Narrow Results

A Universe containing uniform magnetic fields, strings, or domain walls is shown to have an ellipsoidal expansion. This case has motivations from observational cosmology especially the anomaly concerning the low quadrupole amplitude compared to the best-fit $\mathrm{\Lambda }$CDM prediction in Planck data. It is shown that a Universe with eccentricity at decoupling of order $10^{-2}$ can reduce the quadrupole amplitude without affecting higher multipoles of the angular power spectrum of the temperature anisotropy. We study the evolution of ellipsoidal Universes using dynamical system techniques for the first time. The determined critical points vary between saddle and past attractors depending on dark energy state equation parameter $w_{\mathrm{\Lambda }}$, with no future attractors. Important results are shown with numerical integrations of this dynamical system done using several initial conditions. For instance, a tendency for high expansion differences between planar and perpendicular axes is observed which contradicts previous assumption on the evolution behavior of ellipsoid Universes. Considering dark energy as a Chaplygin gas solves this contradiction by controlling cosmic shear evolution.

In this work, we investigate cosmological perturbations of viscous modified chaplygin gas model. Using $1+3$ covariant formalism, we define covariant and gauge invariant gradient variables, which after the application of scalar decomposition and harmonic decomposition techniques together with redshift transformation method, provide the energy overdensity perturbation equations in redshift space, responsible for large-scale structure formation. In order to analyze the effect of the viscous modified chaplygin gas model on matter overdensity contrast, we numerically solve the perturbation equations in both long and short wavelength limits. The numerical results show that the energy overdensity contrast decays with redshift. However, the perturbations which include amplitude effects due to the viscous modified chaplygin model do differ remarkably from those in the $\mathrm{\Lambda }$CDM. In the absence of viscous modified chaplygin model, the results reduce to those of $\mathrm{\Lambda }$CDM.

Based on recently proposed unified models for dark matter and energy which are constructed by introducing a generalized Chaplygin vacuum fluid, we suggest in this paper a new scenario for quintessential inflation which does not rely on the existence of a specific potential for the vacuum field, but on a generalization of the definition of the Hilbert–Einstein action for a massless scalar field conformally-coupled to gravity that contains an extra Lagrangian term for the generalized vacuum Chaplygin fluid and the corresponding new surface term.

Recently Gambini and Pullin proposed a new consistent discrete approach to quantum gravity and applied it to cosmological models. One remarkable result of this approach is that the cosmological singularity can be avoided in a general fashion. However, whether the continuum limit of such discretized theories exists is model dependent. In the case of massless scalar field coupled to gravity with Λ=0, the continuum limit can only be achieved by fine tuning the recurrence constant. We regard this failure as the implication that cosmological constant should vary with time. For this reason we replace the massless scalar field by Chaplygin gas which may contribute an effective cosmological constant term with the evolution of the universe. It turns out that the continuum limit can indeed be reached in this case.

In this letter, we consider a model of the universe filled with modified Chaplygin gas and another fluid (with barotropic equation of state) and its role in accelerating phase of the universe. We have assumed that the mixture of these two fluid models is valid from (i) the radiation era to ΛCDM for -1 ≤ γ ≤ 1 and (ii) the radiation era to quintessence model for γ < -1. For these two fluid models, the statefinder parameters describe different phase of the evolution of the universe.

Some new class of Chaplygin wormholes are investigated in the framework of a Chaplygin gas with the equation of state . Since empty spacetime (p = ρ = 0) does not follow Chaplygin gas, so the interior Chaplygin wormhole solutions will never be asymptotically flat. For this reason, we have to match our interior wormhole solution with an exterior vacuum solution, i.e. Schwarzschild solution at some junction interface, say r = a. We also discuss the total amount of matter characterized by Chaplygin gas that supplies fuel to construct a wormhole.

Hybrid Chaplygin gas model is put forward, in which the gases play the role of dark energy. For this model the coincidence problem is greatly alleviated. The effective equation of state of the dark energy may cross the phantom divide w = -1. Furthermore, the crossing behavior is decoupled from any gravity theories. In the present model, w < -1 is only a transient behavior. There is a de Sitter attractor in the future infinity. Hence, the big rip singularity, which often afflicts the models with matter whose effective equation of state less than -1, naturally disappears. There exist stable scaling solutions, both at the early universe and the late universe. We discuss the perturbation growth of this model. We find that the index is consistent with observations.

The purpose of the present study is to show that in a particular cosmological model, with an affine equation of state, one can obtain, besides the background given by the scale factor, Hubble and deceleration parameters, a representation in terms of scalar fields and, more important, explicit mathematical expressions for the density contrast and the power spectrum. Although the model so obtained is not realistic, it reproduces features observed in some previous numerical studies, and as such it indicates that the Chaplygin gas by itself is not competitive when compared to the usual Λ-CDM model.

In this paper, we investigate the role played by dark energy (DE) in the form of Generalized cosmic Chaplygin gas in an accelerating universe described by FRW cosmology. We have tried to describe the model from the theoretical point of view of a field, by introducing a scalar field ϕ and a self-interacting potential V(ϕ). The corresponding expressions for the field are obtained for the given model. Statefinder parameters have been used to characterize the dark energy model. Plots have been generated for characterizing different phases of universe diagrammatically and a comparative study is performed with the Modified Chaplygin gas model. As an outcome of the study, Generalized cosmic Chaplygin gas is identified as a much less constrained form of dark energy as compared to modified Chaplygin gas.

In this paper, the implications of considering interaction between Chaplygin gas and a barotropic fluid with constant equation of state have been explored. The unique feature of this work is that assuming an interaction $Q\propto H{\rho }_d$, analytic expressions for the energy density and pressure have been derived in terms of the hypergeometric ${}_2{\text{F}}_1$ function. It is worthwhile to mention that an interacting Chaplygin gas model was considered in 2006 by Zhang and Zhu, nevertheless, analytic solutions for the continuity equations could not be determined assuming an interaction proportional to $H$ times the sum of the energy densities of Chaplygin gas and dust. Our model can successfully explain the transition from the early decelerating phase to the present phase of cosmic acceleration. Arbitrary choice of the free parameters of our model through trial and error shows that recent observational data strongly favors $w_m=0$ and $w_m=-\frac{1}{3}$ over the $w_m=\frac{1}{3}$ case. Interestingly, the present model also incorporates the transition of dark energy into the phantom domain, however, future deceleration is forbidden.

In this paper, we study thermodynamic curvature of Chaplygin gas which is one of the dark energy candidates in terms of entropy and temperature. So, we get some information about past universe by considering thermodynamic curvature of Chaplygin gas. In addition to thermodynamic curvature, we study behaviors of test particle in the universe filled with Chaplygin gas.

In this work, we intend to address the matter–antimatter asymmetry via the gravitational baryogenesis mechanism in the background of a quantum theory of gravity. We investigate this mechanism under the framework of Hořava–Lifshitz gravity. We will compute the baryon-to-entropy ratio in the chosen framework and investigate its physical viability against the observational bounds. We also conduct the above study for various sources of matter like scalar field and Chaplygin gas as specific examples. We speculate that quantum corrections from the background geometry will lead to interesting results.

A stellar model with an electrically charged anisotropic fluid as a source of matter is presented. The radial pressure is described by a Chaplygin state equation, $P_r(\rho )=\mu c^2\rho -\nu /(c^2\rho )$, while the anisotropy $\mathrm{\Delta }\equiv P_t-P_r=r^{2}f(r)$ is annulled in the center of the star $(f(r)$ is regular and $f(0)\ne 0)$, the electric field, is also annulled in the center. The density pressures and the tangential speed of sound are regular, while the radial speed of sound is monotonically increasing. The model is physically acceptable and meets the stability criteria of Harrison–Zeldovich–Novikov and in respect of the cracking concept the solution is unstable in the region of the center and potentially stable near the surface. A graphic description is presented for the case of an object with a compactness rate $u=0.27336$, mass $M=1.77M_{\odot }$ and radius $R=9.56$ km that matches the star Vela X-1. Also, the interval of the central density ${\rho }_c\in [1.176977292,1.308791129]10^{18}\mathrm{\text{kg}}/{\mathrm{\text{m}}}^3$, which is consistent with the expected magnitudes for this type of stars, which shows that the behavior is accurate for describing compact objects.

Starting from a regular, static and spherically symmetric spacetime, we present a stellar model formed by two sources of ordinary and quintessence matter both with anisotropic pressures. The ordinary matter, with density $\rho$, is formed by a fluid with a state equation type Chaplygin $P_r(\rho )=\mu c^2\rho -\nu /(c^2\rho )$ for the radial pressure. And the quintessence matter, with density ${\rho }_q$, has a state equation $P_r({\rho }_q)=-c^2{\rho }_q$ for the radial pressure and $P_r({\rho }_q)=-(1+3w)c^2{\rho }_q/2$ for the tangential pressure with $-1<w<-\frac{1}{3}$. The model satisfies the required conditions to be physically acceptable and additionally the solution is potentially stable, i.e. $v_t^2-v_r^2<0$ according to the cracking concept, and it also satisfies the Harrison–Zeldovich–Novikov criteria. We describe in a graphic manner the behavior of the solution for the case in which the mass is $1.3M_{\odot }$ and radius $R=8.849$km which matches the star EXO 1785-248, from where we obtain the maximum density ${\rho }_c=1.206510^{18}\mathrm{\text{kg}}/{\mathrm{\text{m}}}^3$ for the values of the parameters $\mu =0.15535$, $w=-0.33334$.

In this paper, we have assumed that the thermodynamic pressure may be generated by the negative cosmological constant and the thermodynamic parameters of the n-dimensional asymptotically Anti-de Sitter (AdS)-Tangherlini black holes are identical with the polytropic gas and modified Chaplygin gas separately. The black hole mass, volume, entropy, and temperature have been written due to the assigned gases’ thermodynamic system. We found the solutions of Einstein’s field equations of n-dimensional black hole for polytropic gas and modified Chaplygin gas. The null energy condition, weak energy condition, strong energy condition, and dominant energy condition have been examined for the fluid sources in n-dimensional black hole system due to both the gases. We found that the four energy conditions completely depend on the model parameters and the dimensions of the spacetime. We have studied the heat engine phenomena for both the black holes and analyzed the work done with the efficiencies due to the Carnot and other cycles.

In this paper, we study the Chaplygin gas equation in curvature-matter coupling gravity to describe the observed expansion of the universe. We consider the Chaplygin gas equation of state in terms of the energy density $\rho$ and pressure $p$, by the relationship $p=-\frac{A}{\rho }$ and the curvature-matter coupling gravity as $f(R,T)$, where $A$ is a positive constant, $R$ and $T$ denote, respectively, the Ricci scalar and the trace of energy–momentum tensor. Considering the gravitational lagrangian in the form $f(R,T)=R+h(T)$, where we used two forms of $h(T)$ and by assuming a perfect fluid as matter source, we determined through the modified Friedmann equations, the corresponding energy and pressure for each model. By using the free parameters of our models, we performed the numerical results of the energy conditions to study its viability. Furthermore, we have derived some cosmological parameters like deceleration parameter and statefinder parameters and graphically investigated the nature of these parameters in Chaplygin gas model.

In this paper, we study the evolution of the FRW universe filled with variable modified Chaplygin gas (VMCG). We begin with a thermodynamical treatment of VMCG described by the equation of state $P=A\rho -B{\rho }^{-\alpha }$, and obtain its temperature as a function of redshift z. We show that the results are consistent with similar works on other types of Chaplygin gas models. In addition to deriving the exact expression of temperature of the fluid in terms of the boundary conditions and redshift, we also used observational data to determine the redshift at the epoch of transition from the decelerated to the accelerated phase of expansion of the universe. The values of other relevant parameters like the Hubble parameter, the equation-of-state parameter and the speed of sound are obtained in terms of the redshift parameter, and these values are compared with the results obtained from previous works on MCG and other Chaplygin gas models for the various values of n permitted by thermodynamic stability. We assume the present value of temperature of the microwave background radiation to be given by $T_0=2.7K$, and the parameter A in the equation of state is taken as $1/3$ since it corresponds to the radiation-dominated phase of the universe. The value of the parameter ${\mathrm{\Omega }}_x$ has been assumed to be $0.7$ in our calculation. Since it is known that the redshift of photon decoupling is $z\simeq 1100$, we used this value to calculate the temperature of decoupling.

In this paper, we study the 2D Euler equations for Chaplygin gas, which is a model for dark energy of the universe in some of cosmology theories. By using an ansatz developed by Yuen, we present some rotational and self-similar analytical solutions. We obtain the global existence of the constructed solutions and investigate the qualitative properties of the gas density according to various parameters.

This paper deals with some wormhole solutions which are obtained by taking two different shape functions along with zero tidal force. For obtaining wormhole solutions, anisotropic fluid and a equation of state $p_t=-\frac{a}{\rho }$ related by Chaplygin gas are considered, where $\rho$ is the energy density, $p_t$ is tangential pressure and $a$ is positive constant. Energy conditions are examined for two different models, and it is found that major energy conditions are satisfied in a region.

In this paper, we have considered the generalized cosmic Chaplygin gas (GCCG) in the background of Brans–Dicke (BD) theory and also assumed that the Universe is filled in GCCG, dark matter and radiation. To investigate the data fitting of model parameters, we have constrained the model using recent observations. Using ${\chi }^2$ minimum test, the best-fit values of the model parameters are determined by OHD+CMB+BAO+SNIa joint data analysis. We have drawn the contour figures for different confidence levels $1\sigma$, $2\sigma$ and $3\sigma$. To examine the viability of the GCCG model in BD theory, we have also determined △AIC and △BIC using the information criteria (AIC and BIC). Graphically, we have analyzed the natures of the equation of state parameter and deceleration parameter for our best-fit values of model parameters. Also, we have studied the square speed of sound $v_s^2$ which lies in the interval $(0,1)$ for expansion of the Universe. So, our considered model is classically stable by considering the best-fit values of the model parameters due to the data analysis.