String Cosmological Models Research Articles

Higher Dimensional Bianchi Type-I String Cosmological Model in f(R) Theory of Gravity

The field equations of Kaluza-Klein (KK) theory have been used in the area of cosmology. The researchers have scrutinized KK theory in the existence of cosmic string as a source in f(R) theory of gravity which is an extension of 5-dimensional KK string cosmological models. Certain features, physical and geometrical properties of the model are also investigated.

String cosmological model in bianchi type IX inflationary universe with flat potential

String cosmology has been investigated in a spatially symmetric Bianchi-IX line element under consideration of flat potential. To illustrate the nonlinear system of field equations an appropriate relation between the metric coefficient and the hybrid expansion law (HEL) for the scalar factor a (t) is considered i.e. a(t) = a 0 t γ 1 e γ 2 t where γ1 and γ2 are the non-negative constants and a 0 denotes current value of a(t). It has been observed that the energy density and particle density diverge initially and become finite for large t. The tension in string clouds tends to infinite when t = 0 and constant value at a large value of t for m>1. The proper volume is also increases with time in exponentially way favorable to inflationary criteria. Expansion and shear infinite large initially and approach to finite value at late time. Some structural aspects of the model and their importance are pointed.

Behaviour of viscous fluid in string cosmological models in the framework of Lyra geometry

In this communication, we studied the aspects of bulk viscous fluid cosmological model with quadratic equation of state in the presence of strings loaded with particles in a higher dimensional (5- dimensional) Bianchi type-III geometry in Lyra’s Manifold (Lyra, 1951). Using physically plausible circumstances, an exact model of the universe is presented by obtaining the solutions of the Einstein’s field equations. Important geometrical and dynamical parameters of the model universes are premeditated and physical significance regarding their prospect in modern cosmology are discussed in details. Interestingly it is seen that both bulk viscosity and quadratic equation of state are acting crucial jobs throughout the evolution of the model which is expanding with acceleration so it represents dark energy model universe. Hence our model can be thought as a realistic universe.

Higher Dimensional LRS Bianchi Type-I String Cosmological Model with Bulk Viscosity in General Relativity

Objective: To present a new solution to the field equations obtained for higher dimensional LRS Bianchi type-I universe generated by means of a cloud of strings with particles connected to them with bulk viscosity in general relativity. Methods: To obtain the solutions of field equations of higher dimensional LRS Bianchi type-I universe we consider that the shear scalar of the model is proportional to the scalar expansion of the model (saq ), which leads to, c=b^n. The physical and geometrical behaviors of the model universe are studied by comparing with the present cosmological scenario and observations. Findings: It is observed that our model is anisotropic, expanding and decelerates at early stage and then accelerates in late universe giving the inflation model universe. Novelty: We obtained new solution to the field equations for higher dimensional LRS Bianchi type-I generated by means of a cloud of strings with bulk viscous fluid in general relativity. Keywords: LRS Bianchi TypeI Metric; Bulk Viscous Fluid; Strings

Dynamics of Bulk Viscous String Cosmological Model in f(R,G) Theory of Gravity

Existing investigations dedicated to the self-propelling investigation of dynamics of bulk viscous string in LRS Bianchi type-I cosmological model within the circumstance of alternative theory of gravity with Langrangian be the impulsive perform of Ricci scalar R and Gouss-Bonnet invariant G , say f(R,G) gravity. Exact solutions of the field equations correspond to special law of variation which provides singular model. Also some physical and kinematical aspects and its behavior with the present day universe of the cosmological model have been discussed.

Five dimensional Bianchi type-I string cosmological model with electromagnetic field

In this paper we defined the five dimensional Bianchi type-I string cosmological models with electromagnetic field. Here we considered that the energy momentum tensor is equal to the sum of the energy density, tension density and electromagnetic field. In order to get determinate solutions of the Einstein's field equations, we assume the average scale factor as a connecting function of time. Some physical and kinematical parameters are also discussed.

Viscous string anisotropic cosmological model in scalar–tensor theory

We investigate the spatially homogeneous and anisotropic Kantowski–Sachs space time in the presence of viscous string and new holographic dark energy cosmological model in Saez–Ballester scalar tensor theory. It is found that the density ( $$\rho $$ ) of new holographic dark energy is positive and pressure (p) of the new holographic dark energy is negative. The EoS parameter ( $$\omega $$ ) shows phantom behavior. The physical and geometrical aspects of the statefinder parameters (r, s) and $$ \omega $$ – $$\omega ^\prime $$ plane are also discussed.

Interaction of Bianchi type-I anisotropic cloud string cosmological model universe with electromagnetic field

Here, we have investigated the interaction of Bianchi type-I anisotropic cloud string cosmological model universe with electromagnetic field in the context of general relativity. In this paper, the energy-momentum tensor is assumed to be the sum of the rest energy density and string tension density with an electromagnetic field. To obtain exact solution of Einstein’s field equations, we take the average scale factor as an integrating function of time. Also, the dynamics and significance of various physical parameters of model are discussed.

Decelerating to accelerating scenario for Bianchi type-II string Universe in f(R,T)-gravity theory

In this paper, we have discussed string cosmological model within the framework of [Formula: see text] theory of gravity in homogeneous but anisotropic Bianchi Type-II space-time. We have considered cosmic string as a source of energy–momentum tensor. We get the solution of the corresponding field equations by assuming deceleration parameter [Formula: see text], which is time-dependent (here, [Formula: see text] and [Formula: see text] are arbitrary constants). This particular form of scale factor enables us to explain the two scenarios, (i) By using recent constraints ([Formula: see text], [Formula: see text]) from supernovae type Ia union data (Cunha, Kinematic constraints to the transition redshift from supernovae type Ia union data, Phys. Rev. D 79 (2009) 047301), we find the values of arbitrary constants [Formula: see text] and [Formula: see text] for which we have derived a cosmological model showing accelerating expansion universe ([Formula: see text]) only throughout the evolution and (ii) By using the recent constraints ([Formula: see text], and [Formula: see text]) from SNIa data in combination with BAO and CMB observations (Giostri et al. From cosmic deceleration to acceleration: new constraints from SN Ia and BAO/CMB, J. Cosmol. Astrophys. 3 (2012) 27), we find the values of arbitrary constants for which we have derived a cosmological model with phase transition from early decelerating ([Formula: see text]) to the present accelerating ([Formula: see text]) universe. Also, for the model, we have evaluated and discussed the various physical and kinematic parameters. We have also shown the variation of these cosmological parameters graphically for specific values of the constants. The stability and physical viability are also discussed for the derived models using some recently developed diagnostic tools.

Bianchi type-V string cosmological model with a massive scalar field

In this paper we are interested to investigate spatially homogeneous and anisotropic Bianchi type-V cosmological model in the presence of cosmic string source coupled with an attractive massive scalar meson field. We have obtained a deterministic cosmological model using some relevant physical and mathematical conditions. Also we have determined cosmological parameters like average Hubble parameter, shear scalar, scalar expansion, string tension density and the energy density of the string. We have also obtained the massive scalar field in the model. A detailed discussion of the above physical quantities is, also, presented

Bulk viscous string cosmological models in Saez-Ballester theory of gravity

In this communication, a new class of bulk viscous string cosmological models has been constructed in Saez-Ballester theory of gravitation. To obtain the deterministic solution of the field equations, we have considered deceleration parameter as a bilinear function of cosmic time $t$ for model I and special parametrization of Hubble parameter for model II. The presented class of the cosmological models indicate phase conversion from early decelerated expansion phase to present accelerated expansion phase. To discuss the dynamicity of the universe, the behaviour of various physical parameters has also been studied and presented graphically. For stability analysis, the nature of various energy conditions is investigated. Statefinder pair analysis is used to discriminate the constructed models with other dark energy models and it is noticed that the proposed models are in good agreement with recent observational data.

Non-Static Plane Symmetric Dark Energy with String Cosmological Model in Self Creation Theory

In this paper we have investigated the non-static plane symmetric model in the presence of dark energy and one dimensional cosmic string in the frame work of Barber’s second self creation theory by using hybrid scale factor. We have discussed some physical and geometrical properties of the obtained model such as spatial volume(V ), deceleration parameter (q), expansion scalar(θ), Hubble parameter (H), shear scalar (σ), anisotropic parameter (Ah), EoS parameter (ωde) and the state finder parameters (r, s). Here we have observed that our model is compatible with the present day cosmological observations.

Some Bianchi Type Bulk Viscous String Cosmological Models in f(R) Gravity

In this paper, we investigate bianchi type II, V III and IX Bulk viscous string cosmological models in the context of f(R) gravity. Here, we obtained the solutions of the field equations in the presence of cosmic strings under some specific possible physical conditions. Some physical and geometrical features of the models are also discussed.

Kantowski-Sachs bulk viscous string cosmological model in f(R) theory of gravity

The present work deals with a spatially homogeneous and anisotropic Kantowski-Sachs space-time filled with bulk viscous fluid, containing one-dimensional cosmic strings. To obtain a deterministic solution of the field equations, we used the assumption of shear scalar is proportional to expansion scalar of the model. We have investigated geometric and kinematic properties of our bulk viscous string model and discussed the role of bulk viscosity in the evolution of Kantowski-Sachs universe within the framework of f(R) modified theory gravity. A detailed physical discussion of the dynamical parameters is presented through a graphical representation.

Magnetized LRS Bianchi Type-I Massive String Cosmological Model for Perfect Fluid Distribution with Cosmological Term and923

Magnetized LRS Bianchi Type-I Massive String Cosmological Model for Perfect Fluid Distribution with Cosmological Term and923

Exact conformal field theories from mutually T-dualizable σ -models

Exact conformal field theories (CFTs) are obtained by using the approach of Poisson-Lie (PL) T-duality in the presence of spectators. We explicitly construct some non-Abelian T-dual $\sigma$-models (here as the PL T-duality on a semi-Abelian double) on $2+2$-dimensional target manifolds $M \approx O \times \bf G$ and ${\tilde M} \approx O \times {\bf {\tilde G}}$, where $\bf G$ and ${\bf {\tilde G}}$ as two-dimensional real non-Abelian and Abelian Lie groups act freely on $M$ and $\tilde M$, respectively, while $O$ is the orbit of $\bf G$ in $M$. The findings of our study show that the original models are equivalent to Wess-Zumino-Witten (WZW) models based on the Heisenberg $(H_4)$ and $GL(2,\mathbb{R})$ Lie groups. In this way, some new T-dual backgrounds for these WZW models are obtained. For one of the duals of the $H_4$ WZW model, we show that the model is self-dual. In the case of the $GL(2,\mathbb{R})$ WZW model it is observed that the duality transformation changes the asymptotic behavior of solutions from $AdS_{3} \times \mathbb{R}$ to flat space. Then, the structure and asymptotic nature of the dual spacetime of this model including the horizon and singularity are determined. We furthermore get the non-critical Bianchi type III string cosmological model with a non-vanishing field strength from T-dualizable $\sigma$-models and show that this model describes an exact CFT (equivalent to the $GL(2,\mathbb{R})$ WZW model). After that, the conformal invariance of T-dual models up to two-loop order (first order in $\alpha'$) is discussed.

Transit dark energy string cosmological models with perfect fluid in F(R,T)-gravity

In this paper, spatially homogeneous and anisotropic Bianchi type-[Formula: see text] dark energy (DE) cosmological transit models with string fluid source in [Formula: see text] gravity [T. Harko et al., Phys. Rev. D 84 (2011) 024020], where [Formula: see text] is the Ricci scalar and [Formula: see text] the trace of the stress energy–momentum tensor, have been studied in the context of early time decelerating and late-time accelerating expansion of the Universe as suggested by the recent observations. The exact solutions of the field equations are obtained first by using generalized hybrid expansion law (HEL) [Formula: see text] which yields a time-dependent deceleration parameter [Formula: see text] and second by considering the metric coefficient [Formula: see text]. By using recent constraints from supernovae type-Ia union data [Cunha, arXiv:0811.2379[astro-ph]], we obtain [Formula: see text] and [Formula: see text] for transit model [Formula: see text]. The Universe has an initial singularity and is anisotropic closed and it tends to be flat at the late time, i.e. our Universe is in accelerating expansion. Our model shows a phase transition property from decelerating to accelerating. It is remarkable to mention here that our Universe is homogeneous and anisotropic in the early phase whereas it becomes homogeneous and isotropic for [Formula: see text]. We have also discussed the stability of the background solution with respect to perturbations of the metric along with the properties of future singularities in the Universe dominated by DE including the phantom-type fluid. Various physical and dynamical parameters are also calculated and investigated in terms of time and redshift both.

Locally rotationally symmetric Bianchi type-I string cosmological models in f(R) theory of gravity

This study deals with spatially homogeneous and anisotropic locally rotationally symmetric (LRS) Bianchi type-I universe with cosmic string source in the framework of [Formula: see text] theory of gravity [S. Capozziello, S. Carloni and A. Troisi, Quintessence without scalar fields, Recent Res. Dev. Astron. Astrophys. 1 (2003), 625; S. Nojiri and S. D. Odintsov, Modified gravity with negative and positive powers of curvature: Unification of inflation and cosmic acceleration, Phys. Rev. D 68 (2003) 123512]. Solving the field equations using (i) relation between metric potentials, (ii) power law relation between [Formula: see text] and average scale factor [Formula: see text] and (iii) equations of state for string models we have presented Takabayasi [T. Takabayasi, Quantum Mechanics Determinism, Causality, and Particles (Springer, Berlin, 1976)], Nambu [P. S. Letelier, String cosmologies, Phys. Rev. D 28 (1983) 2414–2419] and Reddy [D. R. K. Reddy, A string cosmological model in a scalar–Tensor theory of gravitation, Astrophys. Space Sci. 286 (2003) 359–363] string cosmological models. The dynamical parameters of our models are determined and their physical behavior is discussed. The most interesting result about the models is that the anisotropic effects are wiped out at late times.

Kaluza–Klein string cosmological model in f(R, T) theory of gravity

In this paper we have studied Kaluza–Klein string cosmological model within the framework of f(R, T) theory of gravity, where R is the Ricci scalar and T is the trace of the stress energy momentum tensor. We have obtained the solution of the corresponding field equations by using a time varying deceleration parameter. We also discussed various physical and dynamical properties of the model. The variation of different cosmological parameters are shown graphically for specific values of the parameters of the model.

Magnetized string cosmological models of accelerated expansion of the Universe in f(R,T) theory of gravity

A class of spatially homogeneous and anisotropic Bianchi-V massive string models have been studied in the modified [Formula: see text]-theory of gravity proposed by Harko et al. [Phys. Rev. D 84:024020, 2011] in the presence of magnetic field. For a specific choice of [Formula: see text], where [Formula: see text] and [Formula: see text]; [Formula: see text], [Formula: see text] being arbitrary parameters, solutions of modified gravity field equations have been generated. To find the deterministic solution of the field equations, we have considered the time varying deceleration parameter which is consistent with observational data of standard cosmology (SNIa, BAO and CMB). As a result to study the transit behavior of Universe, we consider a law of variation for the specifically chosen scale factor, which yields a time-dependent deceleration parameter comprising a class of models that depicts a transition of the Universe from the early decelerated phase to the recent accelerating phase. In this context, for the model of the Universe, the field equations are solved and corresponding cosmological aspects have been discussed. The Energy conditions in this modified gravity theory are also studied. Stability analysis of the solutions through cosmological perturbation is performed and it is concluded that the expanding solution is stable against the perturbation with respect to anisotropic spatial direction. Some physical and geometric properties of the models are also discussed.