Gravitation

About: Gravitation is a research topic. Over the lifetime, 29306 publications have been published within this topic receiving 821510 citations.

Black Hole Based Tests of General Relativity

Abstract: General relativity has passed all solar system experiments and neutron star based tests, such as binary pulsar observations, with flying colors. A more exotic arena for testing general relativity is in systems that contain one or more black holes. Black holes are the most compact objects in the Universe, providing probes of the strongest-possible gravitational fields. We are motivated to study strong-field gravity since many theories give large deviations from general relativity only at large field strengths, while recovering the weak-field behavior. In this article, we review how one can probe general relativity and various alternative theories of gravity by using electromagnetic waves from a black hole with an accretion disk, and gravitational waves from black hole binaries. We first review model-independent ways of testing gravity with electromagnetic/gravitational waves from a black hole system. We then focus on selected examples of theories that extend general relativity in rather simple ways. Some important characteristics of general relativity include (but are not limited to) (i) only tensor gravitational degrees of freedom, (ii) the graviton is massless, (iii) no quadratic or higher curvatures in the action, and (iv) the theory is four-dimensional. Altering a characteristic leads to a different extension of general relativity: (i) scalar–tensor theories, (ii) massive gravity theories, (iii) quadratic gravity, and (iv) theories with large extra dimensions. Within each theory, we describe black hole solutions, their properties, and current and projected constraints on each theory using black hole based tests of gravity. We close this review by listing some of the open problems in model-independent tests and within each specific theory.

The Effective Field Theorist's Approach to Gravitational Dynamics

Abstract: We review the effective field theory (EFT) approach to gravitational dynamics. We focus on extended objects in long-wavelength backgrounds and gravitational wave emission from spinning binary systems. We conclude with an introduction to EFT methods for the study of cosmological large scale structures.

Spacetime structure of static solutions in Gauss-Bonnet gravity: Neutral case

Abstract: We study the spacetime structures of the static solutions in the n-dimensional Einstein-Gauss-Bonnet-{lambda} system systematically. We assume the Gauss-Bonnet coefficient {alpha} is non-negative and a cosmological constant is either positive, zero, or negative. The solutions have the (n-2)-dimensional Euclidean submanifold, which is the Einstein manifold with the curvature k=1, 0, and -1. We also assume 4{alpha}-tilde/l{sup 2}{ 0. The divergent behavior around the singularity in Gauss-Bonnet gravity is milder than that around the central singularity inmore » general relativity. There are three types of horizons: inner, black hole, and cosmological. In the k=1,0 cases, the plus-branch solutions do not have any horizon. In the k=-1 case, the radius of the horizon is restricted as r{sub h} {radical}(2{alpha}-tilde)) in the plus (minus) branch. The black hole solution with zero or negative mass exists in the plus branch even for the zero or positive cosmological constant. There is also the extreme black hole solution with positive mass. We briefly discuss the effect of the Gauss-Bonnet corrections on black hole formation in a collider and the possibility of the violation of the third law of the black hole thermodynamics.« less

Gravitational wave asteroseismology reexamined

Abstract: The frequencies and damping times of the non radial oscillations of non rotating neutron stars are computed for a set of recently proposed equations of state (EOS) which describe matter at supranuclear densities. These EOS are obtained within two different approaches, the nonrelativistic nuclear many-body theory and the relativistic mean field theory, that model hadronic interactions in different ways leading to different composition and dynamics. Being the non radial oscillations associated to the emission of gravitational waves, we fit the eigenfrequencies of the fundamental mode and of the first pressure and gravitational-wave mode (polar and axial) with appropriate functions of the mass and radius of the star, comparing the fits, when available, with those obtained by Andersson and Kokkotas in 1998. We show that the identification in the spectrum of a detected gravitational signal of a sharp pulse corresponding to the excitation of the fundamental mode or of the first p-mode, combined with the knowledge of the mass of the star---the only observable on which we may have reliable information---would allow to gain interesting information on the composition of the inner core. We further discuss the detectability of these signals by gravitational detectors.

The place of the vertical gradient in gravitational interpretations

Abstract: It is a generally recognized fact that there is a certain similarity between structural contours and gravity contours. This is particularly true in the case of flat, shallow structures. In fact, in the limit, in the case of a surface distribution of matter, the gravity picture very near this surface becomes identical with the surface density distribution aside from a constant factor.