Showing papers on "Gravitation published in 2001"
TL;DR: In this paper, the authors provide a review of both new experimental and theoretical developments in the Casimir effect, and provide the most recent constraints on the corrections to Newtonian gravitational law and other hypothetical long-range interactions at submillimeter range.
1,388 citations
01 Jan 2001
TL;DR: In this paper, the stability of a quantum superposition of two different stationary mass distributions is examined, where the perturbing effect of each distribution on the space-time structure is taken into account, in accordance with the principles of general relativity.
Abstract: The stability of a quantum superposition of two different stationary mass distributions is examined, where the perturbing effect of each distribution on the space-time structure is taken into account, in accordance with the principles of general relativity. It is argued that the definition of the time-translation operator for the superposed space-times involves an inherent ill-definedness, leading to an essential uncertainty in the energy of the superposed state which, in the Newtonian limit, is proportional to the gravitational self-energyEΔ of the difference between the two mass distributions. This is consistent with a suggested finite lifetime of the order of ħ/EΔ for the superposed state, in agreement with a certain proposal made by the author for a gravitationally induced spontaneous quantum state reduction, and with closely related earlier suggestions by Diosi and by Ghirardiet al.
1,013 citations
TL;DR: In this paper, the authors studied the fluctuation spectrum of linearized gravity around non-fine-tuned branes in AdS4 and AdS5 space, where the warp factor near the brane is essentially that of a Minkowski brane.
Abstract: We study the fluctuation spectrum of linearized gravity around non-fine-tuned branes. We focus on the case of an AdS4 brane in AdS5. In this case, for small cosmological constant, the warp factor near the brane is essentially that of a Minkowski brane. However, far from the brane, the metric differs substantially. The space includes the AdS5 boundary, so it has infinite volume. Nonetheless, for sufficiently small AdS4 cosmological constant, there is a bound state graviton in the theory, and four-dimensional gravity is reproduced. However, it is a massive bound state that plays the role of the four-dimensional graviton.
633 citations
[...]
TL;DR: In this article, it was shown that 11-dimensional supergravity can be described by a nonlinear realization based on the group E11, where the gravitational degrees of freedom are described by two fields which are related by duality.
Abstract: We argue that 11-dimensional supergravity can be described by a nonlinear realization based on the group E11. This requires a formulation of 11-dimensional supergravity in which the gravitational degrees of freedom are described by two fields which are related by duality. We show the existence of such a description of gravity.
539 citations
TL;DR: In this paper, the authors generalize the mechanism proposed in a previous paper and show that a four-dimensional relativistic tensor theory of gravitation can be obtained on a delta-function brane in flat infinite-volume extra space.
Abstract: We generalize the mechanism proposed in a previous paper and show that a four-dimensional relativistic tensor theory of gravitation can be obtained on a delta-function brane in flat infinite-volume extra space. In particular, we demonstrate that the induced Ricci scalar gives rise to Einstein's gravity on a delta-function type brane if the number of space-time dimensions is bigger than five. The bulk space exhibits the phenomenon of infrared transparency. That is to say, the bulk can be probed by gravitons with vanishing four-dimensional momentum square, while it is unaccessible to higher modes. This provides an attractive framework for solving the cosmological constant problem.
529 citations
TL;DR: The gravitational 1/r(2) law is tested at separations ranging down to 218 microm using a 10-fold symmetric torsion pendulum and a rotating 10- fold symmetric attractor to improve previous short-range constraints and find no deviations from Newtonian physics.
Abstract: Motivated by higher-dimensional theories that predict new effects, we tested the gravitational 1/r(2) law at separations ranging down to 218 microm using a 10-fold symmetric torsion pendulum and a rotating 10-fold symmetric attractor. We improved previous short-range constraints by up to a factor of 1000 and find no deviations from Newtonian physics.
472 citations
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TL;DR: The radiation-dominated k = 0 FRW cosmology emerges as the induced metric on a codimension one hypersurface of constant extrinsic curvature in the five-dimensional AdS-Schwarzschild solution as discussed by the authors.
Abstract: The radiation-dominated k=0 FRW cosmology emerges as the induced metric on a codimension one hypersurface of constant extrinsic curvature in the five-dimensional AdS-Schwarzschild solution. That we should get FRW cosmology in this way is an expected result from AdS/CFT in light of recent comments regarding the coupling of gravity to ''boundary'' conformal field theories. I remark on how this calculation bears on the understanding of the Randall-Sundrum ''alternative to compactification.'' A generalization of the AdS/CFT prescription for computing Green's functions is suggested, and it is shown how gravity emerges from it with a strength G{sub 4}=2G{sub 5}/L. Some upper bounds are set on the radius of curvature L of AdS{sub 5}. One of them comes from estimating the rate of leakage of visible sector energy into the CFT. That rate is connected via a unitarity relation to deviations from Newton's force law at short distances. The best bound on L obtained in this paper comes from a match to the parameters of string theory. It is L{approx}<1nm if the string scale is 1GeV. Higher string scales imply a tighter bound on L.
462 citations
TL;DR: In this article, the authors present the theoretical and analytical bases of optimal techniques to measure weak gravitational shear from images of galaxies, which can remove the systematic effects of image distortion, bias from PSF asymmetries and dilution of the signal by the seeing.
Abstract: We present the theoretical and analytical bases of optimal techniques to measure weak gravitational shear from images of galaxies. We first characterize the geometric space of shears and ellipticity, then use this geometric interpretation to analyse images. The steps of this analysis include: measurement of object shapes on images, combining measurements of a given galaxy on different images, estimating the underlying shear from an ensemble of galaxy shapes, and compensating for the systematic effects of image distortion, bias from PSF asymmetries, and `"dilution" of the signal by the seeing. These methods minimize the ellipticity measurement noise, provide calculable shear uncertainty estimates, and allow removal of systematic contamination by PSF effects to arbitrary precision. Galaxy images and PSFs are decomposed into a family of orthogonal 2d Gaussian-based functions, making the PSF correction and shape measurement relatively straightforward and computationally efficient. We also discuss sources of noise-induced bias in weak lensing measurements and provide a solution for these and previously identified biases.
387 citations
TL;DR: In this article, a non-perturbative spacetime regularization of the Lorentzian space-times is proposed to quantize gravity in d = 3,4.
303 citations
TL;DR: In this paper, the ionization fraction of protostellar alpha disks, taking into account vertical temperature structure, and the possible presence of trace metal atoms, has been calculated for both thermal and X-ray ionization.
Abstract: We calculate the ionization fraction of protostellar alpha disks, taking into account vertical temperature structure, and the possible presence of trace metal atoms. Both thermal and X-ray ionization are considered. Previous investigations of layered disks used radial power-law models with isothermal vertical structure. But alpha models are used to model accretion, and the present work is a step towards a self-consistent treatment. The extent of the magnetically uncoupled (``dead'') zone depends sensitively on alpha, on the assumed accretion rate, and on the critical magnetic Reynolds number, below which MHD turbulence cannot be self-sustained. Its extent is extremely model-dependent. It is also shown that a tiny fraction of the cosmic abundance of metal atoms can dramatically affect the ionization balance. Gravitational instabilities are an unpromising source of transport, except in the early stages of disk formation.
245 citations
TL;DR: In this article, the sonic analog of a gravitational black hole in dilute-gas Bose-Einstein condensates is investigated, and it is shown that there exist both dynamically stable and unstable configurations which, in the hydrodynamic limit, exhibit behaviors completely analogous to that of gravitational black holes.
Abstract: The sonic analog of a gravitational black hole in dilute-gas Bose-Einstein condensates is investigated It is shown that there exist both dynamically stable and unstable configurations which, in the hydrodynamic limit, exhibit behaviors completely analogous to that of gravitational black holes The dynamical instabilities involve the creation of quasiparticle pairs in positive and negative energy states We illustrate these features in two qualitatively different one-dimensional models, namely, a long, thin condensate with an outcoupler laser beam providing an ``atom sink,'' and a tight ring-shaped condensate We also simulate the creation of a stable sonic black hole by solving the Gross-Pitaevskii equation numerically for a condensate subject to a trapping potential which is adiabatically deformed A sonic black hole could, in this way, be created experimentally with state-of-the-art or planned technology
TL;DR: In this article, it was shown that magnetic fields cannot prevent local collapse unless they provide magnetostatic support, which is a very severe requirement (see, e.g., paper I).
Abstract: Hydrodynamic supersonic turbulence can only prevent local gravitational collapse if the turbulence is driven on scales smaller than the local Jeans lengths in the densest regions, which is a very severe requirement (see Paper I). Magnetic fields have been suggested to support molecular clouds either magnetostatically or via magnetohydrodynamic (MHD) waves. Whereas the first mechanism would form sheetlike clouds, the second mechanism not only could exert a pressure onto the gas counteracting the gravitational forces but could lead to a transfer of turbulent kinetic energy down to smaller spatial scales via MHD wave interactions. This turbulent magnetic cascade might provide sufficient energy at small scales to halt local collapse. We test this hypothesis with MHD simulations at resolutions up to 2563 zones done with ZEUS-3D. We first derive a resolution criterion for self-gravitating, magnetized gas: to prevent collapse of magnetostatically supported regions caused by numerical diffusion, the minimum Jeans length must be resolved by four zones. Resolution of MHD waves increases this requirement to roughly six zones. We then find that magnetic fields cannot prevent local collapse unless they provide magnetostatic support. Weaker magnetic fields do somewhat delay collapse and cause it to occur more uniformly across the supported region in comparison to the hydrodynamical case. However, they still cannot prevent local collapse for much longer than a global free-fall time.
TL;DR: In this paper, a general model of multidimensional gravity including a Riemann tensor square term (nonzero c case) was considered and the number of brane-worlds in such a model was constructed (mainly in five dimensions).
Abstract: A general model of multidimensional ${R}^{2}$ gravity including a Riemann tensor square term (nonzero c case) is considered. The number of brane-worlds in such a model is constructed (mainly in five dimensions) and their properties are discussed. The thermodynamics of a Schwarzschild--anti-deSitter (S-AdS) BH (with boundary) is presented when perturbation on c is used. The entropy, free energy, and energy are calculated. For a nonzero c the entropy (energy) is not proportional to the area (mass). The equation of motion of the brane in a BH background is presented as a FRW equation. Using a dual CFT description it is shown that the dual field theory is not a conformal one when c is not zero. In this case the holographic entropy does not coincide with the BH entropy (they coincide for Einstein gravity or $c=0$ HD gravity where the AdS/CFT description is well applied). An asymmetrically warped background (an analogue of a charged AdS BH) where Lorentz invariance violation occurs is found. The cosmological 4D dS brane connecting two dS bulk spaces is formulated in terms of the parameters of ${R}^{2}$ gravity. Within the proposed dS/CFT correspondence the holographic conformal anomaly from five-dimensional higher derivative gravity in a de Sitter background is evaluated.
TL;DR: In this paper, it was shown that the case cg < c is very tightly constrained by the observation of the highest energy cosmic rays, and that if the cosmic rays have an extragalactic origin the bound is orders of magnitude tighter, of order c−cg < 2 × 10−19c.
Abstract: Recently, interesting 4-D Lorentz violating models have been proposed, in which all particles have a common maximum velocity c, but gravity propagates (in the preferred frame) with a different maximum velocity cg≠c. We show that the case cg < c is very tightly constrained by the observation of the highest energy cosmic rays. Assuming a galactic origin for the cosmic rays gives a conservative bound of c−cg <2 × 10−15c; if the cosmic rays have an extragalactic origin the bound is orders of magnitude tighter, of order c−cg < 2 × 10−19c.
TL;DR: It is shown that the fluctuations of the force are generated locally, in contrast to the variations of the potential, which originate from noise in the whole system.
Abstract: In N-body simulations of collisionless stellar systems, the forces are softened to reduce the large fluctuations owing to shot noise. Softening essentially modifies the law of gravity at r=|xi−xj| smaller than some softening length e. Therefore, the softened forces are increasingly biased for ever larger e, and there is some optimal e which yields the best compromise between reducing the fluctuations and introducing a bias. Here, analytical relations are derived for the amplitudes of the bias and the fluctuations in the limit of e being much smaller than any physical scale of the underlying stellar system. In particular, it is shown that the fluctuations of the force are generated locally, in contrast to the variations of the potential, which originate from noise in the whole system. Based on these asymptotic relations and using numerical simulations, I study the dependence of the resulting force error on the number of bodies, the softening length, and on the functional form by which Newtonian gravity is replaced. The widely used Plummer softening, where each body is replaced by a Plummer sphere of scale radius e, yields significantly larger force errors than do methods in which the bodies are replaced by density kernels of finite extent. I also give special kernels, which reduce the errors even further. These kernels largely compensate for the errors made with too small interparticle forces at r
TL;DR: In this article, the presence of radio halos and relics in six clusters of galaxies (A115, A520, A773, A1664, A2254, A2744) was confirmed using deep very large array radio observations.
Abstract: Deep Very Large Array radio observations confirm the presence of halo and relic sources in six clusters of galaxies (A115, A520, A773, A1664, A2254, A2744) where a wide diffuse emission was previously found in the NRAO VLA Sky Survey. New images at 1.4 GHz of these six clusters of galaxies are presented and X-ray data obtained from the ROSAT archive are analyzed. The properties of clusters hosting radio halos and relics are analyzed and discussed. A correlation between the halo radio power and the cluster gravitational mass is presented.
TL;DR: In this article, the derivation of the holographic renormalization group given in hep-th/9912012 is reviewed, and some extra background material is included, and various applications are discussed.
Abstract: In this lecture, we review the derivation of the holographic renormalization group given in hep-th/9912012. Some extra background material is included, and various applications are discussed.
TL;DR: In this paper, the authors derived upper limits on the dark matter content of elliptical galaxies using the number and sizes of observed gravitational lenses, and extended the argument to galaxies built from cold dark matter (CDM) mass distributions.
Abstract: (Abridged) Using the number and sizes of observed gravitational lenses, I derive upper limits on the dark matter content of elliptical galaxies. Galaxies built from Cold Dark Matter (CDM) mass distributions are too concentrated to comfortably satisfy these limits; SCDM is ruled out, and LCDM is only marginally consistent with the data. Thus, lensing adds to the evidence that CDM mass distributions are too concentrated on kiloparsec scales to agree with real galaxies, and extends the argument to elliptical galaxies. By contrast, the lack of central images in radio lenses implies that the central densities of CDM galaxies are too low on ~10 parsec scales, even if supermassive black holes are included. Self-interacting dark matter, or any other modification to regular cold dark matter, must simultaneously reduce the densities on kiloparsec scales and increase the densities on parsec scales in order to satisfy the unique constraints from lensing.
TL;DR: In this paper, the authors developed a theory of Einstein rings and demonstrated it using the infrared Einstein ring images of the quasar host galaxies observed in PG 1115+080, B1608+656, and B1938+666.
Abstract: We develop a theory of Einstein rings and demonstrate it using the infrared Einstein ring images of the quasar host galaxies observed in PG 1115+080, B1608+656, and B1938+666. The shape of an Einstein ring accurately and independently determines the shape of the lens potential and the shape of the lensed host galaxy. We find that the host galaxies of PG 1115+080, B1608+656, and B1938+666 have axis ratios of 0.58 ± 0.02, 0.69 ± 0.02, and 0.62 ± 0.14, respectively, including the uncertainties in the lens models. The Einstein rings break the degeneracies in the mass distributions or Hubble constants inferred from observations of gravitational lenses. In particular, the Einstein ring in PG 1115+080 rules out the centrally concentrated mass distributions that lead to a high Hubble constant (H0 > 60 km s-1 Mpc-1) given the measured time delays. Deep, detailed observations of Einstein rings will be revolutionary for constraining mass models and determining the Hubble constant from time-delay measurements.
TL;DR: Two-time physics (2T) as discussed by the authors is a general reformulation of 1T that displays previously unseen hidden symmetries in 1T dynamical systems and establishes previously unknown duality-type relations among them.
Abstract: Two-time physics (2T) is a general reformulation of one-time physics (1T) that displays previously unnoticed hidden symmetries in 1T dynamical systems and establishes previously unknown duality-type relations among them. This may play a role in displaying the symmetries and constructing the dynamics of little understood systems, such as M-theory. 2T-physics describes various 1T dynamical systems as different d-dimensional `holographic' views of the same 2T system in d + 2 dimensions. The `holography' is due to gauge symmetries that tend to reduce the number of effective dimensions. Different 1T evolutions (i.e. different Hamiltonians) emerge from the same 2T-theory when gauge fixing is done with different embeddings of d dimensions inside d + 2 dimensions. Thus, in the 2T setting, the distinguished 1T which we call `time' is a gauge-dependent concept. The 2T-action also has a global SO(d,2) symmetry in flat spacetime, or a more general d + 2 symmetry in curved spacetime, under which all dimensions are on an equal footing. This symmetry is observable in many 1T-systems, but it remained unknown until discovered in the 2T formalism. The symmetry takes various nonlinear (hidden) forms in the 1T-systems, and it is realized in the same irreducible unitary representation (the same Casimir eigenvalues) in their quantum Hilbert spaces. 2T-physics has mainly been developed in the context of particles, including spin and supersymmetry, but some advances have also been made with strings and p-branes, and insights for M-theory have already emerged. In the case of particles, there exists a general worldline formulation with background fields, as well as a field theory formulation, both described in terms of fields that depend on d + 2 coordinates. All 1T particle interactions with Yang-Mills, gravitational and other fields are included in the d + 2 reformulation. In particular, the standard model of particle physics can be regarded as a gauge-fixed form of a 2T-theory in 4 + 2 dimensions. These facts already provide evidence for a new type of higher-dimensional unification.
TL;DR: In this article, a test of the strong equivalence principle and a search for sub-millimetre scale deviations of the Newtonian 1/r2 law were performed by the University of Washington Eot-Wash group.
Abstract: This paper describes recent experimental work by the University of Washington Eot-Wash group on two different topics: a test of the strong equivalence principle and a search for sub-millimetre scale deviations of the Newtonian 1/r2 law. Our strong equivalence principle test was motivated by the resurgence of interest in `gravitational' scalar fields, which typically lead to violation of the equivalence principle for gravitational self-energy. Our sub-millimetre experiment was motivated by predictions of fundamentally new effects from `large' extra dimensions and from the dilaton and moduli scalar particles of string theory.
TL;DR: In this paper, the authors discuss scenarios in which the galactic dark matter in spiral galaxies is described by a long range coherent field which settles in a stationary configuration that might account for the features of the galactic rotation curves.
Abstract: We discuss scenarios in which the galactic dark matter in spiral galaxies is described by a long range coherent field which settles in a stationary configuration that might account for the features of the galactic rotation curves. The simplest possibility is to consider scalar fields, so we discuss, in particular, two mechanisms that would account for the settlement of the scalar field in a nontrivial configuration in the absence of a direct coupling of the field with ordinary matter: topological defects and spontaneous scalarization.
TL;DR: In this article, the authors investigated the modes of oscillation trapped within the inner region of accretion disks by the strong-field gravitational properties of a black hole (or a compact, weakly magnetized neutron star), and focused on the "corrugation" (c-) modes, nearly incompressible perturbations of the inner disk.
Abstract: We first briefly review how we investigate the modes of oscillation trapped within the inner region of accretion disks by the strong-field gravitational properties of a black hole (or a compact, weakly magnetized neutron star). Then we focus on the "corrugation" (c-) modes, nearly incompressible perturbations of the inner disk. The fundamental c-modes have eigenfrequencies (ordered by radial mode number) which correspond to the Lense-Thirring frequency, evaluated at the outer trapping radius of the mode, in the slow rotation limit. This trapping radius is a decreasing function of the black hole angular momentum, so a significant portion of the disk is modulated only for slowly rotating black holes. The eigenfrequencies are thus strongly increasing functions of black hole angular momentum. The dependence of the eigenfrequencies on the speed of sound (or the luminosity) within the disk is very weak, except for slowly rotating black holes.
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TL;DR: In this article, a practical model for relativistic reduction of positional observations with an accuracy of 1 \muas which is expected to be attained in the future space astrometry missions is presented.
Abstract: This paper is devoted to a practical model for relativistic reduction of positional observations with an accuracy of 1 \muas which is expected to be attained in the future space astrometry missions. All relativistic effects which are caused by the gravitational field of the Solar system and which are of practical importance for this accuracy level are thoroughly calculated and discussed. The model includes relativistic modeling of the motion of observer, modeling of relativistic aberration and gravitational light deflection as well as a relativistic treatment of parallax and proper motion suitable for the accuracy of 1 \muas. The model is formulated both for remote sources (stars, quasars, etc.) and for Solar system objects (asteroids, etc.). The suggested model is formulated within the framework of Parametrized Post-Newtonian Formalism with parameters $\beta$ and $\gamma$. However, for general relativity ($\beta=\gamma=1$) the model is fully compatible with the IAU Resolutions (2000) on relativity in celestial mechanics, astrometry and metrology. The model is presented in a form suitable for implementation in a software system for data processing or simulation. The changes which should be applied to the model to attain the accuracy of 0.1 \muas are reviewed. Potentially important relativistic effects caused by additional gravitational fields which are generated outside of the Solar system are also briefly discussed.
TL;DR: In this article, a full cycle numerical simulation of the Radio Science Experiments of the BepiColombo mission has been tested with simulated tracking and accelerometer data, and the determination, by least squares fit, of a long list of variables including the initial conditions for each observed arc, calibration parameters, gravity field harmonic coefficients, and corrections to the orbit of Mercury.
TL;DR: In this paper, it was shown that the gravity induced proton decay can be suppressed up to a desired level, while the GUT origin of the proton instability is rather problematic.
TL;DR: In this article, the exact forms of final spectra of the two types of structures generated during a generalized slow-roll inflation are derived, and the classical evolution and the quantum generation processes of the scalar and tensor-type cosmological perturbations in the context of generalized gravity theories are presented in unified forms.
TL;DR: In this article, it was shown that the symmetry U(1,D−1) is consistent and one can obtain complexified gravity by gauging the symmetric metric and antisymmetric tensor.
Abstract: The presence of a constant background antisymmetric tensor for open strings or D-branes forces the space-time coordinates to be noncommutative. This effect is equivalent to replacing ordinary products in the effective theory by the deformed star product. An immediate consequence of this is that all fields get complexified. The only possible noncommutative Yang–Mills theory is the one with U(N) gauge symmetry. By applying this idea to gravity one discovers that the metric becomes complex. We show in this article that this procedure is completely consistent and one can obtain complexified gravity by gauging the symmetry U(1,D−1) instead of the usual SO(1,D−1). The final theory depends on a Hermitian tensor containing both the symmetric metric and antisymmetric tensor. In contrast to other theories of nonsymmetric gravity the action is both unique and gauge invariant. The results are then generalized to noncommutative spaces.
TL;DR: In this paper, the authors investigated the susceptibility of gaseous, magnetized galactic disks to formation of self-gravitating condensations using two-dimensional, local models.
Abstract: We investigate the susceptibility of gaseous, magnetized galactic disks to formation of self-gravitating condensations using two-dimensional, local models. We focus on two issues: (1) determining the threshold condition for gravitational runaway, taking into account nonlinear effects, and (2) distinguishing the magneto-Jeans instability (MJI) that arises under inner-galaxy rotation curves from the modified swing amplification (MSA) that arises under outer-galaxy rotation curves. For axisymmetric density fluctuations, instability is known to require a Toomre parameter Q<1. For nonaxisymmetric fluctuations, any nonzero shear $q \equiv -d\ln \Omega /d \ln R$ winds up wavefronts such that in linear theory amplification saturates. Any Q threshold for nonaxisymmetric gravitational runaway must originate from nonlinear effects. We use numerical magnetohydrodynamic simulations to demonstrate the anticipated threshold phenomenon, to analyze the nonlinear processes involved, and to evaluate the critical value $Q_c$ for stabilization. We find $Q_c \sim 1.2-1.4$ for a wide variety of conditions, with the largest values corresponding to nonzero but subthermal mean magnetic fields. Our findings for $Q_c$ are similar to those inferred from thresholds for active star formation in the outer regions of spiral galaxies. MJI is distinct from MSA in that opposition to Coriolis forces by magnetic tension, rather than cooperation of epicyclic motion with kinematic shear, enables nonaxisymmetric density perturbations to grow. We suggest that under low-shear inner-disk conditions, $Q_c$ will be larger than that in outer disks by a factor $\sim (v_A/q c_s)^{1/2}$, where $v_A$ and $c_s$ are the respective Alfven and sound speeds.