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Galaxies, Volume 3, Issue 1 (March 2015) – 4 articles , Pages 1-71

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282 KiB  
Article
Thermodynamic Relations for the Entropy and Temperature of Multi-Horizon Black Holes
by Wei Xu, Jia Wang and Xin-he Meng
Galaxies 2015, 3(1), 53-71; https://doi.org/10.3390/galaxies3010053 - 02 Feb 2015
Cited by 19 | Viewed by 3999
Abstract
We present some entropy and temperature relations of multi-horizons, even including the “virtual” horizon. These relations are related to the product, division and sum of the entropy and temperature of multi-horizons. We obtain the additional thermodynamic relations of both static and rotating black [...] Read more.
We present some entropy and temperature relations of multi-horizons, even including the “virtual” horizon. These relations are related to the product, division and sum of the entropy and temperature of multi-horizons. We obtain the additional thermodynamic relations of both static and rotating black holes in three- and four-dimensional (A)dS spacetime. Especially, a new dimensionless, charge-independence and T+S+ = T_S_-like relation is presented. This relation does not depend on the mass, electric charge, angular momentum and cosmological constant, as it is always a constant. These relations lead us to obtaining some interesting thermodynamic bounds of entropy and temperature, including the Penrose inequality, which is the first geometrical inequality of black holes. Besides, based on these new relations, one can obtain the first law of thermodynamics and the Smarr relation for all horizons of a black hole. Full article
(This article belongs to the Special Issue Advances in Gravitational Research)
110 KiB  
Editorial
Acknowledgement to Reviewers of Galaxies in 2014
by Galaxies Editorial Office
Galaxies 2015, 3(1), 52; https://doi.org/10.3390/galaxies3010052 - 08 Jan 2015
Cited by 10 | Viewed by 3084
Abstract
The editors of Galaxies would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2014:[...] Full article
2096 KiB  
Article
The 5D Fully-Covariant Theory of Gravitation and Its Astrophysical Applications
by Tianxi Zhang
Galaxies 2015, 3(1), 18-51; https://doi.org/10.3390/galaxies3010018 - 26 Dec 2014
Cited by 1 | Viewed by 4995
Abstract
In this paper, we comprehensively review the five-dimensional (5D) fully-covariant theory of gravitation developed by Zhang two decades ago and its recent applications in astrophysics and cosmology. This 5D gravity describes not only the fields, but also the matter and its motion in [...] Read more.
In this paper, we comprehensively review the five-dimensional (5D) fully-covariant theory of gravitation developed by Zhang two decades ago and its recent applications in astrophysics and cosmology. This 5D gravity describes not only the fields, but also the matter and its motion in a 5D spacetime. The greatest advantage of this theory is that there does not exist any unknown parameter, so that we can apply it to explain astrophysical and cosmological issues by quantitatively comparing the results obtained from it with observations and to predict new effects that could not be derived from any other gravitational theories. First, the 5D covariant description of matter and its motion enabled Zhang to analytically derive the fifteenth component of the 5D energy-momentum tensor of matter ( T - 44 ), which significantly distinguishes this 5D gravity from other 5D gravitational theories that usually assumed a T - 44 with an unknown parameter, called the scalar charge s, and, thus, to split the 5D covariant field equation into (4 + 1) splitting form as the gravitational, electromagnetic, and scalar field equations. The gravitational field equation turns into the 4D Einstein’s field equation of general relativity if the scalar field is equal to unity. Then, Zhang solved the field equations and obtained an exact static spherically-symmetric external solution of the gravitational, electromagnetic and scalar fields, in which all integral constants were completely determined with a perfect set of simple numbers and parameters that only depend on the mass and electric charge of the matter, by comparing with the obtained weak internal solution of the fields at a large radial distance. In the Einstein frame, the exact field solution obtained from the 5D fully-covariant theory of gravitation reduces to the Schwarzschild solution when the matter is electrically neutral and the fields are weak in strength. This guarantees that the four fundamental tests (light deflection, gravitational redshift, perihelion advance and radar echo delay) of the 4D Einstein’s general relativity in the case of weak fields are also the tests of the 5D fully-covariant theory of gravitation. In the case of strong fields, especially when the matter is highly charged, however, the results from the 5D fully-covariant theory of gravitation are significantly different from the 4D Einstein’s general relativity. Applying this 5D gravity and its exact field solution, Zhang has recently developed a new redshift mechanism, called electric redshift, a new supernova explosion mechanism with gravitational field shielding, a new gravitationless black hole model, a modified neutron star mass-radius relation, a modified Friedmann equation for the accelerating universe, and so on. This paper provides an overview of this 5D fully-covariant theory of gravitation, including also its solution properties and astrophysical applications. Full article
(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)
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287 KiB  
Article
Nonlocal General Relativity
by Bahram Mashhoon
Galaxies 2015, 3(1), 1-17; https://doi.org/10.3390/galaxies3010001 - 26 Dec 2014
Cited by 18 | Viewed by 4235
Abstract
A brief account of the present status of the recent nonlocal generalization of Einstein’s theory of gravitation is presented. The main physical assumptions that underlie this theory are described. We clarify the physical meaning and significance of Weitzenböck’s torsion and emphasize its intimate [...] Read more.
A brief account of the present status of the recent nonlocal generalization of Einstein’s theory of gravitation is presented. The main physical assumptions that underlie this theory are described. We clarify the physical meaning and significance of Weitzenböck’s torsion and emphasize its intimate relationship with the gravitational field, characterized by the Riemannian curvature of spacetime. In this theory, nonlocality can simulate dark matter; in fact, in the Newtonian regime, we recover the phenomenological Tohline–Kuhn approach to modified gravity. To account for the observational data regarding dark matter, nonlocality is associated with a characteristic length scale of order 1 kpc. The confrontation of nonlocal gravity with observation is briefly discussed. Full article
(This article belongs to the Special Issue Advances in Gravitational Research)
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