark matter (DM) in such a way that Newton's constant of gravity shifts as $G\rightarrow G\left(1+ζ\right)$, where $ζ$ is a dimensionless small parameter. Using the flat rotating curves we estimate $ζ\sim 10^{-7}$. For such a model, we find out that by rescaling the radial coordinate, $r$, the curvature space constant, $k$, and the scale factor of the universe, $a$, the effect of apparent DM can change the geometry of the universe and can shift the curvature space constant as $k_{\star}=k (1-ζ)$. To this end, we study a more realistic model with evolving densities where we obtain the $Λ$CDM model in Verlinde's gravity and comment on the Hubble tension problem with evolving densities.

gr-qc↗

arXiv2023

Constraining study of circular orbits and accretion disk around nonlinear electrodynamics black hole

The very latest observation of $M87$ supermassive black hole (BH) by the Event Horizon Telescope (EHT) provides the accretion onto BHs is an interesting study in the theory of gravity. We study the geodesics structure and accretion near a nonlinear electrodynamics BH in strong and weak field approximations. These approximations provide the disc-like structure under the geodesic motion and accretion around the BH. Near the equatorial plane, we provide some new reasons to make circular orbits and accretion of test particles around the BH. Then we investigate perturbations, the critical speed of the fluid and the mass accretion rate of particles around the central object. The physical validity of this study shows that the parameter $β$ and $Q$ play an important role in the circular orbits and the mass accretion rate in strong and weak field approximations.

gr-qc↗

arXiv2023

Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A$^*$

Horizon-scale images of black holes (BHs) and their shadows have opened an unprecedented window onto tests of gravity and fundamental physics in the strong-field regime. We consider a wide range of well-motivated deviations from classical General Relativity (GR) BH solutions, and constrain them using the Event Horizon Telescope (EHT) observations of Sagittarius A$^*$ (Sgr A$^*$), connecting the size of the bright ring of emission to that of the underlying BH shadow and exploiting high-precision measurements of Sgr A$^*$'s mass-to-distance ratio. The scenarios we consider, and whose fundamental parameters we constrain, include various regular BHs, string-inspired space-times, violations of the no-hair theorem driven by additional fields, alternative theories of gravity, novel fundamental physics frameworks, and BH mimickers including well-motivated wormhole and naked singularity space-times. We demonstrate that the EHT image of Sgr A$^*$ places particularly stringent constraints on models predicting a shadow size larger than that of a Schwarzschild BH of a given mass, with the resulting limits in some cases surpassing cosmological ones. Our results are among the first tests of fundamental physics from the shadow of Sgr A$^*$ and, while the latter appears to be in excellent agreement with the predictions of GR, we have shown that a number of well motivated alternative scenarios, including BH mimickers, are far from being ruled out at present.

gr-qc↗

arXiv2023

Einstein-Rosen bridge from the minimal length

We use a string T-duality corrected pair of regular black holes to construct an Einstein-Rosen (ER) bridge with the wormhole throat proportional to the zero-point (Planck) length. This may be a geometric realization of quantum entanglement for particle/antiparticle pairs. We point out that for an extreme mass configuration consisting of a black hole pair, one can have an ER bridge with a horizon area that coincides with the Bekenstein minimal area bound along with a wormhole mass proportional to the Planck mass. This could be related to gravitational self-completeness with quantum mechanical mass limits. We also discuss the ER bridge for sub-Planckian mass horizonless wormholes and show this admits a region of negative energy at the throat, which we posit to be related to a particle of negative mass generated by quantum fluctuations or the Casimir effect. We argue that Hawking radiation could be the best way for the geometric realization of quantum entanglement for particle/antiparticle pairs emitted by black holes. This sheds new light on the ER=EPR conjecture.

gr-qc↗

arXiv2023

Epicyclic motions and constraints on the charged stringy black hole spacetime

The purpose of this paper is to examine the epicyclic motion of charged particles in the vicinity of a magnetically charged stringy black hole subject to an electromagnetic field. This investigation is motivated by the established fact that magnetic fields have a significant impact on the motion of charged particles in the vicinity of a black hole. It has been observed that the magnetic coupling parameter has a comparable impact on the motion of magnetic monopole charge, leading to an increase/decrease in the radius of the innermost stable circular orbit (ISCO) that plays a crucial role in setting the accretion disk's the inner edge. Further, we analyze the characteristics of the epicyclic motion and determine the generic form for the epicyclic frequencies. Our results demonstrate that the existence of the magnetic coupling parameter can account for the observed resonance of high frequency QPOs from selected three specific microquasars. We have applied the Monte Carlo simulations with the observed frequencies for three selected microquasars to constrain the parameters of the magnetically charged stringy black holes as well as coupling parameters that describe the interaction between the particle and the black hole. Finally, we study in detail the radiation properties of the accretion disk in the close vicinity of black hole. We show that the magnetic coupling parameter can have significant influence on the radiation flux of the accretion disk.

gr-qc↗

arXiv2023

Phase space analysis of the bouncing universe with stringy effects

We use the recently modified Friedmann equations obtained from string T-duality effects that encode the zero-point length (Phys. Lett. B 836 (2023), 137621) to study the phase space analyses of a bouncing early universe. An important implication of such stringy effects is that they can alleviate the initial singularity since the Raychaudhuri equation is modified. We investigate if the Universe can undergo an accelerated expansion phase for a specific domain of the equation of state parameter. The stringy effects are encoded in the parameter $Γ$, which depends not only on the zero-point length but also on the state parameter $ω$. We construct two dynamical systems depending on whether $-1 < ω\leq -1/3$ and $ω\geq -1/3$, and we classify the equilibrium points of each system. Exact solutions and cosmological implications are discussed.

gr-qc↗

arXiv2023

Black holes surrounded by Einstein clusters as models of dark matter fluid

We construct a novel class of spherically symmetric and asymptotically flat black holes and naked singularities surrounded by anisotropic dark matter fluid with the equation of state (EoS) of the form $P_t=ωρ$. We assume that dark matter is made of weakly interacting particles orbiting around the supermassive black hole in the galactic center and the dark matter halo is formed by means of Einstein clusters having only tangential pressure. In the large distance from the black hole, we obtain the constant flat curve with the upper bound for the dark matter state parameter $ω\lesssim 10^{-7}$. To this end, we also check the energy conditions of the dark matter fluid outside the black hole/naked singularity, the deflection of light by the galaxy, and the shadow images of the Sgr A$^\star$ black hole using the rotating and radiating particles. For the black hole case, we find that the effect of dark matter fluid on the shadow radius is small, in particular the angular radius of the black hole is shown to increase by the order $10^{-4} μ$arcsec compared to the vacuum solution. For the naked singularity we obtain significantly smaller shadow radius compared to the black hole case. Finally, we study the stability of the S2 star orbit around Sgr A$^\star$ black hole under dark matter effects. It is argued that the motion of S2 star orbit is stable for values $ω\lesssim 10^{-7}$, however further increase of $ω$ leads to unstable orbits. Using the observational result for the shadow images of the Sgr A$^\star$ reported by the EHT along with the tightest constraint for $ω$ found from the constant flat curve, we show that the black hole model is consistent with the data while the naked singularity in our model can be ruled out.

gr-qc↗

arXiv2023

Avoidance of singularity during the gravitational collapse with string T-duality effects

In this paper, we explore the gravitational collapse of matter (dust) under the effect of zero-point length $l_0$. During the gravitational collapse, we have neglected the backreaction effect of the pre-Hawking radiation (in the sense that it is small effect and cannot prevent the formation of apparent horizon), then we recast the internal metric of a collapsing star as a closed FRW universe for any spherically symmetric case and, finally, we obtain the minimal value for the scale factor meaning that the particles never hit the singularity. We argue that the object emerging at the end of the gravitational collapse can be interpreted as Planck stars (black hole core) hidden inside the event horizon of the black hole with radius proportional to $(GMl_0^{2}/c^2)^{1/3}$. Quite interestingly, we found the same result for radius of the Planck star using a free falling observer point of view. In addition, we pointed out a correspondence between the modified Friedmann's equations in loop quantum gravity and the modified Friedmann's equation in string T-duality. In the end, we discuss two possibilities regarding the final stage of the black hole. The first possibility is that we end up with a Planck-size black hole remnants. The second possibility is that the inner core can be unstable and, due to the quantum tunnelling effect, the spacetime can undergo a black hole-to-white hole transition (a bouncing Planck star).

gr-qc↗

arXiv2022

Unitary symmetries in wormhole geometry and its thermodynamics

From a geometric point of view, we show that the unitary symmetries $U(1)$ and $SU(2)$ stem fundamentally from Schwarzschild and Reissner-Nordström wormhole geometry through spacetime complexification. Then, we develop quantum tunneling which makes these wormholes traversable for particles. Finally, this leads to wormhole thermodynamics.

hep-th↗

arXiv2022

Quantum effects on the black hole shadow and deflection angle in presence of plasma

In this paper, optical properties of renormalization group improved (RGI) Schwarzschild black hole (BH) are investigated in plasma medium. Starting from the equations of motions in plasma medium, we have shown how the shadow radius of the RGI black hole is modified. To this end, we have computed the deflection angle of light in the weak gravity regime for uniform and non-uniform plasma medium. Importantly, due to the plasma medium, we find that the equations of motions for light obtained from the radiating and infalling/rest gas has to be modified. This in turn, changes and modifies the expression for the intensity observed far away from the black hole. Finally, we have obtained the shadow images for the RGI black hole for different plasma models. Even thought, quantum effect changes the background geometry, these effects are very small and, practically, it is impossible to detect them with the present technology using the supermassive black hole shadows. The parameter $Ω$ encodes the quantum effects and, in principle, one expects such quantum effects to play a significant role only for very small black holes. On the other hand, the effect of plasma medium can play important role on the optical appearance of black holes since they effect and modify the equations of motions.

gr-qc↗

arXiv2022

Charged AdS black holes with finite electrodynamics in 4D Einstein-Gauss-Bonnet gravity

Using a modified expression for the electric potential in the context of T-duality (Gaete and Nicolini, Phys. Lett. B, 2022) we obtain an exact charged solution within 4D Einstein-Gauss-Bonnet (4D EGB) theory of gravity in the presence of a cosmological constant. We show that the solution exists in the regularized 4D EGB theory as well and we point out a correspondence between the black hole solution in 4D EGB theory and the solution in the non-relativistic Horava--Lifshitz theory. The black hole solution is regular and free from singularity and as a special case we recover a class of well-known solutions in the literature.

gr-qc↗

arXiv2022

Regular solutions for black strings and torus-like black holes

In this letter, we extend the recent black hole solution with spherical symmetry [Phys. Lett. B \textbf{835} (2022), 137546], to a new class of black hole solutions with different topology, namely, a torus-like black hole and a black string in the presence of the cosmological constant. We have used the regularized expressions for the gravitational and the electromagnetic potentials, then we derived the corrections to the energy density and, finally, the black hole solutions are obtained which are regular in the limit $r \to 0$. Interestingly, it is found that the total mass of the black hole is corrected due to the regularized self-energy of the electrostatic field. In the special limit, we recover the well-known solutions in the literature.

gr-qc↗

arXiv2022

Regular black holes in Verlinde's emergent gravity

In this paper, we construct a class of charged and regular black hole solutions in Verlinde's Emergent Gravity (VEG). In particular, these solutions include, black holes with asymptotically Minkowski core, T-duality, Frolov/Simpson-Visser type solutions, as well as the Bardeen/Hayward type solutions as a special case. Using the relation between the apparent dark matter and the baryonic matter we find the effect of apparent dark matter on the spacetime geometry. We show that in general, the apparent dark matter leads to non-asymptotically flat spacetime geometry and, in the special limit of a point like mass distribution, all the black hole solutions in VEG resembles the global monopole-like solution with a deficit angle. To this end, we extend the solutions by including the cosmological constant (de Sitter space) and we elaborate different energy conditions of black hole solutions. Finally, having in mind that the Einstein's field equation are not modified in this theory while the apparent dark matter is encoded in the energy-momentum tensor, we used the modified Newman--Janis-Azreg-Aïnou algorithm to obtain the corresponding effective rotating metrics.

gr-qc↗

arXiv2022

Charged black holes from T-duality

In this paper, we present a family of regular black hole solutions in the presence of charge and angular momentum. We also discuss the related thermodynamics and we comment about the black hole life cycle during the balding and spin down phases. Interestingly the static solution resembles the Ayón-Beato--García spacetime, provided the T-duality scale is redefined in terms of the electric charge, $l_0\to Q$. The key factor at the basis of our derivation is the employment of Padmanabhan's propagator to calculate static potentials. Such a propagator encodes string T-duality effects. This means that the regularity of the spacetimes here presented can open a new window on string theory phenomenology.

hep-th↗

arXiv2022

Testing Born-Infeld $f(T)$ teleparallel gravity through Sgr A$^\star$ observations

We use observational data from the S2 star orbiting around the Galactic Center to constrain a black hole solution of extended teleparallel gravity models. Subsequently, we construct the shadow images of Sgr A$^{\star}$ black hole. In particular, we constrain the parameter $α=1/λ$ which appears in the Born-Infeld $f(T)$ model. In the strong gravity regime we find that the shadow radius increases with the increase of the parameter $α$. Specifically, from the S2 star observations, we find within 1$σ$ that the parameter must lie between $0 \leq α/M^2 \leq 6 \times 10^{-4}$. Consequently, we used the best fit parameters to model the shadow images of Sgr A$^{\star}$ black hole and then using the Gauss-Bonnet theorem we analysed the deflection angle for leading order expansions of the parameter $α$. It is found that within the parameter range, these observables are very close to the Schwarzschild case. Furthermore, using the best fit parameters for the Born-Infeld $f(T)$ model we show the angular diameter is consistent with recent observations for the Sgr A$^{\star}$ black hole angular diameter $(51.8 \pm 2.3) μ$arcsec and difficult to be distinguished from the GR. For the deflection angle of light, in leading order terms, we find that the deflection angle expressed in the ADM mass coincides with the GR, but the ADM mass in the Born-Infeld $f(T)$ gravity increases with the increase of $α$ and the overall deflection angle is expected to me greater in $f(T)$ gravity. As a consequence of this fact, we have shown that the electromagnetic intensity observed in shadow images is smaller compared to GR.

gr-qc↗

arXiv2022

Regular black holes in three dimensions and the zero point length

In this paper, by means of regularisation procedure via $r\to \sqrt{r^2+l_0^2}$ (where $l_0$ can play the role of zero point length), we first modify the gravitational and electromagnetic potentials in two dimensions and then we solve the Einstein field equations to end up with an exact and regular black hole solution in three dimensions with a negative cosmological constant. We show that, the black hole solution is asymptotically AdS, non-singular at the origin and, under specific conditions, it has a flat de Sitter core at the origin. As a special case, we obtain the charged Banados-Teitelboim-Zanelli (BTZ) solution. Finally, using a dimensional continuation and the NJ algorithm, we end up with a legitimate rotating black hole solution in three dimensions.

gr-qc↗