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Search Results (3)

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Keywords = ZAMOs

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19 pages, 891 KB  
Article
Analytic Investigation of the Imprints of Dark Energy and Charge on the Kerr–Newmann–De Sitter Black-Hole Photon Ring
by James Mugambi, Eunice Omwoyo and Dismas Wamalwa
Astronomy 2025, 4(2), 9; https://doi.org/10.3390/astronomy4020009 - 21 May 2025
Viewed by 1195
Abstract
In 2019, the Event Horizon Telescope (EHT) released the first image of a black hole, sparking huge interest in the study of black-hole images. We present analytical solutions to the null geodesic equations for Kerr–Newman–de Sitter black holes derived using Jacobi elliptic functions. [...] Read more.
In 2019, the Event Horizon Telescope (EHT) released the first image of a black hole, sparking huge interest in the study of black-hole images. We present analytical solutions to the null geodesic equations for Kerr–Newman–de Sitter black holes derived using Jacobi elliptic functions. Using these solutions, we have performed an analytic ray-tracing simulation to model direct images, lensing rings, and photon rings, considering standard observers and zero angular momentum observers (ZAMOs). Additionally, we have derived analytic expressions for the critical parameters governing the structure of the photon ring and analyzed them in detail. From the foregoing, an increase in charge leads to a decrease in both time delay and Lyapunov exponent, while the change in azimuthal angle is insignificant. These findings improve our understanding of the effects of charge on black-hole photon rings and provide a foundation for future studies. Full article
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13 pages, 301 KB  
Article
Inertial Frame Dragging and Relative Rotation of ZAMOs in Axistationary Asymptotically Flat Spacetimes
by Simen Braeck
Universe 2023, 9(3), 120; https://doi.org/10.3390/universe9030120 - 26 Feb 2023
Cited by 2 | Viewed by 1961
Abstract
In axistationary, asymptotically flat spacetimes, zero angular momentum observers (ZAMOs) define an absolute standard of non-rotation locally, as can be verified by the absence of any Sagnac effect for these observers. Nevertheless, we argue that on a global scale the only physically meaningful [...] Read more.
In axistationary, asymptotically flat spacetimes, zero angular momentum observers (ZAMOs) define an absolute standard of non-rotation locally, as can be verified by the absence of any Sagnac effect for these observers. Nevertheless, we argue that on a global scale the only physically meaningful concept is that of relative rotation. The argument is substantiated by solving Einstein’s equations for an approximate thin shell model, where we maintain a degree of freedom, by relaxing the natural assumption of vanishing rotation at asymptotic infinity, at the outset of the analysis. The solution reveals that Einstein’s equations only determine differences in the rotation rate of ZAMOs, thereby establishing the concept of relative rotation globally. The interpretation of rotation as relative in a global context is inherently linked to the freedom to transform between coordinate systems rotating relative to each other, implying that an arbitrary ZAMO located at any radius may claim to be the one that is non-rotating on a global scale, and that the notion of an asymptotic Lorentz frame relative to which one may measure absolute rotation is devoid of any meaning. The concept of rotation in Kerr spacetime is then briefly discussed in the context of this interpretation. Full article
(This article belongs to the Special Issue Entropy, Cosmic Inflation and the Theory of Relativity: In Honour of Prof. Øyvind Grøn)
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27 pages, 2361 KB  
Article
Frame-Dragging: Meaning, Myths, and Misconceptions
by L. Filipe. O. Costa and José Natário
Universe 2021, 7(10), 388; https://doi.org/10.3390/universe7100388 - 18 Oct 2021
Cited by 24 | Viewed by 8143
Abstract
Originally introduced in connection with general relativistic Coriolis forces, the term frame-dragging is associated today with a plethora of effects related to the off-diagonal element of the metric tensor. It is also frequently the subject of misconceptions leading to incorrect predictions, even of [...] Read more.
Originally introduced in connection with general relativistic Coriolis forces, the term frame-dragging is associated today with a plethora of effects related to the off-diagonal element of the metric tensor. It is also frequently the subject of misconceptions leading to incorrect predictions, even of nonexistent effects. We show that there are three different levels of frame-dragging corresponding to three distinct gravitomagnetic objects: gravitomagnetic potential 1-form, field, and tidal tensor, whose effects are independent, and sometimes opposing. It is seen that, from the two analogies commonly employed, the analogy with magnetism holds strong where it applies, whereas the fluid-dragging analogy (albeit of some use, qualitatively, in the first level) is, in general, misleading. Common misconceptions (such as viscous-type “body-dragging”) are debunked. Applications considered include rotating cylinders (Lewis–Weyl metrics), Kerr, Kerr–Newman and Kerr–dS spacetimes, black holes surrounded by disks/rings, and binary systems. Full article
(This article belongs to the Special Issue Frame-Dragging and Gravitomagnetism)
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