Open Universe Models Research Articles

Stability analysis of Einstein’s universe in loop quantum cosmology

A stability analysis of Einstein’s universe in both classical general relativity and semiclassical loop quantum cosmology regimes is presented. The stability properties of the general relativity model are significantly altered due to loop quantum gravity corrections. Comparisons between both dynamical systems are considered on the basis of these modifications. The loop quantum cosmology solutions are restricted to an open universe model [Formula: see text] and represent a cyclic universe. The integrals of motion for both systems are found and their Hamiltonian structure determined.

Inhomogeneous perturbations and stability analysis of the Einstein static universe in $f(R,T)$ gravity

The purpose of this paper is to analyze the existence of static stable Einstein universe using inhomogeneous linear perturbations in the context of $f(R,T)$ gravity ($R$ and $T$ denote the scalar curvature and trace of the stress-energy tensor, respectively). The static and perturbed field equations are constructed for perfect fluid parameterized by linear equation of state parameter. We obtain solutions manifesting the Einstein static state by considering peculiar $f(R,T)$ forms for vanishing and non-vanishing conservation of the stress-energy tensor. It is observed that stable static Einstein regions exist for both closed as well as open FLRW universe models for an appropriate choice of parameters. We conclude that this theory is efficient for presenting such cosmological solutions leading to emergent universe scenario.

Open Universe Models as Analogs of Multidimensional Electric Capacitors

A new approach to obtaining open Universes models as exact solutions of gravitational equations is considered. The proposed method is based on an analogy between electrostatics of conductors and open cosmological models which have a conformally-flat 4-metric in the Fock form. These cosmological models are solutions of the Einstein equations with the energy-momentum tensor of a Pascal perfect fluid. As a result, it is shown that the capacity of the multidimensional "ball" electric capacitor in multidimensional Euclidean auxiliary space is connected with harmonic functions by which the potentials of multidimensional capacitors are described. In turn, the conformal factor of the metric describing an open universe model in the Fock representation is a power function of this potential for each dimensionality of the Euclidean space. In particular, for 3D auxiliary Euclidean space in 4D space-time, there is the Friedman solution for an open universe filled with incoherent dust (with flat metric at spatial infinity). Further for 4D Euclidean space, we have an analogue of the open cosmological model with the equation of state of an ultra-relativistic gas. Other cases are listed in a table. A table of the matter states which generalizes this approach to multidimensional space-times is also constructed. Thus, the possibility of replacing the modelling problem in cosmology with the equivalent electrostatic problem for finding the capacity of capacitors is shown under specified boundary conditions. Exceptions from the general approach are also presented in the article.

Inhomogeneous perturbations and stability in f ( G , T ) $f(\mathcal{G},T)$ gravity

In this paper, we study stability of the Einstein static universe against inhomogeneous scalar perturbations parameterized by equation of state parameter in \(f(\mathcal{G},T)\) gravity (\(\mathcal{G}\) and \(T\) represent the Gauss-Bonnet invariant and trace of energy-momentum tensor, respectively). We formulate static as well as perturbed field equations in the presence of perfect fluid and analyze the stability regions. This is accomplished for particular \(f(\mathcal{G},T)\) models corresponding to zero as well as non-zero covariant divergence of the energy-momentum tensor. It is found that stable Einstein universe exists both for spatially closed as well as open universe models for suitable choice of parameters.

Interacting and noninteracting dark energy models in f(R) gravity

In this paper, we study cosmic evolution and current expansion via generalized ghost pilgrim dark energy (PDE) model for FRW universe in [Formula: see text] gravity. For this purpose, we consider both interacting and non-interacting forms of dark energy (DE) model and reconstruct [Formula: see text] model using red-shift parameter. In order to analyze the effects of the DE model, we formulate some standard cosmological parameters such as Hubble, effective equation of state (EoS) and statefinder parameters as well as [Formula: see text]–[Formula: see text] plane. We also investigate the stability criteria for both interacting and noninteracting models through the squared speed of sound. It is concluded that the best-fit results are found for the positive curvature parameter in the presence of the noninteracting model. For the interacting model, the cosmic evolution is well explained by closed and open universe models while the flat geometry effectively describes the current cosmos.

Dark energy cosmology with nonlinear spin-one fields

We study a dynamical dark energy candidate emerging from nonlinear electrodynamics in a Friedmann-Lemaître-Robertson-Walker open universe, that is, a homogeneous and isotropic cosmological model whose spatial section has hyperbolic geometry k = −1. We investigate the possibility that a specific model satisfies current observational data of the Hubble parameter as a function of the redshift. Our results show that the accelerated scale factor obtained in this model accounts well for the observed cosmic expansion history of the universe. This suggests that nonlinear spin-one fields are possible candidates to model dark energy.

Mode spectrum of the electromagnetic field in open universe models

We examine the mode functions of the electromagnetic field on spherically symmetric backgrounds with special attention to the subclass which allows for the foliation as open Friedmann-Lemaitre (FL) spacetime. It is well-known that in certain scalar field theories on open FL background there can exist so-called supercurvature modes, their existence depending on parameters of the theory. Looking at specific open universe models, such as open inflation and the Milne universe, we find that no supercurvature modes are present in the spectrum of the electromagnetic field. This excludes the possibility for superadiabatic evolution of cosmological magnetic fields within these models without relying on new physics or breaking the conformal invariance of electromagnetism.

Open inflationary universes in a brane world cosmology

In this paper, we study a type of one-field model for open inflationary universe models in the context of brane world models. In the scenario of a one-bubble universe model, we determine and characterize the existence of the Coleman--De Lucia instanton, together with the period of inflation after tunneling has occurred. Our results are compared to those found in the Einstein theory of relativistic models.

String bit models of two-dimensional quantum gravity coupled with matter

We extend the formalism of Hamiltonian string bit models of quantum gravity type in two spacetime dimensions to include couplings to particles. We find that the single-particle closed and open universe models, respectively, behave like empty open and closed universes, and that a system of two distinguishable particles in a closed universe behaves like an empty closed universe. We then construct a metamodel that contains all such models, and find that its transition amplitude is exactly the same as the sl(2) gravity model.

Black hole versus cosmological horizon entropy

The generalized second law of thermodynamics states that entropy always increases when all event horizons are attributed with an entropy proportional to their area. We test the generalized second law by investigating the change in entropy when dust, radiation and black holes cross a cosmological event horizon. We generalize for flat, open and closed Friedmann–Robertson–Walker universes by using numerical calculations to determine the cosmological horizon evolution. In most cases, the loss of entropy from within the cosmological horizon is more than balanced by an increase in cosmological event horizon entropy, maintaining the validity of the generalized second law of thermodynamics. However, an intriguing set of open universe models shows an apparent entropy decrease when black holes disappear over the cosmological event horizon. We anticipate that this apparent violation of the generalized second law will disappear when solutions are available for black holes embedded in arbitrary backgrounds.

Extended open inflationary universes

In this paper we study a type of one-field model for open inflationary universe models in the context of the Jordan-Brans-Dicke theory. In the scenario of a one-bubble universe model we determine and characterize the existence of the Coleman-De Lucia instanton, together with the period of inflation after tunnelling has occurred. Our results are analogous to those found in the Einstein General Relativity models.

Big bang of the brane universe

The big bang of the Friedmann-Robertson-Walker- (FRW-)brane universe is studied. In contrast with the spacelike initial singularity of the usual FRW universe, the initial singularity of the FRW-brane universe is pointlike from the viewpoint of causality including gravitational waves propagating in the bulk. The existence of null singularities (``seam singularities'') is also shown in the flat and open FRW-brane universe models.

Acoustics of the early universe: flat versus open universe models

A simple perturbation description unique for all signs of curvature, andbased on the gauge-invariant formalisms is proposed to demonstrate that:(a) the density perturbations propagate in the flat radiation-dominateduniverse in exactly the same way as electromagnetic or gravitationalwaves propagate in the epoch of the matter domination;(b) in the open universe, sounds are dispersed by curvature. The spacecurvature defines the minimal frequency ωc below whichthe propagation of perturbations is forbidden.Gaussian acoustic fields are considered and the curvature imprint in theperturbations spectrum is discussed.

Detection limits for super-Hubble suppression of causal fluctuations

We investigate to what extent future microwave background experiments might be able to detect a suppression of fluctuation power on large scales in flat and open universe models. Such suppression would arise if fluctuations are generated by causal processes, and a measurement of a small suppression scale would be problematic for inflation models, but consistent with many defect models. More speculatively, a measurement of a suppression scale of the order of the present Hubble radius could provide independent evidence for a fine-tuned inflation model leading to a low-density universe. We find that, depending on the primordial power spectrum, a suppression scale modestly larger than the visible Horizon can be detected, but that the detectability drops very rapidly with increasing scale. For models with two periods of inflation, there is essentially no possibility of detecting a causal suppression scale.

Constraints on structure formation models from the Sunyaev-Zel'dovich effect

In the context of cold dark matter (CDM) cosmological models, we have simulated images of the brightness temperature fluctuations in the cosmic microwave background (CMB) sky owing to the Sunyaev-Zeldovich (S-Z) effect in a cosmological distribution of clusters. We compare the image statistics with recent ATCA limits on arcmin-scale CMB anisotropy. The S-Z effect produces a generically non-Gaussian field and we compute the variance in the simulated temperature-anisotropy images, after convolution with the ATCA beam pattern, for different cosmological models. All the models are normalized to the 4-yr COBE data. We find an increase in the simulated-sky temperature variance with increase in the cosmological density parameter Omega(0). A comparison with the upper limits on the sky variance set by the ATCA appears to rule out our closed- universe model: low-Omega(0) open-universe models are preferred. The result is independent of any present day observations of sigma(8).

Constraining cosmological parameters through Sunyaev-Zel’dovich surveys

For cold dark matter models, images of temperature fluctuations in the cosmic microwave background (CMB), due to Sunyaev Zel’dovich (SZ) effect have been been simulated taking a cosmolgical distribution of clusters into account. All the models are normalised to the 4-year COBE data. The image statistics are compared with the ATCA limits on arcmin scale anisotropy. The comparison appears to favour low-Ω0 open universe models.

Small-scale fluctuations in the cosmic x-ray background: A power spectrum approach

Equations to investigate fluctuations in cosmic x-ray background radiation due to pointlike sources at high-redshift are formulated in a systematic way. The angular power spectrum of x-ray background fluctuations is investigated from large scales to small scales in various cosmological models such as open universe models and models with the cosmological constant, assuming a simple evolution model of the sources. The effect of the epoch-dependent bias is demonstrated for small-angle fluctuations. The contribution from shot noise fluctuations is also discussed.

Where do all the supercurvature modes go?

In the hyperbolic slicing of de Sitter space appropriate for open universe models, a curvature scale is present and supercurvature fluctuations are possible. In some cases, the expansion of a scalar field in the Bunch-Davies vacuum includes supercurvature modes, as shown by Sasaki, Tanaka and Yamamoto. We express the normalizable vacuum supercurvature modes for a massless scalar field in terms of the basis modes for the spatially-flat slicing of de Sitter space.

Fluctuations in the Hawking-Turok model

Scalar and gravity wave subcurvature fluctuations are calculated for a background approximating the Hawking and Turok open universe model. The gravity wave cosmic microwave background contribution is finite and it appears that a normalizable scalar supercurvature mode is possible in some regions of parameter space.

A NEW COSMOLOGICAL CONSTANT MODEL

We propose a new cosmological model with a time-dependent cosmological constant (Λ∝1/t2), which starting at the Planck time as [Formula: see text], evolves to the present-day allowed value of [Formula: see text]. This scenario is supported by noncritical string theory considerations. We compute the age of the Universe and the time dependence of the scale factor in this model, and find general agreement with recent determinations of the Hubble parameter for substantial values of ΩΛ. This effectively low-density open Universe model differs from the traditional cosmological constant model, and has observable implications for particle physics and cosmology.