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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 analyzed unusual features of Einstein's theory of gravitation in a three-dimensional space-time and showed that the standard correspondence of the theory with Newton's theory breaks down.
Abstract: As a preparation for studying quantum models, we analyze unusual features of Einstein's theory of gravitation in a three-dimensional space-time. In three dimensions, matter curves space-time only locally and the gravitational field has no dynamical degrees of freedom. The standard correspondence of Einstein's theory with Newton's theory breaks down. A dust distribution moves without any geodesic deviation between the particles. The cosmological models and relativistic stars behave in a qualitatively different way from their Newtonian counterparts. These features are important for the correct understanding of mini-superspace models.
198 citations
TL;DR: In this article, the authors evaluate rates of stellar capture and disruption in triaxial nuclei and find that rates substantially exceed those in collisionally-resupplied loss cones, as long as an appreciable fraction of the orbits are centrophilic.
Abstract: In classical loss cone theory, stars are supplied to a central black hole via gravitational scattering onto low angular momentum orbits. Higher feeding rates are possible if the gravitational potential near the black hole is non-axisymmetric and the orbits are chaotic. Motivated by recently published, self-consistent models, we evaluate rates of stellar capture and disruption in triaxial nuclei. Rates are found to substantially exceed those in collisionally-resupplied loss cones, as long as an appreciable fraction of the orbits are centrophilic. The mass captured by a black hole after a given time in a steep nucleus scales as the fifth power of the velocity dispersion, and the accumulated mass in 10^10 yr is of the correct order to reproduce the M-sigma relation. Triaxiality can solve the "final parsec problem" of decaying black hole binaries by increasing the flux of stars into the binary's loss cone.
198 citations
TL;DR: In this paper, the authors used distance measurements in the nearby universe to carry out new tests of gravity, surpassing other astrophysical tests by over two orders of magnitude for chameleon theories.
Abstract: We use distance measurements in the nearby universe to carry out new tests of gravity, surpassing other astrophysical tests by over two orders of magnitude for chameleon theories. The three nearby distance indicators—cepheids, tip of the red giant branch (TRGB) stars, and water masers—operate in gravitational fields of widely different strengths. This enables tests of scalar-tensor gravity theories because they are screened from enhanced forces to different extents. Inferred distances from cepheids and TRGB stars are altered (in opposite directions) over a range of chameleon gravity theory parameters well below the sensitivity of cosmological probes. Using published data, we have compared cepheid and TRGB distances in a sample of unscreened dwarf galaxies within 10 Mpc. We use a comparable set of screened galaxies as a control sample. We find no evidence for the order unity force enhancements expected in these theories. Using a two-parameter description of the models (the coupling strength and background field value), we obtain constraints on both the chameleon and symmetron screening scenarios. In particular we show that f(R) models with background field values f {sub R0} above 5 × 10{sup –7} are ruled out at the 95% confidence level. We also compare TRGB and maser distancesmore » to the galaxy NGC 4258 as a second test for larger field values. While there are several approximations and caveats in our study, our analysis demonstrates the power of gravity tests in the local universe. We discuss the prospects for additional improved tests with future observations.« less
197 citations
TL;DR: In this paper, a version of the Weak Gravity Conjecture for 6d F-theory or heterotic string compactifications with 8 supercharges is presented, where the extremality condition of a charged black hole is modified and the test particles required to satisfy the weak gravity conjecture are subject to additional Yukawa type interactions.
197 citations
TL;DR: In this article, the generalized Einstein equations derived from the Lagrangian were investigated and an approximate solution without singularity was constructed using the method of matched asymptotic expansions.
Abstract: We investigate the generalized Einstein equations derived from the Lagrangian which is an arbitrary function ofR. The importance of the saturation phenomenon is underlined, which may replace the role of a cosmological constant. The spherically symmetric homogeneous model is analyzed in more detail, and an approximate solution without singularity is constructed using the method of matched asymptotic expansions.
197 citations