Search Results (6)

This paper discusses the nonlinear stability of a thin-shell wormhole from a regular black hole in Bardeen–de Sitter spacetime in the $f\left(R\right)$ gravity framework. The stability is examined under the linear perturbation about static solution and a nonlinear variable equation of state, such as the modified generalized Chaplygin gas. The stability solutions for a suitable choice of different parameters included in the variable equation of state and $f\left(R\right)$ gravity models, as well as the metric space–time, are illustrated. Full article

This work is focused on studying the cosmology of variable modified Chaplygin gas (VMCG) in the framework of exponential and logarithmic $f\left(T\right)$ theory. The equation of state (EoS) for VMCG in exponential and logarithmic $f\left(T\right)$ gravity shows quintom behavior. Primordial perturbations were studied for VMCG in both exponential and logarithmic $f\left(T\right)$ gravity, and it was observed that the potential increases with cosmic time t, and the scalar field decreases toward the minimum value of the potential. The squared speed of sound was positive, meaning that VMCG in both exponential and logarithmic $f\left(T\right)$ gravity shows stability against small gravitational perturbations. Full article

In this work, we perform a comparative study of the Kantowski–Sachs (KS) and Bianchi-I anisotropic universes with Modified Chaplygin gas (MCG) as matter source. We obtain the volume and scale factors as solutions to the Einstein Field Equations (EFEs) for the anisotropic universes, and check whether the initial anisotropy is washed out or not for different values of the MCG parameters present in the solution by obtaining the anisotropy parameters for each solution. The deceleration parameter is also obtained for each solution, the significance of which is discussed in the concluding section. Interestingly there are a number of notable results that appear from our study which help us to compare and contrast the two different anisotropic models along with proper understanding of the role of MCG as matter source in these models. Full article

In this work, we report a study on bouncing cosmology with modified generalized Chaplygin Gas (mgCG) in a bulk viscosity framework. Reconstruction schemes were demonstrated in Einstein and modified $f\left(T\right)$ gravity framework under the purview of viscous cosmological settings. We also took non-viscous cases into account. We studied the equation of state (EoS) parameter under various circumstances and judged the stability of the models through the sign of the squared speed of sound. We observed the mgCG behaving like avoidance of Big Rip in the presence of bulk viscosity at the turnaround point and in non-viscous cases, a phantom-like behavior appears. The turnaround point equation of state parameter crosses the phantom boundary, violating NEC. The role of the mgCG’s model parameters was also investigated before and after the bounce. A Hubble flow dynamics was carried out and, it was revealed that mgCG is capable of realizing an inflationary phase as well as an exit from inflation. An $f\left(T\right)$ gravitational paradigm was also considered, where the mgCG density was reconstructed in the presence of bulk viscosity. The role of the parameters associated with the bouncing scale factor, describing how fast the bounce takes place, was also studied in this framework. Finally, the reconstructed mgCG turned out to be stable against small perturbations irrespective of the presence of bulk viscosity and modified gravity scenario. Finally, the reconstruction scheme was assessed using statistical analysis, Shannon entropy. Full article

In this paper, we study the inflationary scenario in the realm of k-essence model and swampland conjectures. Taking into account three models of Chaplygin gas, such as generalized, modified, and generalized cosmic Chaplygin gas models, we discuss the equation of state (EoS) parameter $\omega$, slow roll parameters $(ϵ(\varphi ),\eta (\varphi ))$, curvature perturbation $(P_s)$, tensor to scalar ratio $(r)$, and scalar spectral index $(n_s)$. As regards the k-essence model, the coupling function as a function of scalar field $L(\varphi )$ is used. We investigate the swampland conjecture and then find the value of $\zeta (\varphi )$, i.e., bound of second conjecture for these three models by unifying swampland conjecture and k-essence. We plot the EoS parameter $\omega$, inflationary parameters plane $r-n_s$ and bound of swampland conjecture $\zeta (\varphi )-\varphi$, which determine that the values of $\omega <-1$ for each model, r, are $r<0.0094,r\le 0.0065,r\le 0.0067$, and ranges for $n_s$ are $[0.934,0.999],[0.9,0.999],[0.9,0.992]$ for generalized, modified, and generalized cosmic Chaplygin gas models, respectively, and compare their compatibility with the Planck data from 2018. Furthermore, we determine the bound for swampland conjecture as $\zeta (\varphi )\le 0.992,\zeta (\varphi )\le 0.964,\zeta (\varphi )\le 0.964$ for generalized, modified and generalized cosmic Chaplygin gas models, respectively. Full article

A cosmological model with an exotic fluid is investigated. We show that the equation of state of this “modified Chaplygin” gas can describe the current accelerated expansion of the universe. We then reexpress it as FRW cosmological model containing a scalar field $\varphi$ and find its self-interacting potential. Moreover motivated by recent works on tachyon field theory, a map for this exotic fluid as a normal scalar field $\varphi$ with Lagrangian ${\mathcal{L}}_{\varphi }=\frac{{\dot{\varphi }}^2}{2}-U\left(\varphi \right)$ to the tachyonic field T with Lagrangian ${\mathcal{L}}_T=-V\left(T\right)\sqrt{1-{\dot{T}}^2}$ is obtained. Full article