Homogeneous Cosmologies Research Articles

Freidmann cosmology with a cosmological 'constant' in the scale covariant theory

Homogeneous isotropic cosmologies in the presence of a cosmological 'constant' are studied in the scale covariant theory. A class of solutions is obtained for k=0 for models filled with dust, radiation or stiff matter. For k not=0, solutions are presented for the radiation models.

Cosmic nucleosynthesis and nonlinear inhomogeneities

We present a number of numerical simulations of the synthesis of the light elements in an inhomogeneous big-bang cosmology. A completely relativistic, nonlinear plane-symmetric hydrodynamics and general relativity code provides the geometrical evolution, within which the nucleosynthesis occurs. We argue that our plane symmetry provides realistic modelling of the evolution during this epoch, and that local enhancements or depressions of the element abundances may persist to the present. In any case, the average abundances we find are not identical to the predictions of homogeneous cosmologies. Local abundances fluctuating in the range 22%−22% are typically produced in our models. These models are consistent with all observational constraints, in particular with the observed isotropy of the microwave radiation background.

Quark-hadron phase transition and thermodynamics of anisotropic cosmology

A first-order quark-hadron phase transition is studied in the most general homogeneous cosmologies, where also curvature anisotropies and velocity fields are present. We have found an entropy conversation law that governs expansion thermodynamics and holds in every model.

Spottiness in the large-scale structure of the microwave background

Barrow et al.1 have recently considered the large-scale temperature variation (ΔT/T) distribution of the microwave background in the open universe. Interest in this topic has been stimulated by new observations from balloons and the satellite Prognoz. Barrow et al.1 seem to have misunderstood the theory that we have developed2–4 and arrived at conclusions directly contradicting our results. Here we argue that the main conclusions reached by Barrow et al.1—that concerning the relationship between inhomogeneous and the most general homogeneous anisotropic cosmologies, and their other claim that the sky ΔT/T pattern of infinite-wavelength inhomogeneities is a superposition of Bianchi type VII models—are erroneous.

Effective action for neutrino and photon fields in homogeneous cosmologies with small anisotropy

The one-loop contributions to the effective action of conformally invariant neutrino and photon fields are calculated for the homogeneous cosmologies with small anisotropy. The problem of finding a gauge for the electromagnetic field in which the calculations can be carried out explicitly is discussed.

Gravitational clustering for a multicomponent system with a distribution of temperatures

view Abstract Citations (2) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gravitational clustering for a multicomponent system with a distribution of temperatures Kandrup, H. E. Abstract This paper examines the onset of gravitational clustering, in the context of a homogeneous and isotropic Newtonian cosmology, for a complicated system of point-mass 'particles' e.g., galaxies, characterized by a distribution of masses and temperatures. The principal conclusion is that, to the extent that nonlinear and collective effects may be neglected, the spatial clustering of any two species of particles may be described as if the two constituents were characterized by a single effective temperature, the value of which depends only upon the temperatures of the two constituents, and the ratio of their masses. It is also seen that the 'natural' scale on which collective effects will be manifest is in fact the Jeans length appropriate for a multicomponent system. Publication: The Astrophysical Journal Pub Date: November 1984 DOI: 10.1086/162571 Bibcode: 1984ApJ...286...15K Keywords: Computational Astrophysics; Galactic Clusters; Galactic Evolution; Gravitational Effects; Nonlinear Systems; Temperature Distribution; Cosmology; Jeans Theory; Mass Distribution; Maxwell-Boltzmann Density Function; Newton Theory; Astrophysics full text sources ADS |

Power law singularities in orthogonal spatially homogeneous cosmologies

We investigate the structure of the so-called power asymptote singularities in orthogonal spatially homogeneous solutions of the Einstein field equations with perfect fluid source. We first give a systematic survey of the different possible power asymptotes, some of which are well known, and some new, and characterize them in a coordinate-independent manner. The known orthogonal spatially homogeneous exact solutions with perfect fluid source are then classified on the basis of which power asymptote they admit. In many cases this leads to simpler forms of the known solutions, and suggests methods for deriving new solutions.

Friedmann cosmology and hadron properties

Following a previous unified classical approach to gravitational and strong interactions, we associate the interior of hadrons with a homogeneous isotropic cosmology. We derive some hadronic properties from our model and from the corresponding (« scaled down ») Friedmann equations.

Closed, homogeneous cosmology with massive magnetic field

We show that all symmetries of the Einstein universe are preserved upon insertion of a uniform, Clifford-parallel massive magnetic field.

On Mø11er's tetrad theory of gravitation cosmology

We use a particular tetrad field to describe homogeneous isotropic cosmologies in the background of Moller's theory of gravitation. We prove that the universe can be closed or open for any possible value of the density of energy. The relation between the apparent magnitudem and the number of galaxies with a magnitude greater thanm is proved to be different from that of general relativity.

Large scale anisotropies and polarization of the microwave background radiation in homogeneous cosmologies

The polarization and anisotropy of the cosmological microwave background radiation on large angular scales are discussed. A quadrupole anisotropy in the expansion of the universe (shear) is considered in realistic cosmological models and the resulting anisotropies and polarization of the radiation are calculated. The role of spatial curvature is considered separately, and it is found to have two profound effects: first, in closed models only, the direction of polarization of the radiation will appear at observation to be twisted relative to the anisotropy; the existence of this twist implies that the closed universe has a handedness property. Second, in open models a quadrupole anisotropy may be distorted by the spatial curvature so that it resembles a dipole; in the extreme case all the aniso­tropy is confined to a region of small angular diameter (a ‘spot’). Following previous work by Dautcourt and Rose, a transfer equation for polarized radiation in a general curved space-time is derived. The effect of Thomson scattering by free electrons is included, and the equation is separated into those for the multipoles up to quadrupole by expanding in polynomials formed from spin-weighted spherical harmonics. A numerical integration of the equations is described, and the results are presented for the twist angle, the dipole and quadrupole anisotropies, the degree of polarization in the quadrupole mode and the ratio of polarization to quadrupole aniso­tropy in all models considered. The twist of polarization in closed models is large and should be easily observable if the polarization could be. This result suggests an important observational test of the spatial curvature of the standard models. The dipole produced in open models, due to distortion by the spatial curvature, is prominent; it appears unlikely that an intrinsic dipole to quadrupole ratio of less than unity arises in any open models in which the effect occurs when the density is below one-half the critical density. Finally, the ratio of polarization to anisotropy is expected to be a good indicator of the presence of shear, and is sensitive to the ionization history of the matter.

Quantum stationary geometries and avoidance of singularities

A recent approach to quantum gravity leading to the concept of quantum stationary geometries is reviewed. A stationary states equation is presented for: (a) homogeneous relativistic cosmologies of the various Bianchi types; (b) a Friedmann universe filled with a massless scalar field. The equation is solved near the singularity to show that stationary states avoid the singularity. The result is discussed and compared with other approaches.

Planar numerical cosmology. II - The difference equations and numerical tests

This paper is the second in a series describing a method for evolving inhomogeneous planar cosmological models on a computer. In Paper I we set forth our coordinate conditions, variables, and equations, and discussed the way our special gauge simplifies the numerical evolution. In this paper we focus on the architecture of the code. We describe the differencing techniques we use to solve the hydrodynamical and, especially, the Einstein equations. We also present the results of testing the code on problems in hydrodynamics, spatially homogeneous cosmologies, and linearized general relativistic perturbation theory. Appendices contain a discussion of Barton's method for monotonic transport and a new method for the solution of one-dimensional special relativistic similarity flow.

Regularised field equations for Bianchi type VI spatially homogeneous cosmology

Using Jantzen's Lie algebra automorphism formalism and the scale invariance of the Einstein equations, it is shown that there is a natural way of compactifying the gravitational phase space for spatially homogeneous cosmology, thereby allowing standard qualitative geometric techniques from the theory of ordinary differential systems to be used to find the asymptotic behaviour of solutions near the initial singularity and in the distant future. In this way the field equations for Bianchi type VIh (including III=VI-1) cosmologies are reduced to a seven-dimensional first-order system on a physical phase space with compact closure. It turns out that the natural time parameter for this system coincides with Misner's Omega -time. Furthermore, the system is analytic in the interior of the phase space, a feature which seems to be limited to Bianchi types III, IV and VI. The extent to which the system can be extended to the boundary is discussed. Some critical points of the reduced system give rise to a new class of exact solutions which includes rotating models. Finally, the system is specialised to a Taub-like four-dimensional subsystem in which some explicit calculations are done.

Homogeneous isotropic Newtonian cosmology and the Clebsch generalized potentials

We construct the kinetic potentials that appear on the generalized Clebsch representation for the velocity field of a perfect fluid with a politropic state equation that describes the matter distribution in a Newtonian homogeneous isotropic universe. We also specify the constants of motion with respect to the HS group.

Coherent scalar-field oscillations in an expanding universe

Motivated by the cosmological importance of coherent (classical), scalar-field oscillations in the context of the invisible axion and the new inflationary-universe scenario, we analyze, in general, the classical evolution of a scalar field in an isotropic and homogeneous cosmology. For a scalar potential of the form $V(\ensuremath{\varphi})=a{\ensuremath{\varphi}}^{n}$, the energy density of the scalar-field oscillations decreases as ${R}^{\ensuremath{-}\frac{6n}{(n+2)}}$ when the oscillations are rapid compared to the expansion rate ($R=\mathrm{cosmic}\mathrm{scale}\mathrm{factor}$). We also investigate the effect of higher-order terms in the potential perturbatively, and analyze the decay of the coherent field oscillations due to quantum particle creation.

The cosmological evolution of general bianchi models in the adiabatic regime

We analyse the evolution of homogeneous anisotropic cosmologies in the adiabatic regime allowing for a cosmic velocity field and taking into account the contribution of the quantum vacuum to the cosmic energy density. We show that a constant or slowly varying energymomentum tensor of vacuum isotropizes the Hubble expansion. We suggest, therefore, that an imperfect tuning of field parameters in grand unified theories may naturally explain the isotropy of the Universe.

Exact solutions for the early Friedmann-Robertson-Walker universe in general scalar-tensor theories

Homogeneous and isotropic cosmologies with trace-free energy-momentum tensors are studied in general scalar-tensor theories. A method is presented which allows one to construct exact solutions for theories with arbitrary coupling functionω(φ). Particular attention is paid to Schwinger's theory.

Hamiltonian cosmology and gravitational solitons: A variational outlook

A general procedure is applied for finding the possible variational principle of a given nonlinear system of partial differential equations. In particular, attention is focused on Hamiltonian homogeneous cosmologies and gravitational solitons. In the first case, it is found that a variational principle exists if and only if the model is of class A, thus complementing previous results of MacCallum and Taub. In the second case, it is shown that the pertinent system arises from a variational principle; furthermore, the explicit form of the Lagrangian is given. Owing to the structure of the Lagrangian an outstanding first integral is determined.

Geometrical perturbation theory: action-principle surface terms in homogeneous cosmology

The coordinate-independent formulation of spacetime perturbation theory is applied to the analysis of surface terms which appear in variations of the Einstein Action of general relativity. Restricted variations of the type used to study the dynamics of homogeneous universes require a corrected action functional. A general expression for the correction is found.