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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 paper, the authors present analytic calculations of angular momentum transport and gas inflow in galaxies, from scales of ∼ kpc to deep inside the potential of a central massive black hole (BH).
Abstract: We present analytic calculations of angular momentum transport and gas inflow in galaxies, from scales of ∼ kpc to deep inside the potential of a central massive black hole (BH). We compare these analytic calculations to numerical simulations and use them to develop a sub-grid model of BH growth that can be incorporated into semi-analytic calculations or cosmological simulations. Motivated by both analytic calculations and simulations of gas inflow in galactic nuclei, we argue that the strongest torque on gas arises when non-axisymmetric perturbations to the stellar gravitational potential produce orbit crossings and shocks in the gas. This is true both at large radii ∼0.01―1 kpc, where bar-like stellar modes dominate the non-axisymmetric potential, and at smaller radii ≲10 pc, where a lopsided/eccentric stellar disc dominates. The traditional orbit-crossing criterion is not always adequate to predict the locations of, and inflow due to, shocks in gas+stellar discs with finite sound speeds. We derive a modified criterion that predicts the presence of shocks in stellar-dominated systems even absent formal orbit crossing. We then derive analytic expressions for the loss of angular momentum and the resulting gas inflow rates in the presence of such shocks. We test our analytic predictions using hydrodynamic simulations at a range of galactic scales, and show that they successfully predict the mass inflow rates and quasi-steady gas surface densities with a small scatter ≃0.3 dex. We use our analytic results to construct a new estimate of the BH accretion rate given galaxy properties at larger radii, for use in galaxy and cosmological simulations and semi-analytic models. While highly simplified, this accretion rate predictor captures the key scalings in the numerical simulations. By contrast, alternate estimates such as the local viscous accretion rate or the spherical Bondi rate fail systematically to reproduce the simulations and have significantly larger scatter.
221 citations
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TL;DR: In this paper, a general model of multidimensional gravity including a Riemann tensor square term (nonzero c case) was considered and the number of brane-worlds in such a model was constructed (mainly in five dimensions).
Abstract: A general model of multidimensional ${R}^{2}$ gravity including a Riemann tensor square term (nonzero c case) is considered. The number of brane-worlds in such a model is constructed (mainly in five dimensions) and their properties are discussed. The thermodynamics of a Schwarzschild--anti-deSitter (S-AdS) BH (with boundary) is presented when perturbation on c is used. The entropy, free energy, and energy are calculated. For a nonzero c the entropy (energy) is not proportional to the area (mass). The equation of motion of the brane in a BH background is presented as a FRW equation. Using a dual CFT description it is shown that the dual field theory is not a conformal one when c is not zero. In this case the holographic entropy does not coincide with the BH entropy (they coincide for Einstein gravity or $c=0$ HD gravity where the AdS/CFT description is well applied). An asymmetrically warped background (an analogue of a charged AdS BH) where Lorentz invariance violation occurs is found. The cosmological 4D dS brane connecting two dS bulk spaces is formulated in terms of the parameters of ${R}^{2}$ gravity. Within the proposed dS/CFT correspondence the holographic conformal anomaly from five-dimensional higher derivative gravity in a de Sitter background is evaluated.
220 citations
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TL;DR: In this article, the Friedmann equations of a Friedmann-Robertson-Walker universe were derived by using the holographic principle together with the equipartition law of energy and the Unruh temperature.
Abstract: In this paper, by use of the holographic principle together with the equipartition law of energy and the Unruh temperature, we derive the Friedmann equations of a Friedmann-Robertson-Walker universe.
220 citations
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TL;DR: A theory of gravitation is constructed in which all homogeneous and isotropic solutions are nonsingular and all curvature invariants are bounded, and it is expected that this model can be generalized to solve the singularity problem also for anisotropic cosmologies.
Abstract: A theory of gravitation is constructed in which all homogeneous and isotropic solutions are nonsingular, and in which all curvature invariants are bounded All solutions for which curvature invariants approach their limiting values approach de Sitter space The action for this theory is obtained by a higher-derivative modification of Einstein's theory We expect that our model can easily be generalized to solve the singularity problem also for anisotropic cosmologies
220 citations
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TL;DR: In this article, the field equations of a generalized f(R) type gravity model, in which there is an arbitrary coupling between matter and geometry, are derived from a variational principle.
220 citations