String Cosmological Models Research Articles

Non-interacting String and Holographic Dark Energy Cosmological Models in f(R) Theory of Gravitation

Non-interacting String and Holographic Dark Energy Cosmological Models in f(R) Theory of Gravitation

(4+1)-D homogeneous anisotropic string cosmological models with dilaton and anti-symmetric matter

(4+1)-D homogeneous anisotropic string cosmological models with dilaton and anti-symmetric matter

Modified [formula omitted]-gravity string cosmological models with observational constraints

In the current study, we have investigated a modified f(Q)-gravity theory in an anisotropic, locally rotationally symmetric (LRS) Bianchi type-I spacetime universe model. The arbitrary function f(Q)=Q+Λ, where Λ is the “cosmological constant,” and Q is the non-metricity scalar, has been taken into consideration. To obtain the exact solution of the modified field equations, we considered a dusty string fluid matter source and the shear scalar (σ) is proportional to the expansion scalar (θ). The resultant Hubble function has been fitted with observational datasets H(z) and SNe Ia datasets of apparent magnitude m(z) in order to obtain the best fit values for the cosmological parameters. And utilizing these best fit values throughout the investigation, the many cosmic phenomena are examined. We have investigated the cosmographic coefficients like H,q,j,s to see if an accelerated dual mode expanding dark energy (DE) model of the universe exists. Our Om diagnostic study shows that our universe model is a quintessence example of a dark energy model. We have also calculated the universe’s present age.

Cloud String Cosmological Models Using Different Decelerating Parameters with Electromagnetic Field in General Theory of Relativity

In this study, we have investigated the Bianchi type-III anisotropic cosmological model in the presence of cloud strings with electro- magnetic field in general theory of relativity. Exact solutions of field equations are obtained using the fact that shear scalar is proportional to scalar expansion and constant decelerating parameter which is derived from variation law of Hubble parameter proposed by Berman[1] and linearly varying decelerating parameter proposed by Akarsu and Dereli [2]. The dynamics and significance of physical parameters of the model are discussed using a graphical representation of these parameters.The dynamics and significance of physical parameters of the models are discussed.

Bianchi type-III bulk-viscous string models in scalar–tensor theory of gravitation

In this paper, we have investigated Bianchi type-III bulk-viscous string cosmological models in the frame-work of scalar–tensor theory of gravitation. To solve the field equations for the bulk-viscous fluid and perfect fluid under the following physically relevant conditions: (i) Expansion scalar ([Formula: see text]) in the model is directly proportional to the shear scalar ([Formula: see text]). (ii) [Formula: see text], where [Formula: see text] is a constant (iii) [Formula: see text], where [Formula: see text] and [Formula: see text] are real constants. The observed phase transition of the presented universe from the early decelerating phase to the present epoch is in good accordance with the astronomical measurements. A detailed discussion on energy conditions and stability of the all models is analyzed through graphical representation.

Plane Symmetric String Cosmological Model with Zero Mass Scalar Field in f (R) Gravity

Plane Symmetric String Cosmological Model with Zero Mass Scalar Field in f (R) Gravity

Perfect Fluid Coupled String Universe in <i>f(R,T)</i> Gravity

In this paper, we have investigated the Bianchi type VI0 space-time in the presence of string of clouds coupled with perfect fluid within the context of f(R,T) gravity. The exact solutions to the field equations have been obtained by considering the expansion scalar Θ is proportional to the shear scalar σ and the exponential form of an average scale factor. Some physical and kinematical parameters of the constructed model have been discussed and presented graphically, and it is interesting to note that the resultant model resembles the recent observational data. Also, we have discussed the energy conditions for a perfect fluid-coupled string cosmological model.

Five-Dimensional Cosmological Model with One Dimensional Cosmic String Coupled with Zero Mass Scalar Field in Lyra Manifold

A five-dimensional Bianchi type-III string cosmological model is studied with a one-dimensional cosmic string in the presence of zero mass scalar field in the context of the Lyra manifold. Exact solutions of Einstein's field equations are obtained by assuming quadratic equation of state (EoS) of the form p = kρ2 + ρ, where k is a constant and strictly k > 0 . The physical and geometrical aspects of the investigated model are analyzed in detail.

Search for stochastic gravitational-wave background from string cosmology with Advanced LIGO and Virgo's O1∼O3 data

String cosmology models predict a relic background of gravitational-wave (GW) radiation in the early universe. The GW energy spectrum of radiated power increases rapidly with the frequency, and therefore it becomes a potential and meaningful observation object for high-frequency GW detector. We focus on the stochastic background generated by superinflation in string theory and search for such signal in the observing data of Advanced LIGO and Virgo O1∼O3 runs in a Bayesian framework. We do not find the existence of the signal, and thus put constraints on the GW energy density. Our results indicate that at f = 100 Hz, the fractional energy density of GW background is less than 1.7 × 1-8 and 2.1 × 10-8 for dilaton-string and dilaton only cases respectively, and further rule out the parameter space restricted by the model itself due to the non-decreasing dilaton and stable cosmology background (β bound).

String cosmological scenario in periodic time-varying deceleration parameter in Lyra geometry

In this paper, we have studied the bulk viscous fluid connected with string cosmological model in a higher dimensional (five dimensions) Bianchi type-III space–time in the framework of Lyra’s manifold. To get the deterministic solution of the field equations, we assume two physically appropriate conditions: ( i) θ in the model parameter that is proportional to the eigen value of [Formula: see text] of the shear tensor [Formula: see text] and ( ii) the periodic time-varying deceleration parameter q specified as q = mcos ( kt) − 1, where m, k > 0. In this paper, we have studied the periodic time-varying behaviour of a few quantities, such as the deceleration parameter q, the energy density ρ, the proper pressure p, the string tension density λ, the total pressure [Formula: see text], the energy density for particle ρp, and the displacement vector β( t), and we have discussed the physical significance of these quantities. It has been found that in the early stages of the universe’s evolution, string prevails over particles, whereas the universe is dominated by massive string at late time. It is also noted that the string phase of the cosmos does not vanish in the model since particle density is constantly positive throughout the universe’s history. In normal gauge treatment, β( t) acts like cosmological constant (Λ), and the solutions are compatible with observations. For the stability analysis, we have investigated the nature of various energy conditions (ECs). The positive behaviour of DEC indicates the model’s validation; on the other hand, SEC and WEC are violating, indicating the universe’s accelerated expansion.

Bulk Viscous String Cosmological Model with Power Law Volumetric Expansion in Teleparallel Gravity

Bulk Viscous String Cosmological Model with Power Law Volumetric Expansion in Teleparallel Gravity

HIGHER DIMENSIONAL TOPOLOGICAL DEFECT SOLUTIONS WITH MASSIVE SCALAR FIELD IN GENERAL RELATIVITY

In this paper, we investigate a ve dimensional Locally Rotationally Symmetric (LRS) Bianchi type- V string cosmological model with massive scalar eld in general relativity. In order to obtain an exact solution of the field equations, we used the following conditions: (i) the shear scalar is proportional to the expansion scalar, resulting in a relationship between metric potentials, and (ii) the average scale factor is proportional to the massive scalar gield, resulting in a power law relationship. In addition, the physical and kinematical parameters are discussed in detail.

Anisotropic cloud string cosmological model with five-dimensional kaluza-klein space-time

Anisotropic cloud string cosmological models has been investigated in the context of five dimensional Kaluza- Klein space time. In this paper the energy momentum tensor is generated by rest energy density and tension density of the string with particle density attached to them. To obtained the exact solutions of the Einstein field equations we assumed a scale factor a(t)=e1β2βt+c where β and c are positive constant, which yields a variable deceleration parameter (DP) q=−aäȧ2=βH+α. The physical and geometrical behavior of the models is also discussed in detail.

Magnetized Bianchi Type-VI0 String Cosmological Model for Anti-Stiff Fluid in General Relativity

In this paper, we have investigated magnetized Bianchi type-VI0 string cosmological model for anti-stiff fluid in general relativity. We assume that F23 is the only survival component of electromagnetic field tensor Fij. For the absolute determination of the model, we assume the expansion in the model is proportional to the shear. The common measures of the Einstein’s field equations for the cosmological model have been acquired under the assumption of anti-stiff fluid i.e. p + ρ = 0, where ρ and p are the rest energy density and the pressure of the fluid, respectively. The physical and geometrical outcomes of the model in the presence of magnetic field are discussed.

Five Dimensional Bianchi Type-I Anisotropic Cloud String Cosmological Model With Electromagnetic Field in Saez-Ballester Theory

Within the context of Saez-Ballester theory, we explored the interaction of a five-dimensional Bianchi type-I anisotropic cloud string cosmological model Universe with an electromagnetic field. With an electromagnetic field, the energy momentum tensor is assumed to be the sum of the rest energy density and string tension density in this paper. We use the average scale factor as an integrating function of time to get exact answers to Saez-Ballester equations. The dynamics and importance of the model’s many physical parameters are also examined.

SOME BIANCHI TYPE III STRING COSMOLOGICAL MODELS FOR PERFECT FLUID DISTRIBUTION IN GENERAL RELATIVITY WITH AN ALTERNATE APPROACH

Some Bianchi type III string cosmological models for perfect fluid distribution with an alternate approach are discussed, assuming a condition between energy density and string tension density. We have also assumed a relation between metric potentials to get deterministic solution. The physical and geometrical aspects of the models are also discussed

Anisotropic minimally interacting dark energy models with cosmic strings and a massive scalar field

This paper deals with the construction of locally rotationally symmetric (LRS) Bianchi type-II (B-II) cosmological models obtained by solving Einstein field equations coupled with an attractive massive scalar field (MSF) when the source of gravitation is the mixture of cosmic string cloud and anisotropic dark energy (DE) fluid which are minimally interacting. We have obtained exact cosmological models by using (i) shear scalar is proportional to the scalar expansion of the space–time and (ii) a power-law relation between the average scale factor of the universe and the scalar field. Our models represent string cosmological model and DE model in the presence of MSF. Using our model, we determine cosmological parameters such as energy densities, deceleration parameter, statefinders and equation of state parameter. We, also, present the tension density and energy density of the string. We discuss the physical aspects of these cosmological parameters. It is observed that our models represent accelerated expansion phenomenon of our universe as confirmed by Supernova Ia experiment.

Higher Dimensional Bianchi Type-I Cosmological Models With Massive String in General Relativity

Here we studied Bianchi type-I cosmological models with massive strings in general relativity in five dimensional space time. Out of the two different cases obtained here, one case leads to a five dimensional Bianchi type-I string cosmological model in general relativity while the other yields the vacuum Universe in general relativity in five dimensional space time. The physical and geometrical properties of the model Universe are studied and compared with the present day’s observational findings. It is observed that our model is anisotropic, expanding, shearing, and decelerates at an early stage and then accelerates at a later time. The model expands along x, y, and z axes and the extra dimension contracts and becomes unobservable at t → ∞. We also observed that the sum of the energy density (ρ) and the string tension density (λ) vanishes (ρ + λ = 0).

Higher dimensional Bianchi type-I string cosmological model in f(R) theory of gravity

In this paper, we have studied Bianchi type-I string cosmological model by combining Kaluza-Klein (KK) theory and f(R) theory of gravity which is an extension of 5-dimensional KK string cosmological models. We have used equation of state in the form of p-string or Takabayasi string given by ρ=(1+ω)λ, where ρ and λ denote the rest density of energy cloud of strings, and the tension density of the system of strings, respectively and ω is a constant. In order to get physically significant and viable solution various forms of the function f(R) are assumed, in this paper we assume f(R)=R+αR2 (e.g. Astashenok et al. (2017) [1]), where α is real number. Some physical and geometrical properties of the model are also 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.