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Träfflista för sökning "L773:0001 7701 OR L773:1572 9532 "

Search: L773:0001 7701 OR L773:1572 9532

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1.
  • Abramowicz, Marek A, 1945, et al. (author)
  • The perihelion of Mercury advance and the light bending calculated in (enhanced) Newton's theory
  • 2014
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 46:1
  • Journal article (peer-reviewed)abstract
    • We show that results of a simple dynamical gedanken experiment interpreted according to standard Newton's gravitational theory, may reveal that threedimensional space is curved. The experiment may be used to reconstruct the curved geometry of space, i.e. its non-Euclidean metric 3gik. The perihelion of Mercury advance and the light bending calculated from the Poisson equation 3gik∇i∇kΦ = -4πGρ and the equation of motion Fi = mai in the curved geometry 3gik have the correct (observed) values. Independently, we also show that Newtonian gravity theory may be enhanced to incorporate the curvature of three dimensional space by adding an extra equation which links the Ricci scalar 3R with the density of matter ρ. Like in Einstein's general relativity, matter is the source of curvature. In the spherically symmetric (vacuum) case, the metric of space 3gik that follows from this extra equation agrees, to the expected accuracy, with the metric measured by the Newtonian gedanken experiment mentioned above. © 2013 The Author(s).
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2.
  • Akrami, Yashar, et al. (author)
  • The nature of spacetime in bigravity : Two metrics or none?
  • 2015
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 47:1, s. 1838-
  • Journal article (peer-reviewed)abstract
    • The possibility of matter coupling to two metrics at once is considered. This appears natural in the most general ghost-free, bimetric theory of gravity, where it unlocks an additional symmetry with respect to the exchange of the metrics. This double coupling, however, raises the problem of identifying the observables of the theory. It is shown that if the two metrics couple minimally to matter, then there is no physical metric to which all matter would universally couple, and that moreover such an effective metric generically does not exist even for an individual matter species. By studying point particle dynamics, a resolution is suggested in the context of Finsler geometry.
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3.
  • Aksteiner, Steffen, et al. (author)
  • A space-time calculus based on symmetric 2-spinors
  • 2023
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 55:3
  • Journal article (peer-reviewed)abstract
    • In this paper we present a space-time calculus for symmetric spinors, including a product with a number of index contractions followed by symmetrization. As all operations stay within the class of symmetric spinors, no involved index manipulations are needed. In fact spinor indices are not needed in the formalism. It is also general because any covariant tensor expression in a 4-dimensional Lorentzian spacetime can be translated to this formalism. The computer algebra implementation SymSpin as part of xAct for Mathematica is also presented.
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4.
  • Barausse, Enrico, et al. (author)
  • Prospects for fundamental physics with LISA
  • 2020
  • In: General Relativity and Gravitation. - : SPRINGER/PLENUM PUBLISHERS. - 0001-7701 .- 1572-9532. ; 52:8
  • Journal article (other academic/artistic)abstract
    • In this paper, which is of programmatic rather than quantitative nature, we aim to further delineate and sharpen the future potential of the LISA mission in the area of fundamental physics. Given the very broad range of topics that might be relevant to LISA,we present here a sample of what we view as particularly promising fundamental physics directions. We organize these directions through a "science-first" approach that allows us to classify how LISA data can inform theoretical physics in a variety of areas. For each of these theoretical physics classes, we identify the sources that are currently expected to provide the principal contribution to our knowledge, and the areas that need further development. The classification presented here should not be thought of as cast in stone, but rather as a fluid framework that is amenable to change with the flow of new insights in theoretical physics.
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5.
  • Bengtsson, Ingemar, et al. (author)
  • A Toy Penrose Inequality and Its Proof
  • 2016
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 48:12
  • Journal article (peer-reviewed)abstract
    • We formulate and prove a toy version of the Penrose inequality. The formulation mimics the original Penrose inequality in which the scenario is the following: A shell of null dust collapses in Minkowski space and a marginally trapped surface forms on it. Through a series of arguments relying on established assumptions, an inequality relating the area of this surface to the total energy of the shell is formulated. Then a further reformulation turns the inequality into a statement relating the area and the outer null expansion of a class of surfaces in Minkowski space itself. The inequality has been proven to hold true in many special cases, but there is no proof in general. In the toy version here presented, an analogous inequality in (2+1)-dimensional anti-de Sitter space turns out to hold true.
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6.
  • Bengtsson, Ingemar, 1957- (author)
  • The Hawking energy on photon surfaces
  • 2020
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 52:5
  • Journal article (peer-reviewed)abstract
    • The Hawking energy has a monotonicity property under the inverse mean curvature flow on totally umbilic hypersurfaces with constant scalar curvature in Einstein spaces. It grows if the hypersurface is spacelike, and decreases if it is timelike. Timelike examples include Minkowski and de Sitter hyperboloids, and photon surfaces in Schwarzschild.
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7.
  • Bini, Donato, et al. (author)
  • Particle motion in a photon gas : friction matters
  • 2012
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 44:10, s. 2669-2680
  • Journal article (peer-reviewed)abstract
    • The motion of a particle in the Tolman metric generated by a photon gas source is discussed. Both the case of geodesic motion and motion with nonzero friction, due to photon scattering effects, are analyzed. In the Minkowski limit, the particle moves along a straight line segment with a decelerated motion, reaching the endpoint at zero speed. The curved case shows a qualitatively different behavior; the geodesic motion consists of periodic orbits, confined within a specific radial interval. Under the effect of frictional drag, this radial interval closes up in time and in all our numerical simulations the particle ends up in the singularity at the center.
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8.
  • Bradley, Michael, et al. (author)
  • Invariant classification and the generalised invariant formalism: conformally flat pure radiation metrics
  • 2010
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 42:1, s. 155-182
  • Journal article (peer-reviewed)abstract
    • Metrics obtained by integrating within the generalised invariant formalism are structured around their intrinsic coordinates, and this considerably simplifies their invariant classification and symmetry analysis. We illustrate this by presenting a simple and transparent complete invariant classification of the conformally flat pure radiation metrics (except plane waves) in such intrinsic coordinates; in particular we confirm that the three apparently non-redundant functions of one variable are genuinely non-redundant, and easily identify the subclasses which admit a Killing and/or a homothetic Killing vector. Most of our results agree with the earlier classification carried out by Skea in the different Koutras-McIntosh coordinates, which required much more involved calculations; but there are some subtle differences. Therefore, we also rework the classification in the Koutras-McIntosh coordinates, and by paying attention to some of the subtleties involving arbitrary functions, we obtain complete agreement with the results obtained in intrinsic coordinates. We have corrected and completed statements and results by Edgar and Vickers, and by Skea, about the orders of Cartan invariants at which particular information becomes available.
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9.
  • Brizuela, David, et al. (author)
  • xPert : computer algebra for metric perturbation theory
  • 2009
  • In: GENERAL RELATIVITY AND GRAVITATION. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 41:10, s. 2415-2431
  • Journal article (peer-reviewed)abstract
    • We present the tensor computer algebra package xPert for fast construction and manipulation of the equations of metric perturbation theory, around arbitrary backgrounds. It is based on the combination of explicit combinatorial formulas for the nth order perturbation of curvature tensors and their gauge changes, and the use of highly efficient techniques of index canonicalization, provided by the underlying tensor system xAct, for Mathematica. We give examples of use and show the efficiency of the system with timings plots: it is possible to handle orders n = 4 or n = 5 within seconds, or reach n = 10 with timings below 1 h.
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10.
  • Clifton, Timothy, et al. (author)
  • The magnetic part of the Weyl tensor, and the expansion of discrete universes
  • 2017
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 49:2
  • Journal article (peer-reviewed)abstract
    • We examine the effect that the magnetic part of the Weyl tensor has on the large-scale expansion of space. This is done within the context of a class of cosmological models that contain regularly arranged discrete masses, rather than a continuous perfect fluid. The natural set of geodesic curves that one should use to consider the cosmological expansion of these models requires the existence of a non-zero magnetic part of the Weyl tensor. We include this object in the evolution equations of these models by performing a Taylor series expansion about a hypersurface where it initially vanishes. At the same cosmological time, measured as a fraction of the age of the universe, we find that the influence of the magnetic part of the Weyl tensor increases as the number of masses in the universe is increased. We also find that the influence of the magnetic part of the Weyl tensor increases with time, relative to the leading-order electric part, so that its contribution to the scale of the universe can reach values of similar to 1%, before the Taylor series approximation starts to break down.
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11.
  • Edgar, Brian, 1945-, et al. (author)
  • Obtaining a class of Type O pure radiation metrics with a negative cosmological constant, using invariant operators
  • 2007
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 39, s. 539-566
  • Journal article (peer-reviewed)abstract
    • Using the generalised invariant formalism we derive a special subclass of conformally flat spacetimes whose Ricci tensor has a pure radiation and a Ricci scalar component. The method used is a development of the methods used earlier for pure radiation spacetimes of Petrov types O and N, respectively. In this paper we demonstrate how to handle, in the generalised invariant formalism, spacetimes with isotropy freedom and rich Killing vector structure. Once the spacetimes have been constructed, it is straightforward to deduce their Karlhede classification: the Karlhede algorithm terminates at the fourth derivative order, and the spacetimes all have one degree of null isotropy and three, four or five Killing vectors.
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12.
  • Edgar, Brian, et al. (author)
  • Symmetry analysis of radiative spacetimes with a null isotropy using GHP formalism
  • 2014
  • In: General Relativity and Gravitation. - : Springer. - 0001-7701 .- 1572-9532. ; 46:10, s. 1797-
  • Journal article (peer-reviewed)abstract
    • A complete and simple invariant classification of the conformally flat pure radiation metrics with a negative cosmological constant that were obtained by integration using the generalised invariant formalism is presented. We show equivalence between these metrics and the corresponding type O subclass of the more general spacetime studied by Siklos. The classification procedure indicates that the metrics possess a one degree of null isotropy freedom which has very interesting repercussions in the symmetry analysis. The Killing and homothetic vector analysis in GHP formalism is then generalised to this case were there is only one null direction defined geometrically. We determine the existing Killing vectors for the different subclasses that arise in the classification and compare these results to those obtained in the symmetry analysis performed by Siklos for a larger class of metrics with Ricci tensor representing a pure radiation field and a negative cosmological constant. It is also shown that there are no homothetic Killing vectors present.
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13.
  • Edgar, Brian, 1945-, et al. (author)
  • Type O pure radiation metrics with a cosmological constant
  • 2007
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 39:11, s. 1749-1772
  • Journal article (peer-reviewed)abstract
    • In this paper we complete the integration of the conformally flat pure radiation spacetimes with a non-zero cosmological constant Λ, and τ ≠ 0, by considering the case Λ +ττ ≠ 0. This is a further demonstration of the power and suitability of the generalised invariant formalism (GIF) for spacetimes where only one null direction is picked out by the Riemann tensor. For these spacetimes, the GIF picks out a second null direction (from the second derivative of the Riemann tensor) and once this spinor has been identified the calculations are transferred to the simpler GHP formalism, where the tetrad and metric are determined. The whole class of conformally flat pure radiation spacetimes with a non-zero cosmological constant (those found in this paper, together with those found earlier for the case Λ +ττ = 0) have a rich variety of subclasses with zero, one, two, three, four or five Killing vectors. © 2007 Springer Science+Business Media, LLC.
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14.
  • Edgar, SB, et al. (author)
  • Integration in the GHP formalism IV : A new Lie derivative operator leading to an efficient treatment of killing vectors
  • 2000
  • In: General Relativity and Gravitation. - 0001-7701 .- 1572-9532. ; 32:4, s. 637-671
  • Journal article (peer-reviewed)abstract
    • In order to achieve efficient calculations and easy interpretations of symmetries, a strategy for investigations in tetrad formalisms is outlined: work in an intrinsic tetrad using intrinsic coordinates. The key result is that a vector field xi is a Killing vector field if and only if there exists a tetrad which is Lie derived with respect to xi, this result is translated into the GHP formalism using a new generalised Lie derivative operator L-xi with respect to a vector field xi. We identify a class of intrinsic GHP tetrads, which belongs to the class of GHP tetrads which is generalised Lie derived by this new generalised Lie derivative operator L-xi in the presence of a Killing vector field xi. This new operator L-xi also has the important property that, with respect to an intrinsic GHP tetrad, it commutes with the usual GHP operators if and only if xi is a Killing vector field. Practically, this means, for any spacetime obtained by integration in the GHP formalism using an intrinsic GHP tetrad, that the Killing vector properties can be deduced from the tetrad or metric using the Lie-GHP commutator equations, without a detailed additional analysis. Killing vectors are found in this manner for a number of special spaces.
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15.
  • Edgar, SB, et al. (author)
  • The Lanczos potential for Weyl-candidate tensors exists only in four dimensions
  • 2000
  • In: General Relativity and Gravitation. - 0001-7701 .- 1572-9532. ; 32:12, s. 2307-2318
  • Journal article (peer-reviewed)abstract
    • We prove that a Lanczos potential L-abc for the Weyl candidate tensor W-abcd does not generally exist for dimensions higher than four. The technique is simply to assume the existence of such a potential in dimension n, and then check the integrability conditions for the assumed system of differential equations, if the integrability conditions yield another non-trivial differential system for L-abc and W-abcd, then this system's integrability conditions should be checked, and so on. When we find a non-trivial condition involving only W-abcd and its derivatives, then clearly Weyl candidate tensors failing to satisfy that condition cannot be written in terms of a Lanczos potential L-abc.
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16.
  • Feenstra, L., et al. (author)
  • Microtraps and Atom Chips : Toolboxes for cold atom physics
  • 2004
  • In: General Relativity and Gravitation. - 0001-7701 .- 1572-9532. ; 36:10, s. 2317-2329
  • Journal article (peer-reviewed)abstract
    • Magnetic microtraps and Atom Chips are safe, small-scale, reliable and flexible tools to prepare ultra-cold and degenerate atom clouds as sources for various atom-optical experiments. We present an overview of the possibilities of the devices and indicate how a microtrap can be used to prepare and launch a Bose-Einstein condensate for use in an atom clock or an interferometer.
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17.
  • Goluchova, K., et al. (author)
  • Horava's quantum gravity illustrated by embedding diagrams of the Kehagias-Sfetsos spacetimes
  • 2015
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 47:11
  • Journal article (peer-reviewed)abstract
    • Possible astrophysical consequences of the Horava quantum gravity theory have been recently studied by several authors. They usually employ the Kehagias-Sfetsos (KS) spacetime which is a spherically symmetric vacuum solution of a specific version of Horava's gravity. The KS metric has several unusual geometrical properties that in the present article we examine by means of the often used technique of embedding diagrams. We pay particular attention to the transition between naked singularity and black-hole states, which is possible along some particular sequences of the KS metrics.
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18.
  • Heinzle, Mark, et al. (author)
  • Spike Statistics
  • 2013
  • In: General Relativity and Gravitation. - : Springer. - 0001-7701 .- 1572-9532. ; 45:5, s. 939-957
  • Journal article (peer-reviewed)abstract
    • In this paper we explore stochastical and statistical properties of so-called recurring spike induced Kasner sequences. Such sequences arise in recurring spike formation, which is needed together with the more familiar BKL scenario to yield a complete description of generic spacelike singularities. In particular we derive a probability distribution for recurring spike induced Kasner sequences, complementing similar available BKL results, which makes comparisons possible. As examples of applications, we derive results for so-called large and small curvature phases and the Hubble-normalized Weyl scalar.
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19.
  • Herberthson, Magnus (author)
  • A C-> 1 completion of the Kerr space-time at spacelike infinity including I+ and I-
  • 2001
  • In: General Relativity and Gravitation. - 0001-7701 .- 1572-9532. ; 33:7, s. 1197-1205
  • Journal article (peer-reviewed)abstract
    • It is well known that, for asymptotically flat spacetimes, one cannot in general have a smooth differentiable structure at spacelike infinity, i(0). Normally, one uses direction dependent structures, whose regularity has to match the regularity of the (rescaled) metric. The standard C->1-structure at i(0) ensures sufficient regularity in spacelike directions, but examples show very low regularity on I+ and I-. The alternative C1+-structure shows that both null and spacelike directions may be treated on an equal footing, at the expense of some manageable logarithmic singularities at i(0). In this paper, we show that the Kerr spacetime may be rescaled and given a structure which is C->1 in both null and spacelike directions from i(0).
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20.
  • Hoppe, Jens (author)
  • Curved space (matrix) membranes
  • 2011
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 43:9, s. 2523-2526
  • Journal article (peer-reviewed)abstract
    • Hamiltonian formulations of M-branes moving in curved backgrounds are given.
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21.
  • Hu, Shi-Qian, et al. (author)
  • A note on Smarr relation and coupling constants
  • 2019
  • In: General Relativity and Gravitation. - : Springer. - 0001-7701 .- 1572-9532. ; 51:5
  • Journal article (peer-reviewed)abstract
    • The Smarr relation plays an important role in black hole thermodynamics. It is often claimed that the Smarr relation can be written down simply by observing the scaling behavior of the various thermodynamical quantities. We point out that this is not necessarily so in the presence of dimensionful coupling constants, and discuss the issues involving the identification of thermodynamical variables.
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22.
  • Jantzen, R.T., et al. (author)
  • Exact hypersurface-homogeneous scalar field models
  • 1993
  • In: General Relativity and Gravitation. - 0001-7701 .- 1572-9532. ; 25:4, s. 409-421
  • Journal article (peer-reviewed)abstract
    • A mechanism is presented for obtaining exact solutions of the Einstein equations for hypersurface-homogeneous scalar fields which unifies and generalizes recent results for inflaton fields in the spatially homogeneous case and for thick domain walls in the timelike-homogeneous case
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23.
  • Jolin, Shan Williams, et al. (author)
  • Analytic analysis of irregular discrete universes
  • 2018
  • In: General Relativity and Gravitation. - : Plenum Publishing. - 0001-7701 .- 1572-9532. ; 50:9
  • Journal article (peer-reviewed)abstract
    • In this work we investigate the dynamics of cosmological models with spherical topology containing up to 600 Schwarzschild black holes arranged in an irregular manner. We solve the field equations by tessellating the 3-sphere into eight identical cells, each having a single edge which is shared by all cells. The shared edge is enforced to be locally rotationally symmetric, thereby allowing for solving the dynamics to high accuracy along this edge. Each cell will then carry an identical (up to parity) configuration which can however have an arbitrarily random distribution. The dynamics of such models is compared to that of previous works on regularly distributed black holes as well as with the standard isotropic dust models of the FLRW type. The irregular models are shown to have richer dynamics than that of the regular models. The randomization of the distribution of the black holes is done both without bias and also with a certain clustering bias. The geometry of the initial configuration of our models is shown to be qualitatively different from the regular case in the way it approaches the isotropic model.
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24.
  • Khosravi, Sh., et al. (author)
  • An inhomogeneous model universe behaving homogeneously
  • 2008
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 40:5, s. 1047-1069
  • Journal article (peer-reviewed)abstract
    • We present a new model universe based on the junction of FRW to flat Lemaitre-Tolman-Bondi (LTB) solutions of Einstein equations along our past light cone, bringing structures within the FRW models. The model is assumed globally to be homogeneous, i.e. the cosmological principle is valid. Local inhomogeneities within the past light cone are modeled as a flat LTB, whereas those outside the light cone are assumed to be smoothed out and represented by a FRW model. The model is singularity free, always FRW far from the observer along the past light cone, gives way to a different luminosity distance relation as for the CDM/FRW models, a negative deceleration parameter near the observer, and correct linear and non-linear density contrast. As a whole, the model behaves like a FRW model on the past light cone with a special behavior of the scale factor, Hubble and deceleration parameter, mimicking dark energy.
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25.
  • Koivisto, Tomi S., et al. (author)
  • A note on viability of nonminimally coupled f(R) theory
  • 2016
  • In: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 48:7
  • Journal article (peer-reviewed)abstract
    • Consistency conditions for nonminimally coupled f(R) theories have been derived by requiring the absence of tachyons and instabilities in the scalar fluctuations. This note confirms these results and clarifies a subtlety regarding different definitions of sound speeds.
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  • Result 1-25 of 43

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