Gravitation

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

Recent developments in gravitation

Abstract: The book consists of a set of lectures which review progress made in gravitational theory. It includes papers on classical general relativity, quantum gravity, recent developments in supergravity, supersymmetry and relationships to quantum field theory.

BlackHoleCam: fundamental physics of the Galactic center

Abstract: Einstein's General Theory of Relativity (GR) successfully describes gravity. The most fundamental predictions of GR are black holes (BHs), but in spite of many convincing BH candidates in the Universe, there is no conclusive experimental proof of their existence using astronomical observations in the electromagnetic spectrum. Are BHs real astrophysical objects? Does GR hold in its most extreme limit or are alternatives needed? The prime target to address these fundamental questions is in the center of our own Galaxy, which hosts the closest and best-constrained supermassive BH candidate in the Universe, Sagittarius A* (Sgr A*). Three different types of experiments hold the promise to test GR in a strong-field regime using observations of Sgr A* with new-generation instruments. The first experiment aims to image the relativistic plasma emission which surrounds the event horizon and forms a "shadow" cast against the background, whose predicted size (~50 microarcseconds) can now be resolved by upcoming VLBI experiments at mm-waves such as the Event Horizon Telescope (EHT). The second experiment aims to monitor stars orbiting Sgr A* with the upcoming near-infrared interferometer GRAVITY at the Very Large Telescope (VLT). The third experiment aims to time a radio pulsar in tight orbit about Sgr A* using radio telescopes (including the Atacama Large Millimeter Array or ALMA). The BlackHoleCam project exploits the synergy between these three different techniques and aims to measure the main BH parameters with sufficient precision to provide fundamental tests of GR and probe the spacetime around a BH in any metric theory of gravity. Here, we review our current knowledge of the physical properties of Sgr A* as well as the current status of such experimental efforts towards imaging the event horizon, measuring stellar orbits, and timing pulsars around Sgr A*.

Entropy, Extremality, Euclidean Variations, and the Equations of Motion

Abstract: We study the Euclidean gravitational path integral computing the Renyi entropy and analyze its behavior under small variations. We argue that, in Einstein gravity, the extremality condition can be understood from the variational principle at the level of the action, without having to solve explicitly the equations of motion. This set-up is then generalized to arbitrary theories of gravity, where we show that the respective entanglement entropy functional needs to be extremized. We also extend this result to all orders in Newton’s constant G N , providing a derivation of quantum extremality. Understanding quantum extremality for mixtures of states provides a generalization of the dual of the boundary modular Hamiltonian which is given by the bulk modular Hamiltonian plus the area operator, evaluated on the so-called modular extremal surface. This gives a bulk prescription for computing the relative entropies to all orders in G N . We also comment on how these ideas can be used to derive an integrated version of the equations of motion, linearized around arbitrary states.

Stellar equilibrium configurations of compact stars in $f(R,T)$ gravity

Abstract: In this article we study the hydrostatic equilibrium configuration of neutron stars and strange stars, whose fluid pressure is computed from the equations of state $p=\omega\rho^{5/3}$ and $p=0.28(\rho-4{\cal B})$, respectively, with $\omega$ and ${\cal B}$ being constants and $\rho$ the energy density of the fluid. We start by deriving the hydrostatic equilibrium equation for the $f(R,T)$ theory of gravity, with $R$ and $T$ standing for the Ricci scalar and trace of the energy-momentum tensor, respectively. Such an equation is a generalization of the one obtained from general relativity, and the latter can be retrieved for a certain limit of the theory. For the $f(R,T)=R+2\lambda T$ functional form, with $\lambda$ being a constant, we find that some physical properties of the stars, such as pressure, energy density, mass and radius, are affected when $\lambda$ is changed. We show that for a fixed central star energy density, the mass of neutron and strange stars can increase with $\lambda$. Concerning the star radius, it increases for neutron stars and it decreases for strange stars with the increment of $\lambda$. Thus, in $f(R,T)$ theory of gravity we can push the maximum mass above the observational limits. This implies that the equation of state cannot be eliminated if the maximum mass within General Relativity lies below the limit given by observed pulsars.

Bayesian Strong Gravitational-Lens Modeling on Adaptive Grids: Objective Detection of Mass Substructure in Galaxies

Abstract: We introduce a new adaptive and fully Bayesian grid-based method to model strong gravitational lenses with extended images. The primary goal of this method is to quantify the level of luminous and dark-mass substructure in massive galaxies, through their effect on highly-magnified arcs and Einstein rings. The method is adaptive on the source plane, where a Delaunay tessellation is defined according to the lens mapping of a regular grid onto the source plane. The Bayesian penalty function allows us to recover the best non-linear potential-model parameters and/or a grid-based potential correction and to objectively quantify the level of regularization for both the source and the potential. In addition, we implement a Nested-Sampling technique to quantify the errors on all non-linear mass model parameters -- ... -- and allow an objective ranking of different potential models in terms of the marginalized evidence. In particular, we are interested in comparing very smooth lens mass models with ones that contain mass-substructures. The algorithm has been tested on a range of simulated data sets, created from a model of a realistic lens system. One of the lens systems is characterized by a smooth potential with a power-law density profile, twelve include a NFW dark-matter substructure of different masses and at different positions and one contains two NFW dark substructures with the same mass but with different positions. Reconstruction of the source and of the lens potential for all of these systems shows the method is able, in a realistic scenario, to identify perturbations with masses >=10^7 solar mass when located on the Einstein ring. For positions both inside and outside of the ring, masses of at least 10^9 solar mass are required (...).