Narrow Results

We investigate the thermodynamics of the universe filled with modified Chaplygin gas (MCG) considering the integrability condition of the laws of thermodynamics. In principle, all thermal variables can be expressed as functions of either volume or temperature. A new equation of state for MCG is obtained during thermal process. The asymptotic analysis shows that the thermal evolution of the universe is possible from radiation era to ΛCDM model, i.e. the thermal history of MCG gives a unified picture of dark matter and dark energy and as a result the universe cools down through expansion without any phase transition (no critical point).

Laws of thermodynamics have been examined for the universe filled with a perfect fluid, obeying an adiabatic equation of state p = γρ-A/ρ^α (called modified Chaplygin gas), where γ, A and α are positive constants and ρ and p are energy density and thermodynamic pressure respectively. Using general thermodynamics, the behavior of temperature and the thermodynamic stability has been discussed for modified Chaplygin gas. A scenario is obtained such that the thermal equation of state depends on both temperature and volume and there will be thermodynamic stability during the expansion process so that the fluid cools down through the expansion without any phase transition (or passing through any critical point).

In this paper, we will continue to study the modified Chaplygin gas (MCG) based on Ref. 25. Concretely, we not only discuss both the change rates of the energy densities and the Hubble parameter H(z) as a function of the model parameters, which is compared and consistent with the related data in Ref. 27, but also perform the Om diagnostic in order to geometrically differentiate the MCG model from the ΛCDM, GCG and CG models. Moreover, we plot the evolutionary trajectories of MCG model with different interaction terms in Om planes, and find that the coupling intensity b² plays an important role in the MCG model. Furthermore, we also reconstruct the potential of MCG scalar field as well as the dynamics of the scalar field according to the evolution of the MCG dark energy. It is worth stressing that the results given by us can include the ones without interaction as the special cases.

We study the evolution of viscous modified Chaplygin gas (MCG) interacting with f(R, T) gravity in flat FRW universe, where T is the trace of energy–momentum tensor. The field equations are formulated for a particular model f(R, T) = R + 2$\chi$T and constraints for the conservation of energy–momentum tensor are obtained. We investigate the behavior of total energy density, pressure and equation of state (EoS) parameter for emergent, intermediate as well as logamediate scenarios of the universe with two interacting models. It is found that the EoS parameter lies in the matter-dominated or quintessence era for all the three scenarios while the bulk viscosity enhances the expansion for the intermediate and logamediate scenarios.

In the course of study of the evolution of the universe, it is seen that perhaps the extra energy generated and particles created due to the accelerated expansion of the universe might be absorbed by the dark energy and dark matter which are already existing in this universe. It is found that the energy density of dark energy can be expressed as a function of the energy density of the remaining matter portion of the universe which shows that the different components of the universe are correlated. According to the forms of the different types of interaction occurring between dark energy and the other different contents of the universe it may be possible to utilize the dark energy in different ways as it may take different forms of energy. As an interesting phenomenon, it is also observed that the concept of negative time may exist in this universe, and it may revolutionize some of the original concepts of nature and the physical world.

Considering the modified viscous Chaplygin gas as the form of dark energy contained in the Universe, we study the interaction of dark energy with other components like electromagnetic field comprising the Universe. It is incidentally found that the interaction effect behaves like the electric charge from an electromagnetic field showing the possibility that a part of the dark energy can be utilized in the form of electrical energy. Further, from studying geometrical and physical behaviors of the evolution of this model Universe, it can be rightly said that this derived model behaves as the actual Universe.

In this work, the stability of static solutions of spherical thin shell wormholes is analyzed when a slight perturbation (which preserves the basic symmetry) is applied to them. The modified Chaplygin gas (with α = 1 in the equation of state) has been chosen as a candidate for exotic matter needed around the throat. Different cases for such thin shell wormhole construction have been studied, viz. wormholes constructed from Schwarzschild, Schwarzschild–de Sitter, Schwarzschild–anti-de Sitter and Reissner–Nordström metrics. Depending upon the values of the parameters and some restrictions obeyed by them, static stable solutions are seen to exist in some cases.

The cosmological model of the modified Chaplygin gas (MCG) interacting with cold dark matter is studied. Our attention is focused on the final state of the universe in the model. It turns out that there exists a stable scaling solution, which provides the possibility of alleviating the coincidence problem. In addition, we investigate the effect of the coupling constants c₁ and c₂ on the dynamical evolution of this model from the statefinder viewpoint. It is found that the coupling constants play a significant role during the dynamical evolution of the interacting MCG model. Furthermore, we can distinguish this interacting model from other dark energy models in the s–r plane.

We investigate holographic dark energy (HDE) correspondence of interacting Generalized Chaplygin Gas (GCG) in the framework of compact Kaluza–Klein (KK) cosmology. The evolution of the modified HDE with corresponding equation of state is obtained here. Considering the present value of the density parameter a stable configuration is found which accommodates Dark Energy (DE). We note a connection between DE and Phantom fields. It reveals that the DE might have evolved from a Phantom state in the past.

In this paper, we study interacting modified Chaplygin gas (MCG) which has shear and bulk viscosities. We consider sign-changeable interaction between MCG and matter, then investigate the effects of shear and bulk viscosities on the cosmological parameters such as energy, density, Hubble expansion parameter, scale factor and deceleration parameter.

In the present paper, the role of modified chaplygin gas models in relation with the Bianchi type VI₀ universe is examined. For obtaining complete solution of Einstein field equations, it is assumed that expansion scalar in the model is proportional to shear scalar and equation of state of this modified model is valid from the radiation era to the Lambda cold dark matter (ΛCDM) model. State finder and various physical, geometrical properties have also been discussed.

The present paper reports a study on modified Chaplygin gas (MCG)-based reconstruction scheme for extended holographic Ricci dark energy (EHRDE) in the presence of viscous type dissipative term. The dissipative effect has been described by using Eckart approach. Under the assumption that the universe is filled with MCG–EHRDE under the influence of bulk viscosity we have studied the cosmological dynamics, where the bulk viscosity coefficient has been chosen in a particular time varying form $\xi ={\xi }_0+{\xi }_{1}H+{\xi }_2(Ḣ+H^2)$, where ${\xi }_0,{\xi }_1$ and ${\xi }_2$ are constant coefficients and $H$ is the Hubble parameter. Furthermore, we have reconstructed the potential and dynamics of viscous MCG–EHRDE as scalar field. Thereafter we have studied the statefinder trajectories to discern its departure from $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }$CDM) and finally investigated validity of the generalized second law (GSL) of thermodynamics considering event horizon as the enveloping horizon of the universe.

In this paper, we study anisotropic universes with Modified Chaplygin gas (MCG) in the context of Randall Sundrum-2 (RS2) braneworld model. The cosmological solutions for Kantowski–Sachs (KS) and Bianchi-I universes with MCG are obtained on the RS2 braneworld. The solutions are found to be dependent on MCG parameters but are modified from the GR solutions due to the braneworld correction term arising from high-energy effects. The anisotropy and deceleration parameters are obtained for each solution and the possibility of occurrence of future singularities is considered. Interestingly, we find that one drawback of the relativistic picture can be overcome in this model giving a universe close to the presently observed state.

A modified and generalised Chaplygin gas (MCG, $B\ne 0.0$ and GCG, $B=0.0$) has been separately chosen here as a constituent of the universe. Concept of state finder and Om diagnostics are introduced to track the dark energy in the models. Here, observed Hubble data (OHD) and binned Pantheon data of supernovae are used to determine the best-fit equation-of-state (EoS) parameters of these models and these are compared with the $\mathrm{\Lambda }$CDM model. The best-fit value and expected values of cosmological jerk parameter $j$, snap parameter $s$ are determined, which are close to each other. A plot of $j,s$ with red-shift, with themselves, as well as with deceleration parameter $q$, shows the evolution of the universe and its possible future. Variations of $q$ and EoS parameter $\omega$ with red-shift show acceleration–deceleration phase transition in the recent past. Lastly, the state finder pair $(j,s)$ and Om diagnostic have been utilized to discriminate the models.

In this work, we investigate the cosmological application of modified Chaplygin gas (MCG) interacting with pressureless dark matter (DM) in the $f(T)$ modified gravity framework, where $T$ is the torsion scalar in teleparallelism. The interaction term has been chosen proportional to the MCG density with positive coupling constant. In the Einstein general relativity (GR) framework, the interacting MCG has been found to have equation of state (EoS) parameter behaving like quintessence. However, the $f(T)$ gravity reconstructed via the interacting MCG has been found to have EoS crossing the phantom boundary of $-1$. Thus, one can generate a quintom-like EoS from an interacting MCG model in flat universe in the modified gravity cosmology framework. The reconstructed $f(T)$ model has been found to interpolate between dust and $\mathrm{\Lambda }$CDM. Stability of the reconstructed $f(T)$ has been investigated and it has been observed that the model is stable against gravitational perturbation. Cosmological evolution of primordial perturbations has also been investigated and the self-interacting potential has been found to increase with cosmic time and the squared speed of sound has been found to be non-negative.

The present paper reports a study on the bouncing behavior of the viscous modified Chaplygin gas (MCG) in Einstein as well as modified gravity framework. For a bouncing scale factor proposed by Cai et al., Class. Quantum Grav.28 (2011) 215011, we have studied the cosmology of MCG in presence of bulk viscosity. In Einstein gravity framework, we have studied the equation of state parameter and it has been found to cross $-1/3$ indicating the end of the early accelerated expansion and it has also been observed that for flat FRW universe the presence of bulk viscosity induces the crossing of phantom boundary. Role of the model parameters of the MCG has also been investigated before and after the bounce. A Hubble flow dynamics has been carried out and, it was revealed that MCG is capable of realizing inflationary phase as well as an exit from inflation. A $f(T)$ gravitational paradigm has also been considered, where the MCG density has been reconstructed in presence of bulk viscosity. Role of $\sigma$ of the bouncing scale factor, describing how fast the bounce takes place, has also been studied in this framework.

The present study reports a reconstruction scheme for $f(T)$ gravity considering the scale factor in the power law form. The equation of state parameter has been studied for this reconstructed model along with the deceleration parameter and the statefinder pair $\{r,s\}$. The statefinder trajectory has been found to interpolate between dust and $\mathrm{\Lambda }$CDM phase of the universe. Cosmological evolution of primordial perturbations has been studied through scalar metric fluctuations and finally the reconstructed $f(T)$ model has been tested for its consistency with the generic expansion of the universe.

Motivated by the work of Astashenok et al. [Causal limit of neutron star maximum mass in f(R) gravity in view of GW190814, Phys. Lett. B816 (2021) 136222], the present work aims at studying the behavior of f in f(R) gravity and attempts to study the causal mass–radius relation of a massive neutron star. To initiate the study, the background evolution is taken as of modified Chaplygin gas. We observed the positive behavior of f. The Tolman–Oppenheimer–Volkoff equation has been taken into consideration which states the spherical stellar structure of non-rotating neutron stars. We also observed that the mass–radius relationship is close to the Tolman–Oppenheimer–Volkoff limit. The change of density of neutron stars with respect to the radius clearly manifests the actual variation of density from the core to the surface of a neutron star in the background evolution of modified Chaplygin gas, which can unify the early inflation with the late time acceleration of the universe.

This work reports the effect of thermal radiation on singularities in the presence of holographic Ricci dark energy (HRDE) with generalized IR cut-off. The Eckart approach has been considered to initiate the study. The density of HRDE, which is a just particular cases of general Nojiri–Odintsov (NO) HDE [S. Nojiri and S. D. Odintsov, Unifying phantom inflation with late-time acceleration: Scalar phantom-non-phantom transition model and generalized holographic dark energy, Gen. Relativ. Gravit.  38(8) (2006) 1285–1304], in non-interacting and non-viscous case has been reconstructed and it is found that with the passage of cosmic time t, it is decreasing. The Equation of State (EoS) parameter of HRDE in non-interacting and viscous scenario, the EoS parameter of modified Chaplygin gas (MCG) in a non-interacting and non-viscous scenario and the EoS parameter of MCG in an interacting and non-viscous scenario have been reconstructed and studied. There is a chance of Type I, Type II, Type III, Type IV and $w$-singularities for the conditions we obtained. For the effect of thermal radiation of inhomogeneous viscous fluid in an interacting scenario, there is a scope of avoidance of Big-Rip Singularity.

The Kantowski–Sachs cosmological model for viscous generalized Chaplygin gas has been investigated in the Brans–Dicke theory of gravitation. To determine the solutions of field equations, we have considered the power-law relation for the average scale factor. We have computed some cosmological parameters and discussed their physical importance. Furthermore, we have discussed the nature of statefinder and $Om$ diagnostics in our model. It is also worth noting that the conclusions of the cosmological parameter are consistent with modern observational data.