Anisotropic viscous universe with variableG and Λ

An anisotropic model with variableG and and bulk viscosity is considered. The model exhibits an inflationary behavior during which the coefficient of bulk viscosity varies lineraly with the energy density. This allows the anisotropy energy to decrease exponentially with time. Other results overlap with our earlier work with a different ansatz for . The gravitational constant was found to increase during the radiation and matter epochs.     

Accelerating universe with time variation of G and Λ

We study a gravitational model in which scale transformations play the key role in obtaining dynamical G and Λ. We take a non-scale invariant gravitational action with a cosmological constant and a gravitational coupling constant. Then, by a scale transformation, through a dilaton field, we obtain a new action containing cosmological and gravitational coupling terms which are dynamically dependent on the dilaton field with Higgs type potential. The vacuum expectation value of this dilaton field, through spontaneous symmetry breaking on the basis of anthropic principle, determines the time variations of G and Λ. The relevance of these time variations to the current acceleration of the universe, coincidence problem, Mach’s cosmological coincidence and those problems of standard cosmology addressed by inflationary models, are discussed. The current acceleration of the universe is shown to be a result of phase transition from radiation toward matter dominated eras. No real coincidence problem between matter and vacuum energy densities exists in this model and this apparent coincidence together with Mach’s cosmological coincidence are shown to be simple consequences of a new kind of scale factor dependence of the energy momentum density as ρ∼a ⁻⁴. This model also provides the possibility for a super fast expansion of the scale factor at very early universe by introducing exotic type matter like cosmic strings.     

Viscous ΛCDM universe models

We explore flat ΛCDM models with bulk viscosity, and study the role of the bulk viscosity in the evolution of these universe models. The dynamical equations for these models are obtained and solved for some cases of bulk viscosity. We obtain differential equations for the Hubble parameter H and the energy density of dark matter ρ _m , for which we give analytical solutions for some cases and for the general case we give a numerical solution. Also we calculate the statefinder parameters for these models and display them in the s–r-plane.     

Do we live in the universe successively dominated by matter and antimatter?

We wonder if a cyclic universe may be dominated alternatively by matter and antimatter. Such a scenario demands a mechanism for transformation of matter to antimatter (or antimatter to matter) during the final stage of a big crunch. By giving an example, we have shown that in principle such a mechanism is possible. Our mechanism is based on a hypothetical repulsion between matter and antimatter, existing at least deep inside the horizon of a black hole. When universe is reduced to a supermassive black hole of a small size, a very strong field of the conjectured force might create (through a Schwinger type mechanism) particle-antiparticle pairs from the quantum vacuum. The amount of antimatter created from the vacuum is equal to the decrease of mass of the black hole and violently repelled from it. When the size of the black hole is sufficiently small, the creation of antimatter may become so fast, that matter of our Universe might be transformed to antimatter in a fraction of second. Such a fast conversion of matter into antimatter may look as a Big Bang. Our mechanism prevents a singularity; a new cycle might start with an initial size more than 30 orders of magnitude greater than the Planck length, suggesting that there is no need for inflationary scenario in Cosmology. In addition, there is no need to invoke CP violation for explanation of matter-antimatter asymmetry. Simply, our present day Universe is dominated by matter, because the previous universe was dominated by antimatter.     

A Bianchi — V universe with radiation

Bianchi models are spatially homogeneous, anisotropic cosmological models which can be used to describe the early stage of the universe. In this paper, we present a solution of the Einstein field equation in a Bianchi-V type universe with incoherent radiation. It is shown that, with the lapse of time, the model approaches the state of isotropic expansion. The initial singularity of the model is discussed.     

Emergent universe in Hor?ava gravity

Recently Hor̆ava has proposed a non-relativistic renormalisable gravity theory with higher spatial derivatives in four dimensions which reduces to Einstein’s gravity at large distances with a non-vanishing cosmological constant but with improved UV behaviour. In this paper, we have considered the Friedman-Lema?tre-Robertson-Walker cosmological model in Hor̆ava gravity and the emergent scenario for all values of the spatial curvature k (=0,±1) has been studied. As a result, there are constraints on the parameters involved.     

Evidence for a low-density inflationary universe?

The inflationary unvierse model predicts the density parameter ₀ to be 1.0 with the cosmological constant ₀ usually taken to be zero, whereas observational estimates give ₀0.2 and ₀10^-57 cm^–2. It was found, however, that the observed variation of angular diameter with redshift for extragalactic radio sources could be interpreted in terms of a low density universe with linear size evolution of the sources for either an inflationary model with 0 or an open model with =0.     

A model of the late universe with viscous Zel’ldovich fluid and decaying vacuum

Many have speculated about the presence of a stiff fluid in very early stage of the universe. Such a stiff fluid was first introduced by Zel’dovich. Recently the late acceleration of the universe was studied by taking bulk viscous stiff fluid as the dominant cosmic component, but the age predicted by such a model is less than the observed value. We consider a flat universe with viscous stiff fluid and decaying vacuum energy as the cosmic components and found that the model predicts a reasonable background evolution of the universe with de Sitter epoch as end phase of expansion. More over, the model also predicts a reasonable value for the age of the present universe. We also performed a dynamical system analysis of the model and found that the end de Sitter phase predicted by the model is stable.     

Did the universe start with the Higgs-field?

In a cosmological model developed by the author in previous articles the universe starts in a geometrical phase transition in Minkowski space. Here the source of the gravitational field is a Higgs-like scalar field $\bar{\phi}$ . A relation of this cosmological field $\bar{\phi}$ with the Higgs-field ? _H in the gauge theory of electroweak interaction is established. This relation leads to two dimensionless constants. One of them is interpreted as a characteristic constant of the phase transition and is connected with the volume of huge bubbles of open universes.     

Multi-product splitting and Runge-Kutta-Nyström integrators

The splitting of e^h(A+B) into a single product of e ^{h A} and e ^hB results in symplectic integrators when A and B are classical Lie operators. However, at high orders, a single product splitting, with exponentially growing number of operators, is very difficult to derive. This work shows that, if the splitting is generalized to a sum of products, then a simple choice of the basis product reduces the problem to that of extrapolation, with analytically known coefficients and only quadratically growing number of operators. When a multi-product splitting is applied to classical Hamiltonian systems, the resulting algorithm is no longer symplectic but is of the Runge-Kutta-Nyström (RKN) type. Multi-product splitting, in conjunction with a special force-reduction process, explains why at orders p = 4 and 6, RKN integrators only need p ? 1 force evaluations.     

Anisotropic universe with cosmic strings and bulk viscosity in a scalar–tensor theory of gravitation

A spatially homogeneous and anisotropic Bianchi type-I cosmological model is obtained in a scalar–tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. Some physical and kinematical properties of the model are discussed. It is observed that the bulk viscosity has a greater role in getting an accelerated expansion of the universe in this theory.     

A general relativistic rotating evolutionary universe—Part II

As a sequel to Berman (Astrophys. Space Sci., 2008b), we show that the rotation of the Universe can be dealt by generalised Gaussian metrics, defined in this paper. Robertson-Walker’s metric has been employed with proper-time, in its standard applications; the generalised Gaussian metric implies in the use of a non-constant temporal metric coefficient modifying Robertson-Walker’s standard form. Experimental predictions are made.     

CO Clouds around SNR G21.8-0.6 and G32.8-0.1

We made the first CO(I—0) mapping to SNR G21.8-0.6 and SNR G32.8-0.1, both associated with OH 1720 MHz maser.Based on the morphological correspondence and velocity and position agreement between the radio remnant and the CO clouds,we tentatively identify the clouds that are respectively interacting with the two SNRs.     

Interacting viscous dark energy in a Bianchi type-Ⅲ universe

We study the evolution of the equation of state of viscous dark energy in the scope of Bianchi type III space-time. We consider a case where the dark energy is minimally coupled to the perfect fluid, as well as in direct interaction with it. The viscosity and the interaction between the two fluids are parameterized by constants ζ0and σ, respectively. We have made a detailed investigation of the cosmological implications of this parametrization. To differentiate between different dark energy models,we have performed a geometrical diagnostic by using the statefinder pair {s, r}.     

Accelerated Brans-Dicke universe in higher-derivative induced gravity-like conformally coupling and dominated by Gauss-Bonnet dark energy

We investigate the late-time dynamics of a four-dimensional universe based on the effective action of a Brans-Dicke scalar field in the presence of the matter source term, conformal coupling of the scalar curvature to the scalar field, a dynamical cosmological constant and Gauss-Bonnet higher-order terms in the scalar curvature. Many new interesting features are revealed and discussed in some details.     

A Spectral Atlas of F and G Stars

We present an atlas of a group of bright stars in the range of spectral classes F-G and luminosity classes I-V. The spectra were obtained with spectral resolution R ～ 15 000 within spectral region 4500-6620/A Typical spectra of stars with different metallicity [Fe/H] are included. We also show the digital version of the spectral data in FITS format.     

Anisotropic cosmological model with variable G and Λ

Anisotropic Bianchi-III cosmological model is investigated with variable gravitational and cosmological constants in the framework of Einstein’s general relativity. The shear scalar is considered to be proportional to the expansion scalar. The dynamics of the anisotropic universe with variable G and Λ are discussed. Without assuming any specific forms for Λ and the metric potentials, we have tried to extract the time variation of G and Λ from the anisotropic model. The extracted G and Λ are in conformity with the present day observations. Basing upon the observational limits, the behavior and range of the effective equation of state parameter are discussed.