Showing papers on "Friedmann–Lemaître–Robertson–Walker metric published in 1990"
TL;DR: This work presents a systematic study of cosmological solutions in the Lovelock theory of gravitation, including maximally symmetric space-times, Robertson-Walker universes, and product manifolds of symmetric subspaces.
Abstract: We present a systematic study of cosmological solutions in the Lovelock theory of gravitation, including maximally symmetric space-times, Robertson-Walker universes, and product manifolds of symmetric subspaces.
233 citations
TL;DR: A new local decomposition of the observed density gradient is defined and it is shown that the scalar variable defined in the decomposition naturally describes density clumping, and satisfies the standard Bardeen second-order equation.
Abstract: In a previous paper, a second-order propagation equation was derived for covariant and gauge-invariant {ital vector} {ital fields} characterizing density inhomogeneities in an almost-Friedmann-Lemaitre-Robertson-Walker (-FLRW) perfect-fluid universe. However, an error there led to omission of a term representing an effect of vorticity on {ital spatial} {ital density} {ital gradients} at linear level. Here we determine this interaction (leading to an extra term in the second-order propagation equation for the spatial density gradient), and examine its geometrical and physical meaning. We define a new local decomposition of the observed density gradient and we show that the scalar variable defined in the decomposition naturally describes density clumping, and satisfies the standard Bardeen second-order equation. The physical meaning of the other variables defined in the decomposition is discussed, and their propagation equations are presented. Finally, the vorticity-induced time growth of the density gradient is derived in the long-wavelength limit.
172 citations
CERN1
TL;DR: In this article, the authors study quantum string propagation in cosmological backgrounds pointing out the possible emergence of Jeans-like instabilities and determine under which conditions the universe expands, when distances are measured by stringy rods.
96 citations
TL;DR: A theoretical model is proposed for decay of the cosmological constant, allowing a natural interpretation that the cosMological constant is small at present simply because the authors' Universe is old.
Abstract: A theoretical model is proposed for decay of the cosmological constant, allowing a natural interpretation that the cosmological constant is small at present simply because our Universe is old. The model can be made consistent with inflation, the observed time nonvariability of the gravitational constant, as well as evidence for the standard cosmology. The crucial ingredients are an extended theory of a scalar field and a careful analysis of conformal transformations.
84 citations
TL;DR: In this paper, the large distance behaviour of quantum corrections to the gravitational field equations with a cosmological constant term is studied within the formalism of the gauge-invariant Vilkovisky-DeWitt effective action.
84 citations
TL;DR: More general solutions than those presented by Bertolami are deduced in the Brans-Dicke cosmology, endowed with a time-dependent cosmological term, for a Robertson-Walker metric and a perfect fluid obeying the perfect gas law of state.
Abstract: More general solutions than those presented by Bertolami are deduced in the Brans-Dicke cosmology, endowed with a time-dependent cosmological term, for a Robertson-Walker metric and a perfect fluid obeying the perfect gas law of state.
75 citations
CERN1
TL;DR: In this article, a dynamical mechanism based on a general scalar-tensor theory of gravity with a function ω(τ) having a singularity at ф=τ c is proposed.
70 citations
TL;DR: In this paper, the separation of variables is presented in the Dirac equation in open, flat, and closed expanding cosmological Robertson-Walker universes, and the equations governing the radial variable and the evolution of the time-dependent factor are obtained.
Abstract: In this paper the separation of variables is presented in the Dirac equation in open, flat, and closed expanding cosmological Robertson–Walker universes. The equations governing the radial variable and the evolution of the time‐dependent factor are obtained. An exact solution to the Weyl equation is derived for an arbitrary expansion factor of the Robertson–Walker metrics. An exact solution to Dirac equation in a universe filled with radiation is also presented.
61 citations
Journal Article•
TL;DR: In this paper, the authors propose a metrique de type Friedman-Robertson-Walker (FRW) for the element a symetrie spherique and dimension elevee.
Abstract: En partant d'un modele de fluide parfait homogene avec ρ = ρ(t) et p = p(t), on montre que la metrique de type Friedman-Robertson-Walker est l'unique solution pour l'element a symetrie spherique et dimension elevee. Sous l'hypothese d'une equation d'etat p = mρ, les solutions explicites du facteur d'echelle sont etablies et les implications cosmologiques sont discutees. Quelques parametres astrophysiques sont calcules et compares avec les cas analogues a 4 dimensions.
55 citations
TL;DR: Berman and Som's solution for a Brans-Dicke cosmology with time-dependent cosmological term, Robertson-Walker metric, perfect fluid, and perfect gas law of state solves the horizon, homogeneity, and isotropy problems without requiring any unnatural fine tuning in the very early universe, thus being an alternative model to inflation as discussed by the authors.
Abstract: Berman and Som's solution for a Brans-Dicke cosmology with time-dependent cosmological term, Robertson-Walker metric, perfect fluid, and perfect gas law of state solves the horizon, homogeneity, and isotropy problems without requiring any unnatural fine tuning in the very early universe, thus being an alternative model to inflation. The model also does not need recourse to quantum cosmology, and solves the flatness and magnetic monopole problems.
31 citations
TL;DR: In this article, exact solutions of the Dirac equation in open and closed Robertson-Walker spaces are presented, and a set of massive solutions for static metrics is given for the case of nontrivial and arbitrary expansion factors.
Abstract: Exact solutions of the Dirac equation in open and closed Robertson–Walker spaces are presented. A set of massive solutions is given for static metrics. In the case of nontrivial and arbitrary expansion factors, massless solutions are obtained via a conformal transformation. The set of massless solutions in open Robertson–Walker spaces is shown to be complete.
TL;DR: An exact solution to the Einstein equations with a shear-free imperfect-fluid source that approaches a locally flat Robertson-Walker one in the large-{ital t} limit and thus serves as a viable candidate for a realistic cosmological model.
Abstract: An exact solution to the Einstein equations with a shear-free imperfect-fluid source is obtained. The solution approaches a locally flat Robertson-Walker one in the large-{ital t} limit and thus serves as a viable candidate for a realistic cosmological model. The model built out of this solution is found to be free of horizon, entropy, and flatness problems.
TL;DR: Ricci collineations of the Robertson-Walker metric with a vector field of the form ξ=(ξ 0(t, r, r),ξ 1 (t,r),0, 0) are presented in this paper.
Abstract: Ricci collineations and contracted Ricci collineations of the Robertson–Walker metric, associated with a vector field of the form ξ=(ξ0(t,r),ξ1(t,r),0,0) are presented.
TL;DR: In this paper, the cosmological equations of a gravitational field, minimally coupled with a scalar field, are exactly integrated, provided that the potential of the scalar fields belongs to a special class of exponentials.
TL;DR: In this paper, the physical nature of the extreme cases, i.e., degenerate vacuum bulkviscous fluid model and bulk viscous stiff fluid model, is studied in detail.
Abstract: Robertson—Walker cosmological models with bulkviscosity are investigated explicitly with equation of statep=(γ-1)e. In particular, the physical nature of the extreme cases, i.e., degenerate vacuum bulkviscous fluid model and bulkviscous stiff fluid model are studied in detail.
TL;DR: In this paper, the distance-redshift relation in an inhomogeneous universe is studied on the basis of relativistic optical equations, and numerical calculations are performed to get a realistic DRR.
Abstract: The distance-redshift relation (DRR) in an inhomogeneous universe is studied. On the basis of relativistic optical equations, numerical calculations are performed to get a realistic DRR. It is shown that the DRR is coincident with that in the standard Friedmann-Robertson-Walker (FRW) model on average if galaxies or clusters of galaxies are assumed to be completely transparent. It is also shown that the effect of the shear along the light path is small if the scale of inhomogeneities is larger than galactic scale, and that these numerical results are consistent with the analytical investigation of Futamase and Sasaki (1989). 26 refs.
TL;DR: In this paper, a solution of the kinetic equations for ultrarelativistic particles with due account of gravitational interactions with massive particles in the Robertson-Walker universe is obtained. And a conclusion is made as to the high degree of the uniformity of the relict radiation on scales of less than 10′.
Abstract: Kinetic equations for ultrarelativistic particles with due account of gravitational interactions with massive particles in the Robertson-Walker universe are obtained. On the basis of an exact solution of the kinetic equations thus obtained, a conclusion is made as to the high degree of the uniformity of the relict radiation on scales whih are less than 10′.
TL;DR: In this article, three alternative forms are presented for the Robertson-Walker line element of the k = -1 Riemannian curvature isotropic cosmological models, as well as two alternative forms for the K = 0 model and one form for K = + 1 model; these forms are directly derived by solving Einstein's field equations.
Abstract: Three alternative forms are presented for the Robertson-Walker line element of the k = -1 Riemannian curvature isotropic cosmological models, as well as two alternative forms for the k = 0 model and one form for the k = +1 model; these forms are directly derived by solving Einstein's field equations. It is then noted that McVittie's (1933) embedding of a Schwarzschild field in the Robertson-Walker spacetimes fits naturally into one each of the alternative line elements. Related solutions of Einstein's equations are derived with a perfect fluid source which fit naturally in the remaining two forms of the k = -1 universe and the one remaining form of the k = 0 universe. Three additional perfect fluid solutions of Einstein's equations that are in a different sense analogous to McVittie's original solutions are also presented. 12 refs.
TL;DR: In this paper, an inflationary solution in a Weyl invariant effective theory by assuming a pure gauge ansatz was found, which requires a very large cosmological constant in the inflationary era.
TL;DR: In this paper, it was shown that the exact solutions of the Einstein-Liouville equations in a universe with a Robertson-Walker k=0 geometry at early times (when the matter content is effectively a distribution of massless particles) can have an anisotropic distribution function.
Abstract: Exact solutions of the Einstein-Liouville equations in a universe with a Robertson-Walker k=0 geometry at early times (when the matter content is effectively a distribution of massless particles) can have an anisotropic distribution function. In general this will force the geometry to evolve away from isotropy and homogeneity at late times, when the particles effectively become massive as the universe cools.
TL;DR: Vilenkin's string loop equation is generalized to the general FRW models and then solved numerically in the radiation-dominated era as mentioned in this paper. And the initial radius dependence of lifetime is also explored.
TL;DR: In this article, a massless conformal scalar field in an Einstein universe is investigated and it is shown that the "gravitational" temperature of the universe is given asTg=1/2π (ħc/k) (1/R0), where R0 is the radius of the Einstein universe.
Abstract: In recent articles we have introduced Friedmann thermodynamics, where certain geometric parameters in Friedmann models were treated like their thermodynamic counterparts (temperature, entropy, Gibbs potential, etc.). This model has the advantage of allowing us to determine the geometry of the universe by thermodynamic stability arguments. In this paper, in search for evidence for the definition of “gravitational” temperature, we will investigate a massless conformal scalar field in an Einstein universe in detail. We will argue that the “gravitational” temperature of the Einstein universe is given asTg=1/2π) (ħc/k) (1/R0), where R0 is the radius of the Einstein universe. This is in accord with our definition of “gravitational” temperature in Friedmann thermodynamics and determines the dimensionless constant as 1/2π. We discuss the limitations of the model we are using. We also suggest a method to generalize our “gravitational” temperature to arbitrary space-times granted that they are sufficiently smooth.
Journal Article•
TL;DR: In this article, un modele cosmologique est developpe en tenant compte des deviations homogenes et isotropes de l'ecoulement Hubble dans l'integration des equations de champ d'Einstein.
Abstract: Un modele cosmologique est developpe en tenant compte des deviations homogenes et isotropes de l'ecoulement Hubble dans l'integration des equations de champ d'Einstein.
TL;DR: In this paper, an Eulerian, explicit computer code for the study of spherically-symmetric cosmological space-times on a Robertson-Walker background is described.
Abstract: We describe an Eulerian, explicit computer code for the study of spherically-symmetric cosmological space-times on a Robertson-Walker background. The Einstein equations and the relativistic-hydrodynamic equations are written down for such a space-time using the “3+1” formalism of Arnowitt, Deser, and Misner [1]. Time slices are selected by the constant-mean-curvature criterion. Numerical techniques utilized in the code, as well as several code tests, are discussed, with emphasis on the difficulties encountered and their possible causes and cures.
TL;DR: In this article, the authors generalize Weinberg's results for the radiation phase and show that any realistic model of this kind contains gravitational instabilities, provided that q > 0.
Abstract: A well-known solution, for a flat model in general relativity obeying the Robertson-Walker metric, a perfect fluid energy-tensor and a perfect gas law of state, with constant deceleration parameter, is now shown to yield growing scalar density perturbations, provided thatq > 0. This study generalizes Weinberg's results for the radiation phase, and shows that any realistic model of this kind contains gravitational instabilities
TL;DR: In this paper, the authors significantly generalize, simplify, and extend, their results and present a family of Robertson-Walker space-times whose most general radiative purely gravitational perturbations could be expressed as progressing waves.
Abstract: In a series of papers Janis and coworkers found a family of Robertson-Walker space-times whose most general radiative purely gravitational perturbations could be expressed as progressing waves. In this paper we significantly generalize, simplify, and extend, their results.
TL;DR: In this paper, the authors investigated the dynamics of two kinds of multidimensional cosmological models: (1) FRW (K =± 1)× T D and (2) FRw ( K =0)× S D, with the hydrodynamical energy-momentum tensor T μ v = diag ( ϱ, mϱ, mπ�, nϱ, etc.
TL;DR: In this paper, the problem of slowly rotating cosmological viscous fluid universe in a homogeneous and isotropic models has been investigated by considering the perturbation in the metric rotation function to the first order of smallness associated with certain physical restrictions imposed on the rotation function and matter angular velocity.
Abstract: The problem of slowly rotating cosmological viscous fluid universe in a homogeneous and isotropic models has been investigated by considering the perturbation in the metric rotation function to the first order of smallness associated with certain physical restrictions imposed on the metric rotation function and matter angular velocity. Some more general solutions for the metric rotation function have been obtained and physical interpretation of the solutions have been investigated.
TL;DR: In this article, the growth of a dipole disturbance in an otherwise unperturbed, flat, zero-pressure, FRW universe is examined, and the rate of growth is obtained by considering a spherical surface far outside the disturbed region and applying Poisson's equation and the conservation of matter and momentum.
Abstract: The growth of a dipole disturbance in an otherwise unperturbed, flat, zero-pressure, FRW universe is examined. The rate of growth is obtained by considering a spherical surface far outside the disturbed region and applying Poisson's equation and the conservation of matter and momentum. It is found that the metric radius of the disturbance grows as t exp 5/6. The mass accreted onto the central dipole grows as the scale factor to the 3/4 power. 14 refs.