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Gravitation
About: Gravitation is a research topic. Over the lifetime, 29306 publications have been published within this topic receiving 821510 citations.
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TL;DR: In this article, it was shown that higher-dimensional operators contribute positively to the entropy of a thermodynamically stable black hole at fixed mass and charge, and that the entropy shift is equivalent to a certain inequality relating the free energies of black holes.
Abstract: We prove that higher-dimension operators contribute positively to the entropy of a thermodynamically stable black hole at fixed mass and charge. Our results apply whenever the dominant corrections originate at tree level from quantum field theoretic dynamics. More generally, positivity of the entropy shift is equivalent to a certain inequality relating the free energies of black holes. These entropy inequalities mandate new positivity bounds on the coefficients of higher-dimension operators. One of these conditions implies that the charge-to-mass ratio of an extremal black hole asymptotes to unity from above for increasing mass. Consequently, large extremal black holes are unstable to decay to smaller extremal black holes and the weak gravity conjecture is automatically satisfied. Our findings generalize to arbitrary spacetime dimension and to the case of multiple gauge fields. The assumptions of this proof are valid across a range of scenarios, including string theory constructions with a dilaton stabilized below the string scale.
167 citations
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TL;DR: In this paper, the authors consider three types of f(T) gravity and find that all of them can give rise to cosmic acceleration with interesting features, respectively, and show that all three types can explain the present cosmic accelerating expansion.
Abstract: Recently f(T) theories based on modifications of teleparallel gravity, where torsion is the geometric object describing gravity instead of curvature, have been proposed to explain the present cosmic accelerating expansion. The field equations are always second order, remarkably simpler than f(R) theories. In analogy to the f(R) theory, we consider here three types of f(T) gravity, and find that all of them can give rise to cosmic acceleration with interesting features, respectively.
167 citations
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01 Jan 1985
TL;DR: In this article, the authors divided their book into the following chapters: Geometry and Gravitation, the Formalism of General Relativity, Gravitational Field Equations, Three Classical Tests of Einstein's Theory, Elements of Cosmology, Relativistic Cosmological Models, Non-Static Models of the Universe; Gravitational Waves; Dense and Collapsed Matter; The Einstein-Cartan Theory; The Strong Gravity Theory; Gauge Theory of Gravity; Supergravity, and Gravitational Theory in the Language of Exterior Forms
Abstract: This book is divided into the following chapters: Contents: Geometry and Gravitation; The Formalism of General Relativity; Gravitational Field Equations; The Three Classical Tests of Einstein's Theory; Elements of Cosmology; Relativistic Cosmological Models; Non-Static Models of the Universe; Gravitational Waves; Dense and Collapsed Matter; The Einstein-Cartan Theory; The Strong Gravity Theory; Gauge Theory of Gravity; Supergravity; Gravitational Theory in the Language of Exterior Forms.
167 citations
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TL;DR: In this article, the observational and theoretical constraints on ''Einstein-aether theory'' were analyzed and the results of a computation of the remaining post-Newtonian parameters were reported.
Abstract: We analyze the observational and theoretical constraints on ``Einstein-aether theory,'' a generally covariant theory of gravity coupled to a dynamical, unit, timelike vector field that breaks local Lorentz symmetry. The results of a computation of the remaining post-Newtonian parameters are reported. These are combined with other results to determine the joint post-Newtonian, vacuum-\ifmmode \check{C}\else \v{C}\fi{}erenkov, nucleosynthesis, stability, and positive-energy constraints. All of these constraints are satisfied by parameters in a large two-dimensional region in the four-dimensional parameter space defining the theory.
166 citations
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TL;DR: In this paper, a three-bracket structure for target space coordinates in general closed string backgrounds has been proposed, which generalizes the appearance of noncommutative/nonassociative gravity theories for open strings in two-form backgrounds to a putative nonsmooth gravity theory for closed strings probing curved backgrounds with non-vanishing three-form flux.
Abstract: In an on-shell conformal field theory approach, we find indications of a three-bracket structure for target space coordinates in general closed string backgrounds. This generalizes the appearance of noncommutative gauge theories for open strings in two-form backgrounds to a putative noncommutative/nonassociative gravity theory for closed strings probing curved backgrounds with non-vanishing three-form flux. Several aspects and consequences of the three-bracket structure are discussed and a new type of generalized uncertainty principle is proposed.
166 citations