Browsing by Author "Zhu, Tao"

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Constraining the generalized uncertainty principle through black hole shadow, S2 star orbit, and quasiperiodic oscillations
(2022) Jusufi, Kimet; Azreg-Ainou, Mustapha; Jamil, Mubasher; Zhu, Tao
In this paper, we study the effect of the Generalized Uncertainty Principle (GUP) on the shadow of GUP-modified Kerr black hole and the correspondence between the shadow radius and the real part of the quasinormal modes (QNMs). We find that the shadow curvature radius of the GUP-modified Kerr black hole is bigger compared to the Kerr vacuum solution and increases linearly monotonically with the increase of the GUP parameter. We then investigate the characteristic points of intrinsic curvature of the shadow from a topological point of view to calculate the angular size for these curvature radii of the shadow. To this end, we have used the EHT data for the M87* black hole to constrain the upper limits of the GUP parameter and our result is beta < 10(95). Finally, we have explored the connection between the shadow radius and the scalar/electromagnetic/gravitational QNMs. Using the orbit of S2 star we have obtained a bound for the GUP parameter beta < 10(87). The GUP-modified Kerr black hole is also used to provide perfect curve fitting of the particle oscillation upper and lower frequencies to the observed frequencies for three microquasars and to restrict the values of the correction parameter in the metric of the modified black hole to very reasonable bound beta < 10(77).
Observational test of R 2 spacetimes with the S2 star in the Milky Way Galactic Center
(Başkent Üniversitesi Mühendislik Fakültesi, 2024-08-06) Yan, Jian-Ming; Zhu, Tao; Azreg-Ainou, Mustapha; Jamil, Mubasher; Nguyen, Hoang Ky
A novel class of vacuum metrics expressible in analytical form was recently found for pure 7Z2 2 gravity, based on a groundwork put forth by Buchdahl in 1962. These Buchdahl-inspired solutions offer a practical framework for testing 7Z2 2 gravity through empirical observations. Within a subclass of asymptotically flat Buchdahl-inspired vacuum spacetimes, we identified a parameter E measuring the deviation from the classic Schwarzschild metric, which corresponds to E = 0. In this paper, we employ observational data from the S2 star's orbit around Sgr A* in the Milky Way galactic center and perform Monte Carlo Markov Chain simulations to probe the effects of the new metrics on the orbit of the S2 star. Our analysis presented herein reports a range at 95% confidence level on the deviation parameter as E is an element of (-0.6690, - 0.6690, 0.4452). While no decisive evidence either in favor or in disfavor of the asymptotically flat Buchdahl-inspired spacetimes has been achieved, the obtained bound is compatible with the tighter results using other data of different nature as recently reported in Eur. Phys. J. C 84 (2024) 330. As a meaningful test probing into a strong-field regime, our present study calls for further observations with prolonged period and improved accuracy in order to tighten the bound for E using the S2 star orbit.
Observational Tests Of Asymptotically Flat r2 Spacetimes
(Başkent Üniversitesi Mühendislik Fakültesi, 2024-04-07) Zhu, Tao; Nguyen, Hoang Ky; Azreg-Ainou, Mustapha; Jamil, Mubasher
A novel class of Buchdahl-inspired metrics with closed-form expressions was recently obtained based on Buchdahl's seminal work on searching for static, spherically symmetric metrics in R-2 gravity in vacuo. Buchdahl-inspired spacetimes provide an interesting framework for testing predictions of R(2 )ravity models against observations. To test these Buchdahl-inspired spacetimes, we consider observational constraints imposed on the deviation parameter, which characterizes the deviation of the asymptotically flat Buchdahl-inspired metric from the Schwarzschild spacetime. We utilize several recent solar system experiments and observations of the S2 star in the galactic center and the black hole shadow. By calculating the effects of Buchdahl-inspired spacetimes on astronomical observations both within and outside of the solar system, including the deflection angle of light by the Sun, gravitational time delay, perihelion advance, shadow, and geodetic precession, we determine observational constraints on the corresponding deviation parameters by comparing theoretical predictions with the most recent observations. Among these constraints, we find that the tightest one comes from the Cassini mission's measurement of gravitational time delay.
Orbital mechanics and quasiperiodic oscillation resonances of black holes in Einstein-AEther theory
(2020) Azreg-Ainou, Mustapha; Chen, Zihang; Deng, Bojun; Jamil, Mubasher; Zhu, Tao; Wu, Qiang; Lim, Yen-Kheng
In this paper, we study the motion of test particles around two exact charged black hole solutions in Einstein-AEther theory. Specifically, we first consider the quasiperiodic oscillations (QPOs) and their resonances generated by the particle moving in the Einstein-AEther black hole and then turn to study the periodic orbits of the massive particles. For QPOs, we drop the usually adopted assumptions nu(U) = nu(theta), nu(L) = nu(r), and nu(U)/nu(L) = 3/2 with nu(U) (nu(L)) and nu(r) (nu(theta)) being the upper (lower) frequency of QPOs and radial (vertical) epicyclic frequency of the orbiting particles, respectively. Instead, we put-forward a new working ansatz for which the Keplerian radius is much closer to that of the innermost stable circular orbit and explore in detail the effects of the aether field on the frequencies of QPOs. We then realize good curves for the frequencies of QPOs, which fit to data of three microquasars very well by ignoring any effects of rotation and magnetic fields. The innermost stable circular orbits (isco) of timelike particles are also analyzed, and we find the isco radius increases with increasing c(13) for the first type black hole while decreases with increasing c(14) for the second one. We also obtain several periodic orbits and find that they share similar taxonomy schemes as the periodic equatorial orbits in the Schwarzschild/Kerr metrics, in addition to exact solutions for certain choices of the Einstein-AE ther parameters. The equations for null geodesics are also briefly considered, where we study circular photon orbits and bending angles for gravitational lensing.
Shadow and quasinormal modes of a rotating loop quantum black hole
(2020) Liu, Cheng; Zhu, Tao; Wu, Qiang; Jusufi, Kimet; Jamil, Mubasher; Azreg-Ainou, Mustapha; Wang, Anzhong
In this paper, we construct an effective rotating loop quantum black hole (LQBH) solution, starting from the spherical symmetric LQBH by applying the Newman-Janis algorithm modified by Azreg-Ainou's noncomplexification procedure, and study the effects of loop quantum gravity (LQG) on its shadow. Given the rotating LQBH, we discuss its horizon, ergosurface, and regularity as r -> 0. Depending on the values of the specific angular momentum a and the polymeric function P arising from LQG, we find that the rotating solution we obtained can represent a regular black hole, a regular extreme black hole, or a regular spacetime without horizon (a non-black-hole solution). We also study the effects of LQG and rotation, and show that, in addition to the specific angular momentum, the polymeric function also causes deformations in the size and shape of the black hole shadow. Interestingly, for a given value of a and inclination angle theta(0), the apparent size of the shadow monotonically decreases, and the shadow gets more distorted with increasing P. We also consider the effects of P on the deviations from the circularity of the shadow, and find that the deviation from circularity increases with increasing P for fixed values of a and theta(0). Additionally, we explore the observational implications of P in comparing with the latest Event Horizon Telescope observation of the supermassive black hole, M87*. The connection between the shadow radius and quasinomial modes in the eikonal limit as well as the deflection of massive particles are also considered.