Transition analysis of the non-OT G 2 stiff fluid spike solution

We use the technique developed in Moughal’s doctoral thesis to analyse the joint spike transition, revealing new groups of world-lines which undergo distinct transitions, and correcting misconceptions about spikes.

The Sharma–Mittal Model’s Implications on FRW Universe in Chern–Simons Gravity

The Sharma–Mittal holographic dark energy model is investigated in this paper using the Chern–Simons modified gravity theory. We investigate several cosmic parameters, including the deceleration, equation of state, square of sound speed, and energy density. According to the deceleration parameter, the universe is in an decelerating and expanding phase known as de Sitter expansion. The Sharma–Mittal HDE model supports a deceleration to acceleration transition that is compatible with the observational data. The EoS depicts the universe’s dominance era through a number of components, such as ω=0, 13, 1, which indicate that the universe is influenced by dust, radiation, and stiff fluid, while −1<ω<13, ω=−1, and ω<−1 are conditions for quintessence DE, ΛCDM, and Phantom era dominance. Our findings indicate that the universe is in an accelerated expansion phase, and this is similar to the observational data.

Homogeneous perfect fluid collapse in five-dimensional spherically symmetric spacetime

In this paper, the higher-dimensional collapse of homogeneous isotropic perfect fluid is studied by considering the geometry of five-dimensional spherically symmetric metric. Using equations of state for different fields like dust, radiation and stiff fluid with and without cosmological constant [Formula: see text], the gravitational collapse is studied. The results are compared with the usual four-dimensional study in [A. V. Astashenok, K. Mosani, S. D. Odintsov and G. C. Samanta, Int. J. Geom. Methods Mod. Phys. 16, 1950035 (2019)]. It is found that the collapse rate is faster in five-dimensional spacetime as compared to four-dimensional case supporting the cosmic censorship hypothesis.

Isotropic Gravastar Model in Rastall Gravity

In the present paper, we have introduced a new model of gravastar with an isotropic matter distribution in Rastall gravity by the Mazur–Mottola (2004) mechanism. Mazur–Mottola approach is about the construction of gravastar which is predicted as an alternative to black hole. By following this convention, we define gravastar in the form of three phases. The first one is an interior phase which has negative density; the second part consists of thin shell comprising ultrarelativistic stiff fluid for which we have discussed the length, energy, and entropy. By the graphical analysis of entropy, we have shown that our proposed thin shell gravastar model is potentially stable. The third phase of gravastar is defined by the exterior Schwarzschild geometry. For the interior of gravastar, we have found the analytical solutions free from any singularity and the event horizon in the framework of Rastall gravity.

Modeling Coupled Heat and Mass Transfer in Peristaltic Cylindrical Flow of Robertson-Stiff Fluid

The Robertson–Stiff fluid (RS) is a yield-pseudo-plastic model which has been used to describe the rheological properties of drilling fluids, cement slurries and bentonite suspensions. Experimentally, several rheological data showed that this model provides more consistently accurate descriptions of the rheology of such fluids than other viscoplastic models. This result motivates us to study theoretically the peristaltic transport for this shear-thinning model and for other viscoplastic models in the presence of heat and mass transfer in a cylindrical tube. For long wavelength and low Reynolds number approximations, an analytical solution is obtained. The results showed that the velocity and the temperature decrease with increasing the yield parameter and the power index while they increase with the increase in the occlusion parameter. We also observed an opposite behavior of the concentration versus these physical parameters. Moreover, all these parameters enhance the mechanical efficiency of pumping. In addition, the comparison shows that the velocity, temperature and the absolute value of concentration are greater for the proposed model than those of Herschel–Bulkley, Bingham and Casson models, respectively.

The Chaplygin gas as a model for modified teleparallel gravity?

This paper explores the possibility of treating the exotic Chaplygin-gas (CG) fluid model as some manifestation of an f(T) gravitation. To this end, we use the different cosmological CG equations of state, compare them with the equation of state for the modified teleparallel gravity and reconstruct the corresponding Lagrangian densities. We then explicitly derive the equation of state parameter of the torsion fluid $$w_T$$wT and study its evolution for vacuum-torsion, radiation-torsion, dust-torsion, stiff fluid-torsion and radiation-dust-torsion multi-fluid systems. The obtained Lagrangians have, in general, matter dependence due to the matter-torsion coupling appearing in the energy density and pressure terms of the modified teleparallel gravity theory. For the simplest CG models, however, it is possibly to reconstruct f(T) Lagrangians that depend explicitly on the torsion scalar T only. The preliminary results show that, in addition to providing Chaplygin-gas-like solutions to the modified teleparallel gravitation, which naturally behave like dark matter and dark energy at early and late times respectively, the technique can be used to overcome some of the challenges attributed to the CG cosmological alternative.