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Search Results (5)

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Keywords = nonlocal gravity (NLG)

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29 pages, 429 KiB  
Review
A Review of Stable, Traversable Wormholes in f(R) Gravity Theories
by Ramesh Radhakrishnan, Patrick Brown, Jacob Matulevich, Eric Davis, Delaram Mirfendereski and Gerald Cleaver
Symmetry 2024, 16(8), 1007; https://doi.org/10.3390/sym16081007 - 7 Aug 2024
Cited by 8 | Viewed by 6889
Abstract
It has been proven that in standard Einstein gravity, exotic matter (i.e., matter violating the pointwise and averaged Weak and Null Energy Conditions) is required to stabilize traversable wormholes. Quantum field theory permits these violations due to the quantum coherent effects found in [...] Read more.
It has been proven that in standard Einstein gravity, exotic matter (i.e., matter violating the pointwise and averaged Weak and Null Energy Conditions) is required to stabilize traversable wormholes. Quantum field theory permits these violations due to the quantum coherent effects found in any quantum field. Even reasonable classical scalar fields violate the energy conditions. In the case of the Casimir effect and squeezed vacuum states, these violations have been experimentally proven. It is advantageous to investigate methods to minimize the use of exotic matter. One such area of interest is extended theories of Einstein gravity. It has been claimed that in some extended theories, stable traversable wormholes solutions can be found without the use of exotic matter. There are many extended theories of gravity, and in this review paper, we first explore f(R) theories and then explore some wormhole solutions in f(R) theories, including Lovelock gravity and Einstein Dilaton Gauss–Bonnet (EdGB) gravity. For completeness, we have also reviewed ‘Other wormholes’ such as Casimir wormholes, dark matter halo wormholes, thin-shell wormholes, and Nonlocal Gravity (NLG) wormholes, where alternative techniques are used to either avoid or reduce the amount of exotic matter that is required. Full article
(This article belongs to the Special Issue Symmetry in Gravity Theories and Cosmology)
16 pages, 342 KiB  
Article
Anisotropic Cosmology in the Local Limit of Nonlocal Gravity
by Javad Tabatabaei, Abdolali Banihashemi, Shant Baghram and Bahram Mashhoon
Universe 2023, 9(9), 377; https://doi.org/10.3390/universe9090377 - 23 Aug 2023
Cited by 6 | Viewed by 1181
Abstract
Within the framework of the local limit of nonlocal gravity (NLG), we investigate a class of Bianchi type I spatially homogeneous but anisotropic cosmological models. The modified field equations are presented in this case, and some special solutions are discussed in detail. This [...] Read more.
Within the framework of the local limit of nonlocal gravity (NLG), we investigate a class of Bianchi type I spatially homogeneous but anisotropic cosmological models. The modified field equations are presented in this case, and some special solutions are discussed in detail. This modified gravity theory contains a susceptibility function S(x) such that general relativity (GR) is recovered for S = 0. In the modified anisotropic cosmological models, we explore the contribution of S(t) and its temporal derivative to the local anisotropic cosmic acceleration. The implications of our results for observational cosmology are briefly discussed. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
12 pages, 320 KiB  
Article
Nonlocal Gravity: Fundamental Tetrads and Constitutive Relations
by Bahram Mashhoon
Symmetry 2022, 14(10), 2116; https://doi.org/10.3390/sym14102116 - 12 Oct 2022
Cited by 9 | Viewed by 1501
Abstract
Nonlocal gravity (NLG) is a classical nonlocal generalization of Einstein’s theory of gravitation based on a certain analogy with the nonlocal electrodynamics of media. The history dependence enters NLG through a constitutive relation involving a causal kernel that should ultimately be determined via [...] Read more.
Nonlocal gravity (NLG) is a classical nonlocal generalization of Einstein’s theory of gravitation based on a certain analogy with the nonlocal electrodynamics of media. The history dependence enters NLG through a constitutive relation involving a causal kernel that should ultimately be determined via observational data. The purpose of this paper is to reformulate nonlocal gravity such that the nonlocal aspect of the constitutive relation directly connects measurable quantities as in the nonlocal electrodynamics of media. The resulting constitutive relation turns out to coincide with the recent suggestion of Puetzfeld, Obukhov, and Hehl. With the new constitutive relation of NLG, it is possible to show that de Sitter spacetime is not a solution of NLG. Full article
(This article belongs to the Special Issue String Field Theory and Nonlocal Gravity)
20 pages, 382 KiB  
Article
Nonlocal Gravity: Modification of Newtonian Gravitational Force in the Solar System
by Mahmood Roshan and Bahram Mashhoon
Universe 2022, 8(9), 470; https://doi.org/10.3390/universe8090470 - 8 Sep 2022
Cited by 5 | Viewed by 2400
Abstract
Nonlocal gravity (NLG) is a classical nonlocal generalization of Einstein’s theory of gravitation developed in close analogy with the nonlocal electrodynamics of media. It appears that the nonlocal aspect of the universal gravitational interaction could simulate dark matter. Within the Newtonian regime of [...] Read more.
Nonlocal gravity (NLG) is a classical nonlocal generalization of Einstein’s theory of gravitation developed in close analogy with the nonlocal electrodynamics of media. It appears that the nonlocal aspect of the universal gravitational interaction could simulate dark matter. Within the Newtonian regime of NLG, we investigate the deviation of the gravitational force from the Newtonian inverse square law as a consequence of the existence of the effective dark matter. In particular, we work out the magnitude of this deviation in the solar system out to 100 astronomical units. Moreover, we give an improved lower limit for the short-range parameter of the reciprocal kernel of NLG. Full article
(This article belongs to the Special Issue Modified Gravity and Dark Matter at the Scale of Galaxies)
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18 pages, 350 KiB  
Article
Nonlocal Gravitomagnetism
by Bahram Mashhoon and Friedrich W. Hehl
Universe 2019, 5(9), 195; https://doi.org/10.3390/universe5090195 - 5 Sep 2019
Cited by 16 | Viewed by 2873
Abstract
We briefly review the current status of nonlocal gravity (NLG), which is a classical nonlocal generalization of Einstein’s theory of gravitation based on a certain analogy with the nonlocal electrodynamics of media. Nonlocal gravity thus involves integro-differential field equations and a causal constitutive [...] Read more.
We briefly review the current status of nonlocal gravity (NLG), which is a classical nonlocal generalization of Einstein’s theory of gravitation based on a certain analogy with the nonlocal electrodynamics of media. Nonlocal gravity thus involves integro-differential field equations and a causal constitutive kernel that should ultimately be determined from observational data. We consider the stationary gravitational field of an isolated rotating astronomical source in the linear approximation of nonlocal gravity. In this weak-field and slow-motion approximation of NLG, we describe the gravitomagnetic field associated with the rotating source and compare our results with gravitoelectromagnetism (GEM) of the standard general relativity theory. Moreover, we briefly study the energy-momentum content of the GEM field in nonlocal gravity. Full article
(This article belongs to the Special Issue Rotation Effects in Relativity)
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