Gravitation

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

Continuous-spontaneous-reduction model involving gravity

Abstract: A continuous-reduction model implying the dynamical suppression of linear superpositions of macroscopically distinguishable states, presented recently by Di\`osi [Phys. Rev. A 40, 1165 (1989)], is investigated. The model exhibits appealing features; in particular, it relates reduction to gravity and contains no constants besides Newton's gravitational constant G. It turns out, however, that the model is not fully consistent. A slight modification of this model is proposed, which overcomes the difficulties and retains partially its appealing features. The resulting model deals with systems containing distinguishable or identical constituents, allows the derivation from microdynamics of wave-packet reduction, and accounts for the emergence of definite macroscopic properties for macro-objects. Reduction is related to gravity in the same way as in Di\`osi's model, but a fundamental length must be introduced to avoid inconsistencies.

Bulk curves from boundary data in holography

Abstract: We embed spherical Rindler space---a geometry with a spherical hole in its center---in asymptotically anti-- de Sitter (AdS) spacetime and show that it carries a gravitational entropy proportional to the area of the hole. Spherical AdS-Rindler space is holographically dual to an ultraviolet sector of the boundary field theory given by restriction to a strip of finite duration in time. Because measurements have finite durations, local observers in the field theory can only access information about bounded spatial regions. We propose a notion of differential entropy that captures uncertainty about the state of a system left by the collection of local, finite-time observables. For two-dimensional conformal field theories we use holography and the strong subadditivity of entanglement to propose a formula for differential entropy and show that it precisely reproduces the areas of circular holes in ${\mathrm{AdS}}_{3}$. Extending the notion to field theories on strips with variable durations in time, we show more generally that differential entropy computes the areas of all closed, inhomogeneous curves on a spatial slice of ${\mathrm{AdS}}_{3}$. We discuss the extension to higher-dimensional field theories, the relation of differential entropy to entanglement between scales, and some implications for the emergence of space from the renormalization group flow of entangled field theories.

Atom-Interferometry Tests of the Isotropy of Post-Newtonian Gravity

Abstract: We present a test of the local Lorentz invariance of post-Newtonian gravity by monitoring Earth's gravity with a Mach-Zehnder atom interferometer that features a resolution of up to 8 x 10{-9}g/sqrt[Hz], the highest reported thus far. Expressed within the standard model extension (SME) or Nordtvedt's anisotropic universe model, the analysis limits four coefficients describing anisotropic gravity at the ppb level and three others, for the first time, at the 10 ppm level. Using the SME we explicitly demonstrate how the experiment actually compares the isotropy of gravity and electromagnetism.

Proposed antimatter gravity measurement with an antihydrogen beam

Abstract: The principle of the equivalence of gravitational and inertial mass is one of the cornerstones of general relativity. Considerable efforts have been made and are still being made to verify its validity. A quantum-mechanical formulation of gravity allows for non-Newtonian contributions to the force which might lead to a difference in the gravitational force on matter and antimatter. While it is widely expected that the gravitational interaction of matter and of antimatter should be identical, this assertion has never been tested experimentally. With the production of large amounts of cold antihydrogen at the CERN Antiproton Decelerator, such a test with neutral antimatter atoms has now become feasible. For this purpose, we have proposed to set up the AEGIS experiment at CERN/AD, whose primary goal will be the direct measurement of the Earth's gravitational acceleration on antihydrogen with a classical Moire deflectometer.

Nonsingular exponential gravity: A simple theory for early- and late-time accelerated expansion

Abstract: A theory of exponential modified gravity which explains both early-time inflation and late-time acceleration, in a unified way, is proposed. The theory successfully passes the local tests and fulfills the cosmological bounds and, remarkably, the corresponding inflationary era is proven to be unstable. Numerical investigation of its late-time evolution leads to the conclusion that the corresponding dark energy epoch is not distinguishable from the one for the $\ensuremath{\Lambda}\mathrm{CDM}$ model. Several versions of this exponential gravity, sharing similar properties, are formulated. It is also shown that this theory is nonsingular, being protected against the formation of finite-time future singularities. As a result, the corresponding future universe evolution asymptotically tends, in a smooth way, to de Sitter space, which turns out to be the final attractor of the system.