No Arabic abstract
In the present work we have searched the existence of the late time acceleration of the universe with string fluid as source of matter in anisotropic Heckmann-Suchking space-time by using 287 high red shift $(0.3 leq zleq 1.4)$ SN Ia data of observed absolute magnitude along with their possible error from Union 2.1 compilation. It is found that the best fit values for $(Omega_{m})_{0}$, $(Omega_{Lambda})_{0}$, $(Omega_{sigma})_{0}$ and $(q)_{0}$ are 0.2820, 0.7177, 0.0002 $&$ -0.5793 respectively. Several physical aspects and geometrical properties of the model are discussed in detail.
In this paper, we have investigated $Lambda$CDM type cosmological model in Heckmann-Schucking space-time, by using 287 high red shift ($ .3 leq z leq 1.4$ ) SN Ia data of observed absolute magnitude along with their possible error from Union 2.1 compilation. We have used $chi^{2}$ test to compare Union 2.1 compilation observed data and corresponding theoretical values of apparent magnitude $(m)$. It is found that the best fit value for $(Omega_{m})_0$, $(Omega_{Lambda})_0$ and $(Omega_{sigma})_0$ are $0.2940$, $0.7058$ and $0.0002$ respectively and the derived model represents the features of accelerating universe which is consistent with recent astrophysical observations.
It was found recently that the anisotropies in the homogeneous Bianchi I cosmology considered within the context of a specific Horndeski theory are damped near the initial singularity instead of being amplified. In this work we extend the analysis of this phenomenon to cover the whole of the Horndeski family. We find that the phenomenon is absent in the K-essence and/or Kinetic Gravity Braiding theories, where the anisotropies grow as one approaches the singularity. The anisotropies are damped at early times only in more general Horndeski models whose Lagrangian includes terms quadratic and cubic in second derivatives of the scalar field. Such theories are often considered as being inconsistent with the observations because they predict a non-constant speed of gravitational waves. However, the predicted value of the speed at present can be close to the speed of light with any required precision, hence the theories actually agree with the present time observations. We consider two different examples of such theories, both characterized by a late self-acceleration and an early inflation driven by the non-minimal coupling. Their anisotropies show a maximum at intermediate times and approach zero at early and late times. The early inflationary stage exhibits an instability with respect to inhomogeneous perturbations, suggesting that the initial state of the universe should be inhomogeneous. However, more general Horndeski models may probably be stable.
In this paper, aniostropic dark energy cosmological models have been constructed in a Bianchi-V space-time with the energy momentum tensor consisting of two non-interacting fluids namely bulk viscous fluid and dark energy fluid. Two different models are constructed based on the power law cosmology and de Sitter universe. The constructed model also embedded with different pressure gradients along different spatial directions. The variable equation of state (EoS) parameter, skewness parameters for both the models are obtained and analyzed. The physical properties of the models obtained with the use of scale factors of power law and de Sitter law are also presented.
Some cosmological solutions of massive strings are obtained in Bianchi I space-time following the techniques used by Letelier and Stachel. A class of solutions corresponds to string cosmology associated with/without a magnetic field and the other class consists of pure massive strings, obeying the Takabayashi equation of state.
We investigate FRW cosmological solutions in the theory of modulus field coupled to gravity through a Gauss-Bonnet term. The explicit analytical forms of nonsingular asymptotics are presented for power-law and exponentially steep modulus coupling functions. We study the influence of modulus field potential on these asymptotical regimes and find some forms of the potential which do not destroy the nonsingular behavior. In particular, we obtain that exponentially steep coupling functions arising from the string theory do not allow nonsingular past asymptotic unless modulus field potential tends to zero for modulus field $psi to pm infty$. Finally, the modification of the chaotic dynamics in the closed FRW universe due to presence of the Gauss-Bonnet term is discussed.