Showing papers on "Gravitation published in 1989"

Models for universal reduction of macroscopic quantum fluctuations

Abstract: This paper adopts the hypothesis that the absence of macroscopic quantum fluctuations is due to a certain universal mechanism. Such a mechanism has recently been proposed by Ghirardi et al. [Phys. Rev. D 34, 470 (1986)], and here we recapitulate a compact version of it. K\'arolyh\'azy [Nuovo Cimento 52, 390 (1966)] showed earlier the possible role of gravity and, along this line, we construct here a new parameter-free unification of micro- and macrodynamics. We apply gravitational measures for reducing macroscopic quantum fluctuations of the mass density. This model leads to classical trajectories in the macroscopic limit of translational motion. For massive objects, unwanted macroscopic superpositions of quantum states become destroyed in very short times. The relation between state-vector and density-operator formalisms has also been discussed. We only anticipate the need for elaborating characteristic predictions of the model in the region separating micro- and macroscopic properties.

Gravitational Field of a Global Monopole

Abstract: We present an approximate solution of the Einstein equations for the metric outside a monopole resulting from the breaking of a global O(3) symmetry. The monopole exerts practically no gravitational force on nonrelativistic matter, but the space around it has a deficit solid angle, and all light rays are deflected by the same angle, independent of the impact parameter.

Extended inflationary cosmology

Abstract: We present a new type of inflationary scenario based on metric formulations of gravity different from that of Einstein, e.g., a Brans-Dicke theory of gravity. Unlike previous inflation models, the inflationary phase transition can be completed via bubble nucleation. Hence, the fine tuning of an effective potential to obtain a slow-rollover transition is not required.

Exact vacuum solution to conformal Weyl gravity and galactic rotation curves

Abstract: The complete, exact exterior solution for a static, spherically symmetric source in locally conformal invariant Weyl gravity is presented. The solution includes the familiar exterior Schwarzschild solution as a special case and contains an extra gravitational potential term which grows linearly with distance. The obtained solution provides a potential explanation for observed galactic rotation curves without the need for dark matter. The solution also has some interesting implications for cosmology.

Gravitational phenomenology in higher-dimensional theories and strings

Abstract: We investigate gravitational phenomenology in compactified higher-dimensional theories, with particular emphasis on the consequences in string theory of tensor-induced spontaneous Lorentz-symmetry breaking. The role played by this mechanism in causing a gravitational version of the Higgs effect and in compactification is explored. The experimental viability of compactified theories with zero modes is considered by examining nonleading but observable gravitational effects. Additional constraints from the observed cosmological properties of the Universe are uncovered. Our investigations significantly constrain many theories involving extra dimensions in their perturbative regime. To resolve the phenomenological difficulties one must generate masses for the higher-dimensional components of the metric while leaving massless the physical spacetime components. Some possibilities for overcoming this metric-mass problem are suggested. An open issue is whether the metric-mass problem is resolved in string theory.

Phenomenological gravitational constraints on strings and higher-dimensional theories.

Abstract: We investigate measurable gravitational and cosmological effects in four dimensions that can arise from the compactification of higher-dimensional theories incorporating gravity. We identify the nature of effects due to massless scalar components of the compactified higher-dimensional metric and due to modifications of cosmological dynamics. Current experimental data impose constraints on the viability of many higher-dimensional theories, including Kaluza-Klein, supergravity, and string theories. The phenomenological problems can be avoided if the components of the metric in the higher dimensions acquire an effective mass. We survey some possible mechanisms for mass generation.

The Concept of Mass

Abstract: Mass is one of the most fundamental concepts of physics. Understanding and calculating the masses of the elementary particles is the central problem of modern physics, and is intimately connected with other fundamental problems such as the origin of CP violation, the mystery of the energy scales that determine the properties of the weak and gravitational interactions, the compositeness of particles, supersymmetry theory and the properties of the not‐yet‐discovered Higgs bosons.

Gravitational Collapse and Spacetime Singularities

Abstract: Physical phenomena in astrophysics and cosmology involve gravitational collapse in a fundamental way. The final fate of a massive star when it collapses under its own gravity at the end of its life cycle is one of the most important questions in gravitation theory and relativistic astrophysics, and is the foundation of black hole physics. General relativity predicts that continual gravitational collapse gives rise to a space-time singularity. Quantum gravity may take over in such regimes to resolve the classical space-time singularity. This book investigates these issues, and shows how the visible ultra-dense regions arise naturally and generically as an outcome of dynamical gravitational collapse. It will be of interest to graduate students and academic researchers in gravitation physics, fundamental physics, astrophysics, and cosmology. It includes a detailed review of research into gravitational collapse, and several examples of collapse models are investigated in detail.

Gravitational Radiation, Close Binary Systems, and the Brans-dicke Theory of Gravity

Abstract: Observational limits on the orbital period change of the binary pulsar PSR 1913+16 and of the 11-minute binary system 4U1820-30 are used to constrain the Brans-Dicke scalar-tensor theory of gravity. In 4U1820-30, dipole gravitational radiation damping is important. The conservative bound on the Brans-Dicke coupling constant is found to be omega(BD) greater than 30. The bounds are sensitive to the neutron-star model used and to the masses of the stars: for masses of 1.4 solar and 0.067 solar the bounds are omega(BD) greater than 140 for a stiff equation of state and omega(BD) greater than 600 for a soft equation of state. The binary pulsar differences between the Brans-Dicke theory and general relativity are suppressed by a factor related to the gravitational energy of the neutron stars, so that the resulting constraint on omega(BD) is uninteresting. 31 refs.

Evolution of cosmic string networks.

Abstract: A discussion of the evolution and observable consequences of a network of cosmic strings is given. A simple model for the evolution of the string network is presented, and related to the statistical mechanics of string networks. The model predicts the long string density throughout the history of the universe from a single parameter, which researchers calculate in radiation era simulations. The statistical mechanics arguments indicate a particular thermal form for the spectrum of loops chopped off the network. Detailed numerical simulations of string networks in expanding backgrounds are performed to test the model. Consequences for large scale structure, the microwave and gravity wave backgrounds, nucleosynthesis and gravitational lensing are calculated.

Gravity as a zero-point-fluctuation force

Abstract: Sakharov has proposed a suggestive model in which gravity is not a separately existing fundamental force, but rather an induced effect associated with zero-point fluctuations (ZPF's) of the vacuum, in much the same manner as the van der Waals and Casimir forces. In the spirit of this proposal we develop a point-particle\char21{}ZPF interaction model that accords with and fulfills this hypothesis. In the model gravitational mass and its associated gravitational effects are shown to derive in a fully self-consistent way from electromagnetic-ZPF-induced particle motion (Zitterbewegung). Because of its electromagnetic-ZPF underpinning, gravitational theory in this form constitutes an ``already unified'' theory.

New variables for gravity: Inclusion of matter.

Abstract: The Lagrangian and Hamiltonian formulations of general relativity in terms of soldering forms and self-dual connections are extended to include matter sources and the cosmological constant. For matter sources we consider minimally coupled Klein-Gordon fields, complex- and Grassmann-valued Dirac fields, and Yang-Mills fields. Somewhat surprisingly, in spite of the derivative coupling in the spin-half fields, the use of only the self-dual part of the connection as a basic variable does not lead to spurious equations or inconsistencies. Furthermore, as in the source-free case considered earlier, all equations of the theory are polynomial in terms of these variables. Therefore, the framework has several potential applications especially to the nonperturbative canonical quantization program.

2+1 quantum gravity as a toy model for the 3+1 theory

Abstract: 2+1 Einstein gravity is used as a toy model for testing a program for nonperturbative canonical quantisation of the 3+1 theory The program can be successfully implemented in the model and leads to a surprisingly rich quantum theory

Quantum gravity: an introduction to some recent results

Abstract: This article presents a general overview of the problems involved in the application of the quantum principle to a theory of gravitation. The ultraviolet divergences that appear in any perturbative computation are reviewed in some detail, and it is argued that it is unlikely that any theory based on local quantum fields could be consistent. This leads in a natural way to a supersymmetric theory of extended objects as the next candidate theory to study. An elementary introduction to superstrings closes the review, and some speculations about the most promising avenues of research are offered.

Is minisuperspace quantization valid?: Taub in mixmaster

Abstract: The paper addresses quantitatively the question of the validity of physical predictions based on minisuperspace quantization of Einstein's theory of gravitation. It studies a homogeneous, anisotropic cosmological model of higher symmetry (the Taub model) embedded in one of lesser symmetry (the mixmaster model). The comparison of the physical behavior of these two models is based on the construction of a non-negative probability density and the associated conserved inner product which allow a consistent probabilistic interpretation of the state function of the Universe in the interesting regime of deep channel penetration. It is shown that the respective behavior is widely different. A program is set for investigating a hierarchy of models with higher symmetry embedded in models of lesser symmetry to spell out the criteria under which minisuperspace quantum results can be expected to make meaningful predictions about full quantum gravity.

Time in semiclassical gravity

Abstract: Some questions concerning the existence of time and the validity of Einstein's equations as a quantum-mechanical mean have arisen in the development of semiclassical gravity applied to cosmology. The first is a technical question which concerns the choice of a certain class of solutions. We here show that this is a ``gauge'' choice where we follow the work of Mead and Berry. The ``Berry phase'' turns out to be related to the time integral of the energy. The second question is concerned with the justification of the use of the mean matter energy to drive the cosmological expansion. This is shown to be valid in an inflationary universe. At the same time one obtains the Schr\"odinger equation for matter in conformity with previous work where these particular questions were not addressed. All of this means that cosmology as a theory of the mean is meaningful.

Relativistic theory of gravitation

Abstract: In the present paper a relativistic theory of gravitation (RTG) is unambiguously constructed on the basis of the special relativity and geometrization principle. In this a gravitational field is treated as the Faraday--Maxwell spin-2 and spin-0 physical field possessing energy and momentum. The source of a gravitational field is the total conserved energy-momentum tensor of matter and of a gravitational field in Minkowski space. In the RTG the conservation laws are strictly fulfilled for the energy-moment and for the angular momentum of matter and a gravitational field. The theory explains the whole available set of experiments on gravity. By virtue of the geometrization principle, the Riemannian space in our theory is of field origin, since it appears as an effective force space due to the action of a gravitational field on matter. The RTG leads to an exceptionally strong prediction: The universe is not closed but just ''flat.'' This suggests that in the universe a ''missing mass'' should exist in a form of matter.

On the equilibrium statistical mechanics of isothermal classical self-gravitating matter

Abstract: The canonical ensemble is investigated for classical self-gravitating matter in a finite containerΛ [d]⊂ℝ d ,d=3 and 2. Starting with modified gravitational interactions (smoothed-out singularity), it is proven by explicit construction that, in thew *-topology, the canonical equilibrium measure converges to a superposition of Dirac measures when the limit of exact Newtonian gravitational interactions between classical point particles is taken. The consequences of this result for more realistic classical systems are evaluated, and the existence of a gravitational phase transition is proven. The results are discussed with view toward applications in astrophysics and space science. Some attention is paid also to the problem of founding thermodynamics by means of statistical mechanics.

Gravitational analogs of the Aharonov-Bohm effect

Abstract: A quantum scalar particle is considered in the following background gravitational fields due to (a) a tubular matter source with axial interior magnetic field and vanishing exterior magnetic field; (b) slowly moving mass currents (weak approximation); and (c) a spinning cosmic string. It is shown that in the flat space‐time around these sources, the energy spectrum and wave function of the particle depend on the amount of matter and magnetic field (tubular matter source case), on the velocity of the moving mass currents, and on the angular momentum in the spinning cosmic string case. These represent gravitational analogs of the Aharonov–Bohm effect in electrodynamics and are due to global (topological) features of the background space‐times under consideration.

Progress in metric-affine gauge theories of gravity with local scale invariance

Abstract: Einstein's general relativity theory describes very well the gravitational phenomena in themacroscopic world. In themicroscopic domain of elementary particles, however, it does not exhibit gauge invariance or approximate Bjorken type scaling, properties which are believed to be indispensible for arenormalizable field theory. We argue that thelocal extension of space-time symmetries, such as of Lorentz and scale invariance, provides the clue for improvement. Eventually, this leads to aGL(4, R)-gauge approach to gravity in which the metric and the affine connection acquire the status ofindependent fields. The Yang-Mills type field equations, the Noether identities, and conformal models of gravity are discussed within this framework. After symmetry breaking, Einstein's GR surfaces as an effective “low-energy” theory.

Linearised R+R 2 gravity: a new gauge and new solutions

Abstract: The author studies the linearised fourth-order field equations for gravitational theories with Lagrangians L=-g(R+1/2aR2+bRmu nu Rmu nu )- kappa Lm. He shows that a suitable choice of coordinate conditions involving third derivatives of the potentials leads directly to the general solution of these equations. He states appropriate junction conditions across a timelike (or spacelike) hypersurface of discontinuity. Using these junction conditions, and assuming the metric to be asymptotically flat at spatial infinity, he determines the potentials of an isolated static body in the case 3a+2b or=0. The new gauge yields the corresponding metric in a spatially isotropic form. These results are applied to a static spherically symmetric body: the potentials are obtained in a simple integral form when the rest-mass density of matter depends upon the distance from the centre. The results previously obtained for a pointlike distribution of matter and for a homogeneous sphere become trivial consequences of the general method.