受物理学院邀请，乌兹别克斯坦科学院院士来我校交流访问，期间进行系列讲座。
主讲人简介：
1.Bobomurat AHMEDOV教授于1993年与塔什干核物理研究所获得博士学位，并于2001年获得乌兹别克斯坦国立大学(NUUz)物理学和数学的最高理学博士学位，2023年当选为乌兹别克斯坦科学院院士。现就职于乌兹别克斯坦塔什干国立大学天文学与理论物理学院、乌兹别克斯坦科学院乌鲁贝格天文研究所。他的研究方向包括广义相对论与万有引力，相对论天体物理，黑洞，磁化中子星，恒星和黑洞的摄动，引力致密物体的能量学和光学等。发表论文200余篇，被引5700余次，H指数42，出版图书12部。
2. Ahmadjon ABDUJABBAROV 研究员于2009年获得乌兹别克斯坦塔乌鲁别克核物理研究所博士学位（Ph.D），并于2016年获得乌兹别克斯坦国立大学获得Doctor of Science学位。现就职于乌鲁贝格天文研究所担任首席研究员。他目前的研究方向包括电动力学、相对论天体物理学和广义相对论领域的理论研究等。在Physical Review D、European Physical Journal C、New Journal of Physics、Chinese Physics C等国际期刊上发表学术论文180余篇，被引5300余次，H指数43。
（一）2025年5月12日，9:00-12:00 讲座，理学楼822
1. STUDY OF IONOSPHERIC PERTURBATIONS IN D- AND F-LAYERS USING VLF RECEIVER AND TASHKENT-KITAB GPS STATIONS (Bobomurat AHMEDOV)
Tashkent International Heliophysical Year (IHY) station is a member of Atmospheric Weather Electromagnetic System for Observation, Modeling and Education (AWESOME) network being operated globally to study the ionosphere and the magnetosphere with the help of electromagnetic waves in Very Low Frequency (VLF) band. Regular monitoring of the D- and F-layers of ionosphere over Central Asia territory is being performed on the permanent basis starting year 2008 when one VLF receiver and two SuperSID receivers were provided to Uzbekistan IHY cite by Stanford University. The results obtained at Tashkent IHY station are applied to earthquake electromagnetic precursors, lightning, and Solar flares and to ionospheric disturbances originating from gamma ray flares of Soft Gamma-Ray Repeaters connected with evolution of strongly magnetized neutron stars believed as magnetars.
Regular monitoring of the D-layer of ionosphere over Central Asia territory has been performed on the permanent basis. Several Solar events are observed and the analysis has shown that there is simultaneous correlation between the times of change of amplitude of the waves and the Solar flares. Features of the lightning discharge generated by radio atmospherics are studied and its effectiveness in D-region ionosphere diagnostics is examined.
We have mainly analysed GPS derived TEC disturbances from two GPS stations located in Tashkent and Kitab, for possible earthquake ionospheric precursors. The ionospheric anomalies were observed during strong local earthquakes (M greater than 5.0) which occurred mostly in and around Uzbekistan in seismically active zone within 1000 km from the observing GPS stations located in Tashkent and Kitab. The solar and geomagnetic conditions were quiet during occurrence of the selected earthquakes. We produced TEC time series over both sites and apply them to detect anomalous TEC signals preceding or accompanying the earthquakes. The results show anomalous enhancements which are examined in the earthquakes. In general the anomalies occurred 1-6 days before the earthquakes as ionospheric electromagnetic precursors. To identify the anomalous values of TEC we calculated differential TEC (dTEC). dTEC is obtained by subtracting 15 days backward running mean of vTEC from the values of observed vTEC at ach epoch.
2. Black hole shadow as useful tool for testing the gravity theories. (Ahmadjon ABDUJABBAROV)
The observation of black hole shadows by the Event Horizon Telescope (EHT) has opened a new window for testing gravity theories in the strong-field regime. The shadow, a dark region caused by photon capture near the event horizon, encodes information about spacetime geometry, making it a powerful probe for alternatives to general relativity (GR). By comparing theoretical predictions of shadow morphology – influenced by parameters such as spin, charge, and modified gravity corrections – with high-resolution imaging data, stringent constraints can be placed on deviations from GR, including scalar-tensor, f(R), and extra-dimensional theories. This review highlights recent advances in shadow of the black holes, the role of gravitational physics, and the coordinate independent way to describe the image.
（二）2025年5月14日，9:00-12:00 讲座，理学楼822
1. GENERAL RELATIVISTIC MAGNETOHYDRODYNAMIC SIMULATIONS OF ACCRETION STRUCTURES (Bobomurat AHMEDOV)
There are many ways to extract information from black holes but in this section, we are focused on the methods that allow getting constraints on the main characteristics of black holes based on the electromagnetic spectrum of the radiation coming from the accretion disk surrounding a central black hole. The main idea of each method is based on the reflection of the final electromagnetic spectrum radiated by the accretion disk around the black hole because astronomical observations indicate that such spectrum is strongly affected by the gravitational field of the central black hole. This in turn allows one to extract information about the main parameters of the black hole situated in the central part of the accretion disk. However, it is worth noting that such methods strongly depend on the model chosen to explain the black hole spacetime, the basic mechanism of phenomena for the method used, and the structure of the accretion disk surrounding the central black hole. One should also notice that there are various accretion disk models in the literature, for example, a model that describes an accretion disk that has a torus-like structure and evolves with time. General relativistic magneto-hydrodynamic (GRMHD) simulations of such a disk model have been investigated by Tashkent group members. For our purpose, we mostly use the so-called Novikov-Thorne accretion disk model, which is the simplest one and widely used in most analytic methods. The Novikov-Thorne thin disc model can be used to obtain the continuum spectrum emitted from the accretion disc of a black hole.
The matter orbiting black hole (BH) in micro-quasar or active galactic nuclei forms toroidal accretion disk structures, and multiple tori structures have been recently described as ringed accretion disks (RADs) in full general relativistic approach. We explore the dynamics of accretion structures encircling spherically symmetric black holes, comparing three accretion disk models with distinct angular momentum profiles: (i) the geometrically thin Keplerian disk, (ii) the Fishbone–Moncrief torus; and (iii) the Polish Doughnut. Employing general relativistic magnetohydrodynamics simulations with the High Accuracy Relativistic Magnetohydrodynamics code, we investigate these three models, considering the magnetic field’s influence on the accretion disk angular momentum redistribution. We realize a full general relativistic magnetohydrodynamic (GRMHD) numerical simulations related to double toroidal structure immersed into the equatorial plane of gravitomagnetic field of central Schwarzschild BH in an asymptotically uniform magnetic field. We show that the magnetic field is a key factor in accretion disk structures, especially in regions with lower mass density. We study the dynamics of the system assuming various initial conditions and we have demonstrated that initial matter density is the relevant factor governing the system evolution. Our investigation verifies the well-established fact that the presence of a magnetic field significantly influences the accretion rate and its temporal variability.
2. Test particles motion around compact objects in the presence of electromagnetic field. (Ahmadjon ABDUJABBAROV)
The motion of test particles around compact objects – such as black holes and/or neutron stars – in the presence of electromagnetic fields provides critical insights into the interplay between gravity and electromagnetism in strong-field regimes. This study explores the dynamics of charged and neutral particles in the vicinity of these objects, where relativistic effects dominate. By analyzing orbital stability, innermost stable circular orbits (ISCO), and chaotic motion under the influence of combined gravitational and electromagnetic forces, we assess how external fields alter trajectories compared to pure general relativistic predictions. Particular attention is given to the role of magnetic fields in particle acceleration, jet formation, and synchrotron radiation, with implications for astrophysical observations (e.g., quasi-periodic oscillations). The results are extended to alternative gravity theories, highlighting deviations from General Relativity. This work underscores the importance of electromagnetic fields in testing fundamental physics through astrophysical probes.
（三）2025年5月16日，9:00-12:00 讲座，理学楼822
1. ENERGETIC PROPERTIES OF GRAVITATIONAL COMPACT OBJECTS: BLACK HOLES, PULSARS AND MAGNETARS (Bobomurat AHMEDOV)
Modern astronomical observations on the international level on the ground and space telescopes, and recent discoveries have provided convincing evidence that black holes have a significant impact on nearby objects, emitting powerful gamma-ray bursts, absorbing the next star, and stimulating the growth of newborn stars in the surrounding areas.
I will discuss end state of evolution of massive stars, various observational properties of magnetized neutron stars. The energetics of rotating black holes and neutron stars is also in the scope of my talk.
The magnetosphere of a slowly rotating magnetized neutron star subject to toroidal oscillations in the relativistic regime is studied. Under the assumption of a zero inclination angle between the magnetic moment and the angular momentum of the star, we analyse the Goldreich–Julian charge density and derive a second-order differential equation for the electrostatic potential. The analytical solution of this equation in the polar cap region of the magnetosphere shows the modification induced by stellar toroidal oscillations on the accelerating electric field and on the charge density. We also find that, after decomposing the oscillation velocity in terms of spherical harmonics, the first few modes with m = 0, 1 are responsible for energy losses that are almost linearly dependent on the amplitude of the oscillation and that, for the mode (l,m) = (2, 1), can be a factor about 8 larger than the rotational energy losses, even for a velocity oscillation amplitude at the star surface small. The results obtained clarify the extent to which stellar oscillations are reflected in the time variation of the physical properties at the surface of the rotating neutron star, mainly by showing the existence of a relation between spindown and the oscillation amplitude.
We propose a qualitative model for the explanation of the phenomenology of intermittent pulsars. The idea is that stellar oscillations, periodically excited by star glitches, can create relativistic winds of charged particles because of the additional electric field. When the stellar oscillations damp, the pulsar shifts below the death line in the P–B diagram, thus entering the OFF invisible state of intermittent pulsars.
The conditions for radio emission in rotating and oscillating magnetars by focusing on the main physical processes determining the position of their death lines, i.e. of those lines that separate the regions where the neutron star may be radio loud or radio quiet. After using the general relativistic expression for the electromagnetic scalar potential in the magnetar magnetosphere, we find that larger compactness parameters of the star as well as larger inclination angles between the rotation axis and the magnetic moment produce death lines well above the majority of known magnetars. This is consistent with the observational evidence of no regular radio emission from the magnetars in the frequency range typical for the ordinary pulsars. On the contrary, when oscillations of the magnetar are taken into account, the death lines shift downward and the conditions necessary for the generation of radio emission in the magnetosphere are met. Present observations showing a close connection between the burst activity of magnetars and the generation of the radio emission in the magnetar magnetosphere are naturally accounted within our interpretation.
2.Gravitational lensing around black holes in different gravity models (Ahmadjon ABDUJABBAROV)
Gravitational lensing around black holes serves as a powerful observational probe to test and constrain alternative theories of gravity in the strong-field regime. This study investigates the deflection of light and the formation of lensing signatures - such as Einstein rings, relativistic images around black holes described by various gravitational models beyond general relativity (GR). We analyze the lensing effects in metrics arising from modified gravity theories, comparing their predictions with those of classical black holes in GR. Numerical and analytical methods are employed to compute deflection angles, magnification profiles, highlighting observable differences that could be resolved by next-generation telescopes (e.g., EHT or LISA). Our results demonstrate how lensing phenomenology can discriminate between competing gravity theories, offering a pathway to probe quantum corrections, extra dimensions, and dark matter interactions near black hole horizons.
（四）2025年5月19日，9:00-12:00 讲座，理学楼822
1. MODERN UNDERSTANDING OF STRUCTURE AND EVOLUTION OF THE UNIVERSE (Bobomurat AHMEDOV)
According to the cosmological Big Bang theory of the Universe space and time emerged together 13.7 billion years ago and the energy and matter initially present have become less dense as the Universe expanded. After an initial accelerated expansion called the inflationary epoch at around 10-32 seconds, and the separation of the four known fundamental forces, the Universe gradually cooled and continued to expand, allowing the first subatomic particles and simple atoms to form. Dark matter gradually gathered, forming a foam-like structure of filaments and voids under the influence of gravity. Giant clouds of hydrogen and helium were gradually drawn to the places where dark matter was most dense, forming the first galaxies, stars, and everything else seen today. From recent observations it has been discovered that the universe contains much more matter than is accounted for by visible objects; stars, galaxies, nebulas and interstellar gas. This unseen matter is known as dark matter (dark means that there is a wide range of strong indirect evidence that it exists, but we have not yet detected it directly). The ΛCDM model is the most widely accepted cosmological model of our universe which is now again in the epoch of the accelerated expansion. It suggests that about 73% of the mass and energy in the universe is a cosmological constant (or, in extensions to ΛCDM, other forms of dark energy, such as a scalar field) which is responsible for the current expansion of space, and about 23% is dark matter. Ordinary ('baryonic') matter is therefore only 4% of the universe which forms stars, planets, and visible gas clouds. The James Webb Space Telescope (JWST) has revolutionized our understanding of the cosmos through its groundbreaking observations, revealing new insights into the fundamental workings of the universe. Recent discoveries with the JWST have provided astronomers with a wealth of cosmological data, uncovering surprises and challenging existing theories.
One of the key findings from JWST observations is the detection of ancient galaxies dating back to the early universe. These observations have shed light on the formation and evolution of galaxies, offering clues about the conditions that prevailed during the universe's infancy. By studying these distant galaxies, astronomers have gained valuable insights into the processes driving cosmic evolution over billions of years.
Additionally, the JWST has played a crucial role in studying the atmospheres of exoplanets, offering tantalizing glimpses into worlds beyond our solar system. These observations have identified potential signs of habitability and provided vital information about the composition and dynamics of exoplanetary atmospheres. Such discoveries have profound implications for our understanding of the prevalence of life in the universe.
Furthermore, the JWST has contributed to our knowledge of star formation and stellar evolution within our own galaxy and beyond. By peering through cosmic dust clouds and observing stellar nurseries, the telescope has provided unprecedented views of the birth and death of stars, illuminating the processes that shape the cosmic landscape.
In addition to these discoveries, the JWST has unveiled unexpected phenomena and cosmic puzzles, challenging astronomers to rethink existing models of the universe. From mysterious dark matter structures to enigmatic cosmic phenomena, the telescope's observations continue to inspire awe and curiosity, driving forward our quest to unravel the mysteries of the cosmos. The James Webb Space Telescope's recent discoveries have transformed our understanding of the universe, offering new insights into its origins, evolution, and composition. As the telescope continues to probe the depths of space, we can anticipate even more groundbreaking discoveries that will shape our understanding of how the universe works.
2.Parametrization of the spacetime metrics as a tool of probing gravity theories (Ahmadjon ABDUJABBAROV)
The parametrization of spacetime metrics offers a powerful and systematic approach to test and constrain alternative theories of gravity using astrophysical observations. Building on recent developments in metric parametrization frameworks – such as the Parametrized Post-Newtonian (PPN) formalism for weak-field regimes and Parametrized Post-Kerr (PPK) approaches for strong-field gravity—this study explores their efficacy in probing deviations from general relativity (GR. We investigate how parametrized metrics can be applied to phenomena such as black hole shadows, gravitational wave signatures, and orbital dynamics to distinguish between GR and modified gravity theories (e.g., scalar-tensor, etc.). By comparing theoretical predictions with high-precision data from the Event Horizon Telescope (EHT) and LIGO-Virgo, one may demonstrate how such parametrizations can quantify possible departures from GR while maintaining observational consistency. The analysis highlights the advantages of this approach in identifying degeneracies among competing theories and proposes future observational strategies to refine constraints on gravity beyond Einstein’s framework.
（五）2025年5月21日，9:00-12:00 讲座，理学楼822
1. Quasinormal Modes of Black Holes and Gravitational Waves (Bobomurat AHMEDOV)
Quasinormal modes (QNMs) are a characteristic feature of black holes and other compact objects. They are complex frequencies that describe the oscillations of fields (such as the gravitational field) around a black hole. The real part of the frequency describes the oscillation frequency, while the imaginary part describes the rate at which the oscillation decays. QNMs are important because they can provide information about the properties of a black hole, such as its mass and angular momentum.
In the case of regular black holes, QNMs have been studied extensively in the context of alternative theories of gravity. Regular black holes are a class of black holes that are proposed to exist in some alternative theories of gravity, such as modified gravity or nonsingular black hole models. These theories attempt to alter the singularity at the center of a black hole, which is a point of infinite density and curvature, to a regular, non-singular object. This can be done by introducing new fields or modifying the Einstein field equations. One of the main challenges in studying QNMs of regular black holes is that these objects are not well understood and there is no consensus on their exact properties. Therefore, it is difficult to make precise predictions about their QNMs. However, several approaches have been proposed to study QNMs of regular black holes. One approach is to use perturbation theory, which involves solving the equations of motion for small perturbations around a black hole. This can provide insight into the behavior of QNMs for a given black hole model. Another approach is to use numerical methods, such as the finite element method or the pseudospectral method, to solve the equations of motion for a black hole. This can provide more accurate predictions for the QNMs of a black hole, but it is a more computationally intensive approach.
Despite the challenges, there have been a number of studies that have attempted to calculate the QNMs of regular black holes. These studies have found that the QNMs of regular black holes can be significantly different from those of singular black holes. For example, the QNMs of regular black holes may have a different frequency dependence on the mass and angular momentum of the black hole, and they may also have a different behavior in the low-frequency limit. Our study shows how electromagnetic perturbations evolve over time in both NED Maxwellian regular black holes and RN black holes. The main difference in how electromagnetic perturbations behave in black holes with linear and nonlinear electrodynamics is that increasing the charge parameter in NED Maxwellian regular black holes prolongs the duration of perturbations, while in linear electrodynamics, it shortens the lifetime of the electromagnetic perturbations. Overall, the study of QNMs of regular black holes is an active area of research that can provide insight into the properties of these objects and the theories of gravity that describe them. While there is still much that is unknown about regular black holes, the study of their QNMs can help to shed light on their nature and may provide a way to test these theories experimentally.
The talk is also focused on tests of Einstein's theory of general relativity versus alternative theories of gravity with gravitational waves that are detectable by ground-based interferometers. Einstein's gravity theory has been greatly constrained in the quasi-linear, quasi-stationary weak field regime, where gravity is weak and velocities are small. Gravitational waves allow us to probe a complementary, previously unexplored strong field regime: the non-linear and dynamical extreme gravity regime. Such a regime is, for example, applicable to compact binaries coalescing, where characteristic velocities can reach fifty per cent of the speed of light and gravitational fields are large and dynamic.
2. Effect of plasma on astrophysical and optical properties of the black holes (Ahmadjon ABDUJABBAROV)
The presence of plasma around black holes significantly alters their astrophysical and optical properties, offering a unique probe to study the interplay between strong gravity and magnetized matter. This work investigates how plasma influences key observable features, including black hole shadows, gravitational lensing by modifying light propagation through dispersive and absorptive effects. Using general relativistic ray-tracing and magnetohydrodynamic (MHD) simulations, we analyze the dependence of shadow morphology and intensity profiles on plasma density distribution. The study demonstrates that plasma can induce frequency-dependent shifts in photon ring diameters, suppress certain polarization modes, and enhance synchrotron radiation features—effects detectable with next-generation instruments like the next-generation Event Horizon Telescope (EHT). Furthermore, we explore implications for testing gravity theories in plasma-rich environments, where traditional vacuum metrics must be extended to include refractive and dispersive plasma contributions. The results highlight plasma as both an obstacle and an opportunity in precision black hole astrophysics, with consequences for interpreting observations of Sgr A* and M87*.
