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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, it was shown that the Dirac equation in a non-inertial frame can be used to predict general relativity in a Riemann-Cartan spacetime with torsion and curvature.
Abstract: We give a short outline, in Sec.\ 2, of the historical development of the gauge idea as applied to internal ($U(1),\, SU(2),\dots$) and external ($R^4,\,SO(1,3),\dots$) symmetries and stress the fundamental importance of the corresponding conserved currents. In Sec.\ 3, experimental results with neutron interferometers in the gravitational field of the earth, as inter- preted by means of the equivalence principle, can be predicted by means of the Dirac equation in an accelerated and rotating reference frame. Using the Dirac equation in such a non-inertial frame, we describe how in a gauge- theoretical approach (see Table 1) the Einstein-Cartan theory, residing in a Riemann-Cartan spacetime encompassing torsion and curvature, arises as the simplest gravitational theory. This is set in contrast to the Einsteinian approach yielding general relativity in a Riemannian spacetime. In Secs.\ 4 and 5 we consider the conserved energy-momentum current of matter and gauge the associated translation subgroup. The Einsteinian teleparallelism theory which emerges is shown to be equivalent, for spinless matter and for electromagnetism, to general relativity. Having successfully gauged the translations, it is straightforward to gauge the four-dimensional affine group $R^4 \semidirect GL(4,R)$ or its Poincare subgroup $R^4\semidirect SO(1,3)$. We briefly report on these results in Sec.\ 6 (metric-affine geometry) and in Sec.\ 7 (metric-affine field equations (\ref{zeroth}, \ref{first}, \ref{second})). Finally, in Sec.\ 8, we collect some models, currently under discussion, which bring life into the metric-affine gauge framework developed.

197 citations

Journal ArticleDOI
TL;DR: In this article, the effects of anisotropic spherically symmetric gravitational sources by means of the gravitational decoupling realised via the minimal geometric deformation approach are investigated.
Abstract: We investigate the extension of isotropic interior solutions for static self-gravitating systems to include the effects of anisotropic spherically symmetric gravitational sources by means of the gravitational decoupling realised via the minimal geometric deformation approach. In particular, the matching conditions at the surface of the star with the outer Schwarzschild space-time are studied in great detail, and we describe how to generate, from a single physically acceptable isotropic solution, new families of anisotropic solutions whose physical acceptability is also inherited from their isotropic parent.

197 citations

Journal ArticleDOI
TL;DR: In this paper, the Hamiltonian for the second-order field equations in d spacetime dimensions is constructed, and the contribution of each dimension is given by the continuation of the Euler characteristic of a lower dimension.
Abstract: The most general gravitational action that yields second-order field equations in d spacetime dimensions is a sum of contributions associated with all even dimensions below d. Each contribution is given by the continuation to dimension d of the Euler characteristic of a lower dimension. The Hamiltonian for that general theory is constructed. At high curvatures it is multiple valued.

196 citations

Journal ArticleDOI
TL;DR: In this paper, the authors study the simplest concrete theory for spontaneous Lorentz violation, the ''New Aether theory'' of Jacobson and Mattingly, which is a vector-tensor gravitational theory with a fixed-modulus condition on the vector field and show that the observation of ultra-high energy cosmic rays (which implies the absence of energy loss via various Cherenkov type processes) places constraints on the parameters of this theory.
Abstract: We study the simplest concrete theory for spontaneous Lorentz violation, the ``New Aether theory'' of Jacobson and Mattingly, which is a vector-tensor gravitational theory with a fixed-modulus condition on the vector field. We show that the observation of ultra-high energy cosmic rays (which implies the absence of energy loss via various Cherenkov type processes) places constraints on the parameters of this theory, which are much stronger than those previously found in the literature and are also stronger than the constraints generically arising when gravity displays sub-luminal propagation.

196 citations

Journal ArticleDOI
TL;DR: In this paper, the authors discuss the efficiency of stellar gravity torques as a mechanism to account for the feeding of the central engines of four low-luminosity active galactic nuclei (AGN): NGC 4321, HII nucleus/LINER, NGC 4826 and NGC 4579.
Abstract: We discuss the efficiency of stellar gravity torques as a mechanism to account for the feeding of the central engines of four low luminosity Active Galactic Nuclei (AGN): NGC 4321 (HII nucleus/LINER), NGC 4826 (HII nucleus/LINER), NGC 4579 (LINER 19/Seyfert 19) and NGC 6951 (Seyfert 2) These galaxies have been observed as part of the NUclei of GAlaxies–(NUGA) CO project, aimed at the study of AGN fueling mechanisms Our calculations allow us to derive the characteristic time-scales for gas flows and discuss whether torques from the stellar potentials are efficient enough to drain the gas angular momentum in the inner 1 kpc of these galaxies The stellar potentials are derived using high-resolution near infrared (NIR) images and the averaged effective torques on the gas are estimated using the high-resolution (∼05 �� –2 �� ) CO maps of the galaxies Results indicate paradoxically that feeding should be thwarted close to the AGNs: in the four cases analyzed, gravity torques are mostly positive inside r ∼ 200 pc, resulting in no inflow on these scales As a possible solution for the paradox, we speculate that the agent responsible for driving inflow to still smaller radii is transient and thus presently absent in the stellar potential Alternatively, the gravity torque barrier associated with the Inner Lindblad Resonance of the bars in these galaxies could be overcome by other mechanisms that become competitive in due time against gravity torques In particular, we estimate on a case-by-case basis the efficiency of viscosity versus gravity torques to drive AGN fueling We find that viscosity can counteract moderate-to-low gravity torques on the gas if it acts on a nuclear ring of high gas surface density contrast and ∼a few 100 pc size We propose an evolutionary scenario in which gravity torques and viscosity act in concert to produce recurrent episodes of activity during the typical lifetime of any galaxy In this scenario the recurrence of activity in galaxies is indirectly related to that of the bar instabilities although the active phases are not necessarily coincident with the maximum strength of a single bar episode The general implications of these results for the current understanding of fueling of low-luminosity AGN are discussed

196 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023745
20221,538
20211,353
20201,587
20191,566
20181,592