Role of Black Holes in Testing Modified Theories of Gravity

A special issue of Symmetry (ISSN 2073-8994). This special issue belongs to the section "Physics".

Deadline for manuscript submissions: closed (31 December 2023) | Viewed by 3489

Special Issue Editor


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Guest Editor
Astrophysics Research Centre, School of Mathematics, Statistics and Computer Science, University of KwaZulu-Natal, Private Bag 54001, Durban 4000, South Africa
Interests: gravitation; black holes; gravitational waves

Special Issue Information

Dear Colleagues,

In the past few years, the spectacular progress made in gravitational wave detection by LIGO/Virgo and shadow observations by the Event Horizon Telescope collaboration has shed light on the previously inaccessible horizon-scale physics. These observations are exciting, not only because of the results they confirm but also because they allow us to test our current understanding of gravitational theory in the strong-field and highly relativistic regimes, as well as probe any deviation from general relativity. Given the large number of modified theories to be tested, a more sophisticated approach would be to develop model-independent ways of testing gravity theory with electromagnetic/gravitational waves and to interpret astrophysical observables within a more general and bias-independent theoretical framework to uniquely determine the nature of the astrophysical black hole. In this Special Issue of the journal Symmetry, we welcome contributions on identifying novel and interesting relativistic phenomena that are absent in their general relativity counterpart, estimating astrophysical black hole’s parameters, examining the compatibility of modified gravity black holes with multi-messenger observational data, and testing no-hair theorem. Testing a theory requires finding tight logical connections between its hypotheses so that one can draw justified conclusions about how to revise a theory in the face of observational findings in conflict with its predictions. Work in this direction is important not only to make progress in finding the correct theoretical model but also to increase the possibility of finding these potential signals through future observations. 

We would like to call for papers sharing ideas on a diversity of topics from a theoretical/observational perspective, including but not limited to:

  • Black hole solutions in modified gravity theories;
  • Dynamical black holes and trapping horizons;
  • Classical and quantum aspects of black holes;
  • Cosmic censorship;
  • Gravitational lensing;
  • Shadow;
  • Stability analyses;
  • Energy Extraction;
  • Accretion flow onto black holes;
  • Gravitational waves;
  • LIGO/Virgo and EHT observational bounds on modified theories.

Dr. Rahul Kumar Walia
Guest Editor

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Published Papers (3 papers)

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Research

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18 pages, 734 KiB  
Article
Frequency Shift of Photons in the Spacetime of Deformed RN BH
by Husan Alibekov, Bakhtiyor Narzilloev, Ahmadjon Abdujabbarov and Bobomurat Ahmedov
Symmetry 2023, 15(7), 1414; https://doi.org/10.3390/sym15071414 - 14 Jul 2023
Cited by 2 | Viewed by 723
Abstract
The motion of photons and the frequency shift of photons emitted by particles orbiting the central black hole described by the deformed Reissner–Nordström spacetime metric is investigated. It has been shown that in spacetime, two stable photon spheres are formed due to the [...] Read more.
The motion of photons and the frequency shift of photons emitted by particles orbiting the central black hole described by the deformed Reissner–Nordström spacetime metric is investigated. It has been shown that in spacetime, two stable photon spheres are formed due to the gravitational attraction of photons by the central gravitating compact object. It has been found that as spacetime parameters increase, charge and deformation photon spheres become smaller in size and the effect of the electric charge is stronger than the effect of the deformation parameter. The frequency shift of photons radiated by the particles at the equatorial plane becomes stronger for the smaller values of the deformation parameter and the electric charge of the deformed Reissner–Nordström black hole. Full article
(This article belongs to the Special Issue Role of Black Holes in Testing Modified Theories of Gravity)
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82 pages, 5931 KiB  
Article
Primordial Gravitational Wave Circuit Complexity
by Kiran Adhikari, Sayantan Choudhury, Hardey N. Pandya and Rohan Srivastava
Symmetry 2023, 15(3), 664; https://doi.org/10.3390/sym15030664 - 06 Mar 2023
Cited by 2 | Viewed by 1195
Abstract
In this article, we investigate the various physical implications of quantum circuit complexity using the squeezed state formalism of Primordial Gravitational Waves (PGW). Recently, quantum information-theoretic concepts, such as entanglement entropy and complexity, have played a pivotal role in understanding the dynamics of [...] Read more.
In this article, we investigate the various physical implications of quantum circuit complexity using the squeezed state formalism of Primordial Gravitational Waves (PGW). Recently, quantum information-theoretic concepts, such as entanglement entropy and complexity, have played a pivotal role in understanding the dynamics of quantum systems, even in diverse fields such as high-energy physics and cosmology. This paper is devoted to studying the quantum circuit complexity of PGW for various cosmological models, such as de Sitter, inflation, radiation, reheating, matter, bouncing, cyclic and black hole gas models, etc. We compute complexity measures using both Covariance and Nielsen’s wave function method for three different choices of quantum initial vacua: Motta-Allen, α and Bunch–Davies. Besides computing circuit complexity, we also compute the Von Neumann entanglement entropy. By making the comparison between complexity and entanglement entropy, we are able to probe various features regarding the dynamics of evolution for different cosmological models. Because entanglement entropy is independent of the squeezing angle, we are able to understand more details of the system using Nielsen’s measure of complexity, which is dependent on both squeezing parameter and angle. This implies that quantum complexity could indeed be a useful probe to study quantum features on a cosmological scale. Quantum complexity is also becoming a powerful technique to understand the chaotic behaviour and random fluctuations of quantum fields. Using the growth of complexity, we are able to compute the quantum Lyapunov exponent for various cosmological models and comment on its chaotic nature. Full article
(This article belongs to the Special Issue Role of Black Holes in Testing Modified Theories of Gravity)
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Review

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16 pages, 1074 KiB  
Review
X-ray Tests of General Relativity with Black Holes
by Cosimo Bambi
Symmetry 2023, 15(6), 1277; https://doi.org/10.3390/sym15061277 - 18 Jun 2023
Cited by 2 | Viewed by 1207
Abstract
General relativity is one of the pillars of modern physics. For decades, the theory has been mainly tested in the weak-field regime with experiments in the solar system and radio observations of binary pulsars. Until 2015, the strong-field regime was almost completely unexplored. [...] Read more.
General relativity is one of the pillars of modern physics. For decades, the theory has been mainly tested in the weak-field regime with experiments in the solar system and radio observations of binary pulsars. Until 2015, the strong-field regime was almost completely unexplored. Thanks to new observational facilities, the situation has dramatically changed in the last few years. Today, we have gravitational wave data of the coalesce of stellar-mass compact objects from the LIGO-Virgo-KAGRA collaboration, images at mm wavelengths of the supermassive black holes in M87* and Sgr A* from the Event Horizon Telescope collaboration, and X-ray data of accreting compact objects from a number of X-ray missions. Gravitational wave tests and black hole imaging tests are certainly more popular and are discussed in other articles of this Special Issue. The aim of the present manuscript is to provide a pedagogical review on X-ray tests of general relativity with black holes and to compare these kinds of tests with those possible with gravitational wave data and black hole imaging. Full article
(This article belongs to the Special Issue Role of Black Holes in Testing Modified Theories of Gravity)
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