arXiv subjects

Kimet Jusufi

Publications and source records attributed to Kimet Jusufi.

At least 37 records · Page 2Linked to original sources

arXiv2025

Circular orbits and accretion disk around a deformed-Schwarzschild black hole in loop quantum gravity

In this paper, we study the motion of neutral and electrically charged particles in the vicinity of a deformed-Schwarzschild black hole inspired by Loop Quantum Gravity (LQG). To examine the motion of an electrically charged test particle, we propose an expression for electromagnetic 4-potential that contains the impacts of loop quantum gravity. This electromagnetic 4-potential satisfies approximately the covariant Maxwell's equations to first order in the loop quantum effects. We explore the effects of the loop quantum correction parameter on the particle geodesics. We investigate the innermost stable circular orbits (ISCOs) for both neutral and electrically charged particles in detail, demonstrating that the loop quantum parameter significantly influences on the ISCO radius, causing it to shrink. Finally, we explore the accretion disk around the loop quantum black hole. We delve into the electromagnetic radiation flux, temperature, differential luminosity, and the spectral luminosity as radiation properties of the accretion disk in detail. We show that the loop quantum correction parameter shifts the profile of the electromagnetic flux and accretion disk temperature towards the central object, leading to a slight increase in these quantities.

gr-qc↗

arXiv2025

Signatures of modified gravity from the gravitational Aharonov-Bohm effect

To date, no observational confirmation of dark matter particles has been found. In this paper, we put forward an alternative approach to inferring evidence for dark matter through modified gravity, without invoking fundamental dark matter particles. Specifically, we explore the possibility of extracting signatures of Kaluza-Klein gravity through the gravitational Aharonov-Bohm effect. Kaluza-Klein theory has recently been proposed as an alternative to the dark sector, and predicts a tower of particles, including spin-0 and spin-1 gravitons alongside the usual spin-2 gravitons, which can gravitationally couple to matter. We thus analyze a quantum system in free fall around a gravitating body in the presence of a modified Yukawa-like gravitational potential, and determine the gravitational phase induced by the additional degrees of freedom introduced by the Kaluza-Klein model. Our results reveal that, in addition to the usual result from General Relativity, the quantum wave function of the system exhibits an additional effect: a splitting of the energy levels with a new quantum number due to the extra vector gravitational degrees of freedom. The energy splitting difference between general relativity and Kaluza-Klein gravity is found to be of the order of meV for an atomic system and eV for a nuclear system. Similar values also arise in generic modified gravity models and can be feasibly tested in the future. Numerical estimates for the graviton mass are also provided, and potential imprints on gravitational waves are mentioned.

gr-qc↗

arXiv2025

Black holes surrounded by massive vector fields in Kaluza-Klein gravity

We present an exact black hole solution surrounded by massive vector fields predicted by Kaluza-Klein (KK) gravity. KK gravity in four dimensions (4D) is of particular interest, as it predicts a tower of particle states, including gravitons with spin-0 and spin-1 components, in addition to the massless spin-2 gravitons of general relativity. The extra degrees of freedom in the gravitational sector modify the law of gravity, allowing the theory to explain the effects attributed to dark matter in the universe. In this paper, we construct a black hole solution surrounded by massive spin-1 gravitons within KK theory. In addition to the influence of the massive vector fields, we incorporate an interaction term between the black hole and the massive vector field. The black hole solution is affected by the mass of the spin-1 graviton and an additional parameter that encodes corrections to Newton's constant, as well as the coupling between the massive vector field and the black hole mass. We find that the coupling between the massive vector field and the black hole mimics the effect of an electric charge. To this end, we investigate the accretion disk, quasinormal modes (QNMs), and the stability of the black hole spacetime. Finally, we use Event Horizon Telescope (EHT) observations of Sgr A* to constrain the black hole parameters.

gr-qc↗

arXiv2025

Thermodynamical properties of nonsingular universe

We disclose the thermodynamical properties of the apparent horizon in a nonsingular universe. We take into account the zero-point length correction to the gravitational potential and derive the modified entropy expression that includes zero-point length correction terms. We apply the first law of thermodynamics on the apparent horizon as well as the emergent gravity scenario to derive the modified Friedmann equations. Further, we examine the time evolution of the total entropy, including the entropy of the apparent horizon and the matter field entropy inside the horizon and find out that the generalized second law of thermodynamics is satisfied. We also investigate the cosmological implications of the modified cosmology through zero-point length. We observe that the zero-point length correction does not change the general profile of the universe evolution, however, it shifts the time of the phase transition in a universe filled with matter and cosmological constant. We explore the age of the universe for our model and observe that the predicted age of the universe becomes larger compared to the standard cosmology. By calculating the explicit form of Ricci and Kretchmann invariants, we confirm that in our model, the initial singularity of the universe is removed. This is an expected result, because the main motivation for considering zero-point length correction in the gravitational potential is to remove singularity at the origin.

gr-qc↗

arXiv2025

Dark Universe inspired by the Kaluza-Klein gravity and impact on Primordial Gravitational Waves

We explore the potential implications of Kaluza-Klein (KK) gravity in unifying the dark sector of the Universe. Through dimensional reduction in KK gravity, the 5D spacetime framework can be reformulated in terms of a 4D spacetime metric, along with additional scalar and vector fields. From the 4D perspective, this suggests the existence of a tower of particle states, including KK gravitons with spin-0 and spin-1 states, in addition to the massless spin-2 gravitons of general relativity (GR). The key idea in the present paper is the analogy with superconductivity theory. By assuming a minimal coupling between an additional complex scalar field and the gauge field, a "mass" term emerges for the spin-1 gravitons. This, in turn, leads to long-range gravitational effects that could modify Newton's law of gravity through Yukawa-type corrections. Assuming an environment-dependent mass for the spin-1 graviton, near the galactic center the repulsive force from this spin-1 graviton is suppressed by an additional attractive component from Newton's constant corrections, resulting in a Newtonian-like, attraction-dominated effect. In the galaxy's outer regions, the repulsive force fades due to its short range, making dark matter appear only as an effective outcome of the dominant attractive corrections. This approach also explains dark matter's emergence as an apparent effects on cosmological scales while our model is equivalent to the scalar-vector-tensor gravity theory. Finally, we examine the impact of dark matter on the primordial gravitational wave (PGW) spectrum and show that it is sensitive to dark matter effects, providing an opportunity to test this theory through future GW observatories.

gr-qc↗

arXiv2024

Exploring a novel Einstein--Rosen BTZ wormhole

We introduce a novel Einstein-Rosen BTZ wormhole metric as a solution to the Einstein field equations with a negative cosmological constant and explore in detail its various phenomenological aspects. We show that the wormhole metric is characterized by a horizon at the throat, resembling a black hole horizon. This implies that our wormhole metric describes a one-way traversable wormhole at the throat, with Hawking radiation observed by an observer located at some distance from the wormhole. It is also found the same Hawking temperature using the BTZ-like coordinates and Kruskal-like coordinates. This temperature is invariant not only on the type of coordinates but also the nature of the spin of quantum fields. Importantly, we find that at the wormhole throat, the spacetime is not a pure vacuum solution, but rather contains an exotic string matter source with negative tension, which may stabilize the wormhole geometry. To this end, we found that the size of the wormhole throat is proportional to the number of quantum bits suggesting a possible implications on ER=EPR. Further we studied the particle dynamics and, finally, we tested the ANEC with a test scalar and vector fields. For the double null-component computed in BTZ coordinates, we found an apparent divergence at the wormhole throat, which is then shown to be regularized by means of Kruskal-like coordinates. The ANEC for such a scalar/vector field is violated at the wormhole throat.

gr-qc↗

arXiv2024

Generalized Uncertainty Principle from the Regularized Self-Energy

We use the Schrödinger--Newton equation to calculate the regularized self-energy of the particle using a regular self-gravitational and electrostatic potential derived in the string T-duality. The particle mass $M$ is no longer concentrated into a point but it is diluted and described by a quantum-corrected smeared energy density resulting in corrections to the energy of the particle which is interpreted as a regularized self-energy. We extend our results and find corrections to the relativistic particles using the Klein-Gordon, Proca, and Dirac equations. An important finding is that we extract a form of generalized uncertainty principle (GUP) from the corrected energy. The form of GUP is shown to depend on the nature of particles; namely, for bosons (spin $0$ and spin $1$) we obtain a quadratic form of GUP, while for fermions (spin $1/2$) we obtain a linear form of GUP. The correlation we found between spin and GUP may offer insights into investigating quantum gravity.

hep-th↗

arXiv2024

Dark Universe Phenomenology from Yukawa Potential?

We argue that the effect of cold dark matter in the cosmological setup can be explained by the coupling between the baryonic matter particles in terms of the long-range force having a graviton mass $m_g$ via the Yukawa gravitational potential. Such a quantum-corrected Yukawa-like gravitational potential is characterized by the coupling parameter $α$, the wavelength parameter $λ$, which is related to the graviton mass via $m_g=\hbar/(λc)$, that determines the range of the force and, finally, a Planck length quantity $l_0$ that makes the potential regular at the centre. The corrected Friedmann equations are obtained using Verlinde's entropic force interpretation of gravity based on the holographic scenario and the equipartition law of energy. The parameter $α$ modifies Newton's constant as $G_{\rm eff}\rightarrow G(1+α)$. We argue that dark matter is an apparent effect as no dark matter particle exists in this picture. Furthermore, the dark energy is also related to graviton mass and $α$; in particular, we point out that the cosmological constant can be viewed as a self-interaction effect between gravitons. We further show that there exists a precise correspondence with Verlinde's emergent gravity theory, and due to the long-range force, the theory can be viewed as a non-local gravity theory. To this end, we performed the phase space analyses and estimated $λ\simeq 10^3 $[Mpc] and $α\in (0.0385,0.0450)$, respectively. Finally, from these values, for the graviton mass, we get $m_g\simeq 10^{-68}$ kg, and cosmological constant $Λ\simeq 10^{-52}\rm m^{-2}$. Further, we argue how this theory reproduces the MOND phenomenology on galactic scales via the acceleration of Milgrom $a_0 \simeq 10^{-10}\rm m/s^2$.

gr-qc↗

arXiv2024

Addressing the Hubble tension in Yukawa cosmology?

In Yukawa cosmology, a recent discovery revealed a relationship between baryonic matter and the dark sector. The relation is described by the parameter α and the long-range interaction parameter λ - an intrinsic property of the graviton. Applying the uncertainty relation to the graviton raises a compelling question: Is there a quantum mechanical limit to the measurement precision of the Hubble constant (H0)? We argue that the uncertainty relation for the graviton wavelength λ can be used to explain a running of H0 with redshift. We show that the uncertainty in time has an inverse correlation with the value of the Hubble constant. That means that the measurement of the Hubble constant is intrinsically linked to length scales (redshift) and is connected to the uncertainty in time. On cosmological scales, we found that the uncertainty in time is related to the look-back time quantity. For measurements with a high redshift value, there is more uncertainty in time, which leads to a smaller value for the Hubble constant. Conversely, there is less uncertainty in time for local measurements with a smaller redshift value, resulting in a higher value for the Hubble constant. Therefore, due to the uncertainty relation, the Hubble tension is believed to arise from fundamental limitations inherent in cosmological measurements. Finally, our findings indicate that the mass of the graviton fluctuates with specific scales, suggesting a possible mass-varying mechanism for the graviton.

astro-ph.CO↗

arXiv2024

Graviton mass due to dark energy as a superconducting medium: theoretical and phenomenological aspects

It is well known that the cosmological constant term in the Einstein field equations can be interpreted as a stress tensor for dark energy. This stress tensor is formally analogous to an elastic constitutive equation in continuum mechanics. As a result, the cosmological constant leads to a "shear modulus" and "bulk modulus" affecting all gravitational fields in the universe. The form of the constitutive equation is also analogous to the London constitutive equation for a superconductor. Treating dark energy as a type of superconducting medium for gravitational waves leads to a Yukawa-like gravitational potential and a massive graviton within standard General Relativity. We discuss a number of resulting phenomenological aspects such as a screening length scale that can also be used to describe the effects generally attributed to dark matter. In addition, we find a gravitational wave plasma frequency, index of refraction, and impedance. The expansion of the universe is interpreted as a Meissner-like effect as dark energy causes an outward "expulsion" of space-time similar to a superconductor expelling a magnetic field. The fundamental cause of these effects is interpreted as a type of spontaneous symmetry breaking of a scalar field. There is an associated chemical potential, critical temperature, and an Unruh-Hawking effect associated with the formulation.

gr-qc↗

arXiv2024

Modified gravity/entropic gravity correspondence due to graviton mass

Some time ago, it has been suggested that gravitons can acquire mass in the process of spontaneous symmetry breaking of diffeomorphisms through the condensation of scalar fields [Chamseddine and Mukhanov, JHEP, 2010]. Taking this possibility into account, in the present paper, first we show how the graviton mass intricately reshapes the gravitational potential akin to a Yukawa-like potential at large distances. Notably, this long-range force modifies the Newton's law in large distances and might explain the phenomena of dark matter. The most important finding in the present paper is the derivation of a modified Newtons law of gravity by modifying the Verlindes entropic force relation due to the graviton contribution. The graviton contribution to the entropy basically measures the correlation of graviton and matter fields which then reproduces the Bekenstein-Hawking entropy at the horizon. This result shows the dual description of gravity: in the language of quantum information and entropy the gravity can be viewed as an entropic force, however in terms of particles and fields, it can be viewed as a longe range force. Further we have recovered the corrected Einstein field equations as well as the $Λ$CDM where dark matter emerges as an apparent effect.

gr-qc↗

arXiv2024

Effect of magnetized plasma on shadow and gravitational lensing of a Reissner-Nordström black hole

We explore the influence of the axion-plasmon on the optical properties of the charged black hole and show how the axion-plasmon coupling modifies the motion of photons around the charged black hole. We then explore in details observational effects such as the black hole shadow, the gravitational deflection angle, Einstein rings and shadow images obtained by radially infalling gas on a black hole within a plasma medium. An important finding is that the intensity of the electromagnetic radiation increases with the increase of charge and the size of the black hole shadow decreases with increase of the electric charge for a fixed axion-plasmon coupling values when observed from sufficiently large distance. Overall, for a constant value of charge the optical appearance of the black hole shadow depends on the surrounding plasma model and the largest shadow radius if found for the case of no plasma, while the smallest shadow radius is found for the case of homogeneous plasma.

gr-qc↗

arXiv2024

Dark matter signatures of black holes with Yukawa potential

This study uses a nonsingular Yukawa--modified potential to obtain a static and spherically symmetric black hole solution with a cosmological constant. Such Yukawa--like corrections are encoded in two parameters, $α$ and $λ$, that modify Newton's law of gravity in large distances, and a deformation parameter $\ell_0$, which plays an essential role in short distances. The most significant effect is encoded in $α$, which modifies the total black hole mass with an extra mass proportional to $αM$, mimicking the dark matter effects at large distances from the black hole. On the other hand, the effect due to $λ$ is small for astrophysical values. We scrutinize the \textit{quasinormal} frequencies and shadows associated with a spherically symmetric black hole and the thermodynamical behavior influenced by the Yukawa potential. In particular, the thermodynamics of this black hole displays a rich behavior, including possible phase transitions. We use the WKB method to probe the \textit{quasinormal} modes of massless scalar, electromagnetic, and gravitational field perturbations. In order to check the influence of the parameters on the shadow radius, we consider astrophysical data to determine their values, incorporating information on an optically thin radiating and infalling gas surrounding a black hole to model the black hole shadow image. In particular, we consider Sgr A* black hole as an example and we find that its shadow radius changes by order of $10^{-9}$, meaning that the shadow radius of a black hole with Yukawa potential practically gives rise to the same result encountered in the Schwarzschild black hole. Also, in the eikonal regime, using astrophysical data for Yukawa parameters, we show that the value of the real part of the QNMs frequencies changes by $10^{-18}$.

gr-qc↗

arXiv2024

Testing Yukawa cosmology at the Milky Way and M31 galactic scales

We address the galaxy rotation curves through the Yukawa gravitational potential emerging as a correction of the Newtonian potential in extended theories of gravity. On the one hand, we consider the contribution of the galactic bulge, galactic disk, and the dark matter halo of the Navarro-Frenk-White profile, in the framework of the standard $Λ$CDM model. On the other hand, we use modified Yukawa gravity to show that the rotational velocity of galaxies can be addressed successfully without the need for dark matter. In Yukawa gravity, we recover MOND and show that dark matter might be seen as an apparent effect due to the modification of the law of gravitation in terms of two parameters: the coupling constant $α$ and the characteristic length $λ$. We thus test our theoretical scenario using the Milky Way and M31 rotation velocity curves. In particular, we place observational constraints on the free parameters of Yukawa cosmology through the Monte Carlo method and then compare our results with the predictions of the $Λ$CDM paradigm by making use of Bayesian information criteria. Specifically, we find that $λ$ is constrained to be of the order of kpc, while cosmological data suggest $λ$ of the order of Gpc. To explain this discrepancy, we argue that there is a fundamental limitation in measuring $λ$ due to the role of quantum mechanics on cosmological scales.

astro-ph.CO↗

arXiv2024

Charged Black Holes with Yukawa Potential

This study derives a novel family of charged black hole solutions featuring short- and long-range modifications. These ones are achieved through a Yukawa-like gravitational potential modification and a nonsingular electric potential incorporation. The short-range corrections encode quantum gravity effects, while the long-range adjustments simulate gravitational effects akin to those attributed to dark matter. Our investigation reveals that the total mass of the black hole undergoes corrections owing to the apparent presence of dark matter mass and the self-adjusted electric charge mass. Two distinct solutions are discussed: a regular black hole solution characterizing small black holes, where quantum effects play a crucial role, and a second solution portraying large black holes at considerable distances, where the significance of Yukawa corrections comes into play. Notably, these long-range corrections contribute to an increase in the total mass and hold particular interest as they can emulate the role of dark matter. Finally, we explore the phenomenological aspects of the black hole. Specifically, we examine the influence of electric charge and Yukawa parameters on thermodynamic quantities, the quasinormal modes for the charged scalar perturbations as well as for the vector perturbations, analysis of the geodesics of light/massive particles, and the accretion of matter onto the charged black hole solution.

gr-qc↗

arXiv2024

Apparent Dark Matter Inspired by Einstein Equation of State

The purpose of this article is twofold. First, by means of Padmanabhan's proposal on the emergence nature of gravity, we recover the $Λ$CDM model and the effect of the dark matter in the context of cosmology. Toward this goal, we use the key idea of Padmanabhan that states cosmic space emerges as the cosmic time progress and link the emergence of space to the difference between the number of degrees of freedom on the boundary and in the bulk. Interestingly enough, we show that the effect of the cold dark matter in the cosmological setup can be understood by assuming an interaction between the numbers of degrees of freedom in the bulk. In the second part, we follow the Jacobson's argument and obtain the modified Einstein field equations with additional dark matter component emerging due to the interaction term between dark energy and baryonic matter related by $Ω_{DM,0}=\sqrt{2 αΩ_{M,0} Ω_{DE,0}}$, where $α$ is a coupling constant. Finally, a correspondence with Yukawa cosmology is pointed out, and the role of massive gravitons as a possibility in explaining the nature of the dark sector as well as the theoretical origin of the Modified Newtonian Dynamics (MOND) are addressed. We speculate that the interaction coupling $α$ fundamentally measures the entanglement between the gravitons and matter fields and there exists a fundamental limitation in measuring the gravitons wavelength.

gr-qc↗

arXiv2023

Superradiance and stability of rotating charged black holes in T-duality

We investigate the shadow images, the relation between Quasinormal Modes (QNMs) and the shadow radius, and the superradiance effect observed in the context of a rotating charged black hole under T-duality. Our investigation places particular emphasis on two key parameters: the electric charge denoted as $Q$ and the quantum deformed parameter represented by the zero-point length, $l_0$. Our findings reveal a distinct pattern: as the electric charge increases, the shadow radius experiences a consistent decrease. Intriguingly, when considering the quantum deformed parameter, we find a noteworthy phenomenona reflecting point. Specifically, we illustrate that the shadow radius initially increases with an increase in $l_0$ and subsequently decreases. Further analysis involves the computation of eikonal equatorial and polar QNMs, where a similar reflecting point emerges upon varying $l_0$. This establishes the inverse correlation between QNMs and shadow radius within our research framework. Our investigation into the effects of Q and $l_0$ on superradiance reveals that the amplification factor initially increases with $Q$ and $l_0$ and then starts decreasing. Moreover, the rotating black holes in T-duality allows superradiance scattering for a wider range of frequency than Kerr black holes, making black holes in T-duality brighter than the Kerr black holes. We also delve into the stability of the combined system of the rotating black hole and scalar field with the help of black hole bomb mechanism. It provides a window to observe the impact of parameters Q and $l_0$ on the stability. It shows that the combined system is stable for a wider regime for the Kerr black hole.

gr-qc↗

arXiv2023

Entropic corrections to Friedmann equations and bouncing universe due to the zero-point length

We employ Verlinde's entropic force scenario to extract the modified Friedmann equations by taking into account the zero-point length correction to the gravitational potential. Starting form the modified gravitational potential due to the zero-point length, we first find the logarithmic corrections to the entropy expression and then we derive the modified Friedman equations. Interestingly enough, we observe that the corrected Friedmann equations are similar to the Friedmann equations in braneworld scenario. In addition, from the corrected Friedmann equations, we pointed out a possible connection to the GUP principle which might have implications on the Hubble tension. To this end, we discuss the evolution of the scale factor under the effect of zero-point length. Finally, having in mind that the minimal length is of the Planck order, we obtain the critical density and the bouncing behavior of the universe with a critical density and a minimal scale factor of the order of Planck length.

gr-qc↗