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Project titleTimming effects of of compact astrophysical objects in theories of gravity

Research area 02 - PHYSICS AND SPACE SCIENCES, 02-601 - General relativity and gravitation

Keywordsblack holes, wormholes, naked singularities, time advancement, time delay, gravitational lensing, accretion, hoop conjecture

Annotation
The only way to have information on the nature of astrophysical objects in the universe is through their signals reaching the Earth. The path lengths of signals are modified by the presence of compact objects on their way leading to different timing effects, including the “new” effect of negative time delay (or gravitational time advancement), to be studied in this project. The Shapiro time delay occurs when signals pass from weaker to stronger gravity but the negative time delay occurs when signals pass from stronger to weaker gravity. This could be regarded as a new test of general relativity that could play a role in testing the isotropicity of light on Earth. Also, all compact objects in the universe are spinning. Therefore, the central goal of this project is to theoretically understand the effects of spin and frame dragging on various observable physical phenomena. The signature of spin is encoded in all the predictions associated with these physical phenomena. A practical example timing effect is the Sagnac delay, associated with the spinning Earth. Relative time delay between arriving signals and gravitational time advancement are two other important and new timing effects due to spin. Other important effects include strong field gravitational lensing and accretion phenomena associated with spinning objects that will also be addressed in this project. All the above phenomena both in the weak and strong field regime can be used to extract information about the nature of the gravitating source and relevant space-time geometry containing free solution parameters of generalized gravity theories. We wish to look for the influence of free parameters on all the physical phenomena stated above applying them to various spinning space-times mentioned above. In particular, spinning binaries provide the best natural laboratory for studying such phenomena. Our studies will provide concrete information helping us understand the nature of the central object by studying the influence of free parameters on: • timing properties of signals, which include positive (Shapiro) and negative time delays (this is a new effect), Sagnac and times of arrivals caused by frame dragging of the spinning sources • lensing signatures including shadows formed on the background of accretion flow around compact astrophysical objects. • accretion dynamics following the Bondi method of fluid flow and the Page-Thorne model. • Thorne’s hoop conjecture concerning black holes in general relativity. We have included it as a tool to verify whether it supports wormhole topologies and to verify whether it can possibly rule out some claimed black hole solutions of generalized gravity theories. The project is extremely relevant for obtaining comprehensive information of gravitating objects and a platform for viewing the distortion of observables caused by the free parameters of generalized gravity theories. Eventually, future experimental data will constrain the deviation parameters of any theory.

Expected results
Expected results and significance: • The work proposed in this project intends to expand the scope of distinguishing gravity theories and types of compact astrophysical objects using timing methods such as times of arrival, strong field time delay and most importantly, the effect of time advancement. The analyses are expected to provide a new method for testing how black holes, wormholes and naked singularities differ in view of the signal timing methods to be adopted. The significance is that this study will throw up observable predictions that can distinguish different types of central objects. • Gravitational lensing is directly linked to the timing of signals, especially time delay from relativistic images. This delay is expected to reveal the influence of free parameters, scalar field etc on the delay and yield verifiable predictions distinguishing the central object. • Another important diagnostics is the accretion phenomena with ordinary, phantom and quintessence matter accreting to the central object. Its application to spinning astrophysical objects such as black hole, wormhole is expected to reveal their new features heretofore unknown. The significance is that, unlike black holes, the last three objects arise from different field theoretic considerations but have not yet been adequately studied in relation to accretion properties. The present study is expected to fill that gap. • Thorne’s hoop conjecture is fundamental to black hole physics of general relativity. We shall examine whether or not the conjecture is validated by the black holes of competing gravity theories and thus the conjecture is expected to serve as Occam’s razor for ruling out other claimed black hole solutions. The significance is that, for the first time, we shall study in detail if the conjecture applies to wormholes as well. The significance of the new scientific topics developed in this project is that it offers excellent vistas for dealing with problems of current physical interest. The outcome is expected to lead to a deeper understanding of the effects of free parameters of spinning (or even static) objects on the motion of light and particles around them in terms of the various physical diagnostics mentioned above. Free parameters can also be constrained in some cases by means of the available observed data. In particular, the study will reveal how exactly the free parameters of the solutions may influence the astrophysical observables and the accuracies needed to observe them in practice. All scientific results will be published in peer-reviewed journals with a high impact factor included in the Scopus and Web of Science databases for evaluation by the world scientific community.

Annotation of the results obtained in 2023
The only way to obtain information about the nature of astrophysical objects in the Universe is through their signals reaching the Earth. The path length of signals is altered by the presence of compact objects in their path, leading to various timing effects, including the “new” effect of negative time delay (or gravitational time advancement) that was studied in this project. Shapiro time delay occurs when signals move from weaker to stronger gravity, and negative time delay occurs when signals move from stronger to weaker gravity. This effect can be seen as a new test of general relativity and may play a role in testing the isotropy of light on Earth. Thus, the main goal of this project is to theoretically understand the influence of various factors such as rotation and frame-dragging on various observable physical phenomena associated with time effects. In addition, it is important to study the influence of additional parameters of solutions of general relativity and modified theories of gravity on the time effects considered in the project. The relative time delay between incoming signals and the acceleration of gravitational time are two other important and novel timing effects associated with rotation. Calculation of the Sagnac delay for rotating compact objects makes it possible to limit the free parameters of solutions of modified theories of gravity using data from global positioning systems. Other important effects include gravitational lensing in weak and strong gravitational fields and accretion phenomena associated with rotating objects and testing of Thorne's hoop hypothesis, which were considered in the project in the first year. The main results of the first year of the project include the following: - a generalized approach was proposed to measure the Shapiro delay and gravitational acceleration of time using a high-precision Michelson-Morley type experiment. When light is directed vertically a short distance from the Earth and reflected back along the same path to the point of emission, it can naturally be influenced by the Earth's gravitational field. It is this influence that leads to negative gravitational time delay, which could be a potential new test of general relativity. One possible way to measure this effect on the Earth's surface would be to measure the anisotropy of the speed of light, which would be detected in a high-precision Michelson–Morley type experiment . - the deflection angle was obtained and the gravitational lensing parameters were studied in detail for two solutions of charged black holes in the string picture. It is shown that the charge of black holes does not affect the deflection angle in the first order. However, the second-order correction for magnetically charged black holes increases with increasing magnetic charge, while it decreases in the case of an electrically charged black hole. Since the charge appears in the first order of the post-post-Newtonian expansion, they do not appear in the case of weak gravitational lensing, as shown in our work. - the validity of the hoop conjecture for solutions for black holes was investigated. During the project, an extended class of black holes was considered, obtained by Lessa, Silva, Malouf and Almeida (LSMA) in the Kalb-Ramond model. This class of solutions offers interesting examples of single- or double-horizon black holes for arbitrary values of the Lorentz invariance breaking parameters. The status of Thorne's hoop conjecture sheds light on the nature of the relevant spacetimes, in particular the Schwarzschild–de Sitter black hole, in which horizons do exist but the conjecture does not hold. LSMA solutions are Lorentz violation-corrected black holes with Schwarzschild mass for arbitrary values of the Lorentz violation parameters . The horizons were divided into four mutually exclusive generalized types (A - generalized solution for the Reissner-Nordström black hole, B - generalized solution for the Schwarzschild-de Sitter black hole, C - generalized solution for the Schwarzschild-anti-de Sitter black hole, D - generalized solution for black hole in the braneworld model). Next, we checked the status of the conjecture for these spacetimes by plotting the corresponding Hod function H, developed by the project team, in a generalized format for arbitrary values. It was found that although the hoop conjecture H ≤ 1 holds for three types (A,C,D), it is violated in space-time type B, i.e. in Schwarzschild-de Sitter type black holes. - when studying the parameters of a thin disk formed around a black hole by Balart and Vagenas with a source in the form of nonlinear electrodynamics, it was found that the velocities of particles and their angular momenta are greater than in the case of a Reissner-Nordström black hole, while the temperature and luminosity of the disks are lower. It was also determined that during accretion non-phantom types of matter onto the Damur- Solodukhin wormhole, its mass increases, and with the accretion of phantom energy, the mass of two objects decreases. The results of the research conducted in the first year were reported at five conferences at the international and all-Russian levels. Petrov School – 2023, VI International Winter School-Seminar on gravity, cosmology, and astrophysics, Kazan, November 27-December 1, 2023 (4 oral presentations); International scientific and practical conference "SPECTRAL THEORY OF OPERATORS AND RELATED ISSUES", dedicated to the 75th anniversary of Professor Ya.T. Sultanaev, Ufa, October 26-27, 2023 (1 oral report); X IV International School-Conference of Students, Postgraduate Students and Young Scientists, dedicated to the 75th anniversary of Professors Ya.T. Sultanaev and M.Kh. Harrasov (Ufa, October 8 - 11, 2023 (2 oral presentations); II All-Russian youth school-conference “MODERN PHYSICS, MATHEMATICS, DIGITAL AND NANOTECHNOLOGIES IN SCIENCE AND EDUCATION (FMTS-23)”, dedicated to the 80th anniversary of birthday of Doctor of Physical and Mathematical Sciences , Professor R.S. Singatullin , Ufa, April 18-20, 2023 (2 oral reports). Three articles were published, two of which are included in the Scopus and WoS databases in the first quartile (The European Physical Journal C (https://link.springer.com/article/10.1140/epjc/s10052-023-12172-9), Universe (https://www.mdpi.com/2218-1997/9/6/263), News of the UFITs RAS (http://journal.ufaras.ru/?p=2976)). Manuscripts of three articles have been prepared.

Publications

1. Bhadra A., Izmailov R. N., Nandi K. K. On the possibility of observing negative Shapiro-like delay using Michelson–Morley-type experiments Universe, Bhadra A, Izmailov RN, Nandi KK. On the Possibility of Observing Negative Shapiro-like Delay Using Michelson–Morley-Type Experiments. Universe. 2023; 9(6):263 (year - 2023) https://doi.org/10.3390/universe9060263

2. Izmailov R.N., Karimov R.Kh., Minakhmetova E.O. Слабое гравитационное линзирование черными дырами с зарядом Известия Уфимского научного центра РАН, Измаилов Р.Н., Каримов Р.Х., Минахметова Э.О. / Cлабое гравитационное линзирование черными дырами с зарядом // Известия Уфимского научного центра РАН. 2023. № 1. С. 32-37. (year - 2023) https://doi.org/10.31040/2222-8349-2023-0-1-32-37

3. Nandi K. K., Izmailov R.N., Karimov R. Kh., Potapov A. A On the Kalb–Ramond modified Lorentz violating hairy black holes and Thorne’s hoop conjecture EUROPEAN PHYSICAL JOURNAL C, Nandi, K.K., Izmailov, R.N., Karimov, R.K. et al. On the Kalb–Ramond modified Lorentz violating hairy black holes and Thorne’s hoop conjecture. Eur. Phys. J. C 83, 984 (2023). (year - 2023) https://doi.org/10.1140/epjc/s10052-023-12172-9

4. - Что не так с машиной времени физика-теоретика из Коннектикута? Разбираемся с российским ученым Вокруг света, - (year - )