Anisotropic Bianchi type-I models with constant deceleration parameter in general relativity

A special law of variation for Hubble’s parameter is presented in a spatially homogeneous and anisotropic Bianchi type-I space-time that yields a constant value of deceleration parameter. Using the law of variation for Hubble’s parameter, exact solutions of Einstein’s field equations are obtained for Bianchi-I space-time filled with perfect fluid in two different cases where the universe exhibits power-law and exponential expansion. It is found that the solutions are consistent with the recent observations of type Ia supernovae. A detailed study of physical and kinematical properties of the models is carried out.     

Bianchi type-I models in self-creation cosmology with constant deceleration parameter

We have studied the evolution of homogeneous and anisotropic Bianchi type-I cosmological models filled with perfect fluid in Barber second self-creation theory by assuming a special law of variation for Hubble’s parameter that yield a constant value of deceleration parameter. Some physical consequences of the models have been discussed in case of Zel’dovich fluid and radiation dominated fluid.     

Bianchi type-V cosmological model with perfect fluid using negative constant deceleration parameter in a scalar tensor theory based on Lyra Manifold

An exact Bianchi type-V perfect fluid cosmological model is obtained in a scalar tensor theory proposed by Sen (Z. Phys. 149:311, 1957) based on Lyra Manifold in case of β is a constant and it is shown that this cosmological model exists only in the case of Radiation Universe (ρ=3p) if β is a function of ‘t’ using negative constant deceleration parameter. Some physical and geometrical properties of these models are discussed.     

Two-fluid cosmological models in a Bianchi type I space-times

In this paper we present anisotropic, homogeneous two-fluid cosmological models in a Bianchi I space-time. These classes of cosmological models picture two different scenarios of cosmic history; viz., when the radiation and matter content of the universe are in interactive phase and another when the two are non-interacting. The universe is highly anisotropic in the initial stages, however, anisotropy tapers out to insignificance in due course of cosmic evolution. In every model the anisotropy of the space-time is determined by the density parameter Ω₀ at the present epoch. For Ω₀=1, the anisotropy is washed out before long. An interesting class of models, having an inflationary epoch in finite future, is discovered.      

Plane symmetric cosmological models with negative constant deceleration parameter in self creation theory

In this paper, we have investigated plane symmetric cosmological models with negative constant deceleration parameter in Barber’s (Gen. Relativ. Gravit. 14:117, 1982) second self-creation theory in presence of perfect fluid source. For this we use a special law of variation for Hubble parameter proposed by Bermann (Nuovo Cim. B 74:182, 1983) that yields a constant deceleration parameter model of the universe. Some physical properties of the models and entropy are discussed and studied.     

LRS Bianchi-V cosmology with decaying vacuum density and heat flow

In this paper we consider a locally-rotationally-symmetric (LRS) Bianchi type-V perfect fluid model with variable cosmological ‘constant’ representing the energy density of vacuum. The field equations are solved with and without heat conduction by using a variation law for the mean Hubble parameter, which is related to the average scale factor of the metric and yields a constant value of the deceleration parameter. A constant value of deceleration parameter generates power-law form of average scale factor which is used to find the exact solutions with and without heat conduction with decaying vacuum density. The solutions presented here satisfy all the necessary conditions for the physically acceptability. The thermodynamical relations in decaying vacuum fluid model are also studied in detail.     

Bianchi Type I string dust cosmological model with magnetic field in general relativity

Bianchi Type I string dust cosmological models in presence and absence of magnetic field following the techniques used by Letelier and Stachel, are investigated. To get the deterministic solution, we have assumed that σ ₁¹ is proportional to the expansion (θ) where σ ₁¹ is the eigen value of shear tensor (σ _i ^j ) and which leads to A=N(BC)⁻ⁿ , n>0 where A,B,C are metric potentials and , N and ℓ are constants. The behaviour of the models in presence and absence of magnetic field are discussed. The other physical and geometrical aspects of the model are also discussed.     

Posterior probability of the deceleration parameter q0 from quasars provided with a luminosity indicator

We have worked out a ’statistical algorithm’ for obtaining the posterior probability density of the deceleration parameter q₀ from quasars where there is a luminosity indicator available. We point out that the role of the luminosity indicator is to provide asecond estimate of individual luminosities after a first estimate has been obtained from measured brightness and redshift together with an assumed q₀. Discrimination of q₀ is to be sought in the statistical properties of the set of differences between the two estimates (the residuals). We show that the variance of the residuals and their correlation with redshifts (further refined to luminosity distances) are two independent test-statistics for q₀, whose known distributions then lead to the probability density sought. We have applied the above algorithm to a sample of flat-spectrum radio quasars with measured CIV, MgII and Ly α lines. A combined Baldwin’s relation was used for all 3 lines. Our result is that log q₀ is normally distributed with a mean value of + 0.270± 0.135 (s.d.), or, q₀ = + 1.86 ± 0.135 dex. This result, we believe, is the sharpest result so far published on q₀.     

A new class of bulk viscous universe with time dependent deceleration parameter and Λ-term

A new class of exact solutions of Einstein’s field equations with a bulk viscous fluid for an LRS Bianchi type-Ia obtained by using a time dependent deceleration parameter and cosmological term Λ. The coefficient of bulk viscosity is assumed to be a power function of mass density (ξ=ξ ₀ ρ ⁿ ). We have obtained a general solution of the field equations from which six models of the universe are derived: exponential, polynomial and sinusoidal form respectively. The behaviour of these models of the universe are also discussed in the frame of reference of recent supernovae Ia observations.      

A Cosmological Model with a Negative Constant Deceleration Parameter in Scale-Covariant Theory of Gravitation

An axially symmetric Bianchi type-I space-time is considered in the presence of perfect fluid source in the scale-covariant theory of gravitation formulated by Canuto et al. [1977a, Phys. Rev. Lett. 39, 429]. With the help of special law of variation for Hubble’s parameter proposed by Bermann [1983, Nuovo Cimento 74B, 182] a cosmological model with a negative constant declaration parameter is obtained in this theory. Some physical properties of the model are also discussed.     

Axially symmetric non-static domain walls in scalar–tensor theories of gravitation

An axially symmetric non-static space-time is considered in the presence of thick domain walls in the scalar–tensor theories formulated by Brans and Dicke (Phys. Rev. 124:925, 1961) and Saez and Ballester (Phys. Lett. A 113:467, 1985). Exact cosmological models, in both the theories, are presented with the help of special law of variation proposed by Berman (Nuovo Cim. B 74:182, 1983), for Hubble’s parameter. Some physical and kinematical properties of the models are discussed.      

Nonlinear spinor field in Bianchi type-I Universe filled with viscous fluid: numerical solutions

We consider a system of nonlinear spinor and a Bianchi type I gravitational fields in presence of viscous fluid. The nonlinear term in the spinor field Lagrangian is chosen to be λ F, with λ being a self-coupling constant and F being a function of the invariants I an J constructed from bilinear spinor forms S and P. Self-consistent solutions to the spinor and BI gravitational field equations are obtained in terms of τ, where τ is the volume scale of BI universe. System of equations for τ and ε, where ε is the energy of the viscous fluid, is deduced. This system is solved numerically for some special cases.      

Interacting holographic dark energy with variable deceleration parameter and tachyon scalar field dark energy model in LRS Bianchi type-II universe

In this work, we have considered the spatially homogeneous and anisotropic Bianchi type-II universe filled with two interacting fluids; dark matter and holographic dark energy components. Assuming the proportionality relation between one of the components of shear scalar and expansion scalar which yields time dependent deceleration parameter, an exact solution to Einstein’s field equations in Bianchi type-II line element is obtained. We have investigated geometric and kinematics properties of the model and the behaviour of the holographic dark energy. It is observed that the mean anisotropic parameter is uniform through the whole evolution of the universe and the coincidence parameter increases with increasing time. The solutions are also found to be in good agreement with the results of recent observations. We have applied the statefinder diagnostics method to study the behaviour of different stages of the universe and to differentiate the proposed dark energy model from the ΛCDM model. We have also established a correspondence between the holographic dark energy model and the tachyon scalar field dark energy model. We have reconstructed the potential and the dynamics of the tachyon scalar field, which describes accelerated expansion of the universe.     

Tilted Bianchi type I cosmological models filled with disordered radiation in general relativity revisitedD

Tilted Bianchi type I cosmological models filled with disordered radiation in presence of a bulk viscous fluid and heat flow are investigated. The coefficient of bulk viscosity is assumed to be a power function of mass density. Some physical and geometric properties of the models are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Zero mass scalar field with bulk viscous cosmological solutions in Lyra geometry

In this paper, we have investigated spatially homogeneous isotropic Friedmann cosmological model with bulk viscosity and zero-mass scalar field in Lyra manifold. The cosmological models are obtained with the help of the special law of variation for Hubble’s parameter proposed by Bermann (Nuovo Cimento 74B:182, 1983) and power law relation. Some physical properties of the models are discussed.     

Modelling of seeing effects in extragalactic astronomy and cosmology

Convolution methods for modelling of astronomical seeing effects have been investigated. The advantages and disadvantages of several techniques are discussed, and particular attention is given to the fast Fourier transform (FFT) method. This method is then applied to two classes of problems, the structure of cores of elliptical galaxies, appearance of distant galaxies and the consequences of seeing effects in some cosmological tests. Estimates are presented for dimming of the central surface brightness and changes in the apparent core radius for elliptical galaxies, as well as seeing-induced changes in ellipticity. Modelling of galaxies with stellar nuclei has also been performed. Some consequences of these effects in investigations of dynamics of elliptical galaxies are addressed briefly. The influence of seeing in observational cosmology is discussed in the context of Hubble diagram (m-z) tests. It is shown that inadequate compensation for seeing effects can seriously distort the conclusions in such tests. Some suggestions for future work in this direction are offered.     

Shear and vorticity in inflationary Brans–Dicke cosmology with lambda-term

We find a solution for exponential inflation in Brans–Dicke cosmology endowed with a cosmological term, which includes time-varying shear and vorticity. We find that the scalar field and the scale factor increase exponentialy while shear, vorticity, energy density, cosmic pressure and the cosmological term decay exponentialy for negative beta, where beta is defined in the text.     

Cosmological models with viscous fluid and time- dependent equation of state in Wesson’s theory

Assuming the time-dependent equation of state p=λ(t)ρ, five dimensional cosmological models with viscous fluid for an open universe (k=−1) and flat universe (k=0) are presented. Exact solutions in the context of the rest mass varying theory of gravity proposed by Wesson (Astron. Astrophys. 119, 145, 1983) are obtained. It is found that the phenomenon of isotropisation takes place in this theory, i.e. the mass scale factor A(t) which characterizes the rest mass of a typical particle is evolving with cosmic time just as the spatial scale factor R(t). It is further found that rest mass is approximately constant in the present universe.     

Five-dimensional cosmological model with a time-dependent equation of state in Wesson’s theory

Exact solution for a homogeneous cosmological model in 5D space-time-mass gravity theory proposed by Wesson (Astron. Astrophys. 119:145, 1983) is obtained by assuming the time-dependent equation of state. The behavior of the solution is discussed for the two cases k<0 and k=0. It is found that the observed constancy of the rest mass of an isolated particle in the present era may be interpreted as a consequence of the decreasing rate of change of rest mass with time. Moreover, a spontaneous compactification-like phenomenon of an extra dimension takes place in the case of k=0. It is also found that with decrease in extra space the observable three-dimensional space entropy increases, thus accounting for the large value of entropy observable at present.