Xin-Zhou Li

Phantom-like GCG and the constraints of its parameters via cosmological dynamics

Abstract We extend the equation of state of GCG (Generalized Chaplygin Gas) to w − 1 regime and show, from the point of view of dynamics, that the parameters of GCG should be in the range of 0 α 1 . Also, dynamical analysis indicates that the phase w g = − 1 is a dynamical attractor and the equation of state of GCG approaches it from either w g > − 1 or w g − 1 depending on the choice of its initial cosmic density parameter and the ratio of pressure to critical energy density.

Viscous Ricci dark energy

We investigate the viscous Ricci dark energy (RDE) model by assuming that there is bulk viscosity in the linear barotropic fluid and the RDE. In the RDE model without bulk viscosity, the universe is younger than some old objects at certain redshifts. Since the age of the universe should be longer than any objects living in the universe, the RDE model suffers the age problem, especially when we consider the object APM 08279 + 5255 at z = 3.91 with age t = 2.1 Gyr. In this Letter, we find that once the viscosity is taken into account, this age problem is alleviated

Quintessence with O(N) symmetry

We study a new class of quintessence models, in which the scalar field possesses an O(N) internal symmetry. We give a critical condition of instability for the potential against Q ball formation. We find that the most widely used potentials of quintessence do not satisfy the above condition, and therefore, the O(N) quintessence with these potentials will not lead to Q ball formation. It is worth noting that the O(N) quintessence with a cosine-type potential is especially interesting in that the angular contribution is not negligible.

Dynamics of quintessence with thermal interactions

The cosmological dynamics of minimally coupled scalar field that couple to the background matter with thermal interactions is investigated in exponential potential. The conditions for the existence and stability of various critical points as well as their cosmological implications are obtained. Although we show that the effects of thermal interaction such as depressing the equation-of-state parameter of quintessence, is only important at the early time, the evolution of equation-of-state parameter of quintessence is manifested. The upper bound is required on the coupling between quintessence and relativistic relic particles such as photons and neutrinos.

Statefinder diagnostic in a torsion cosmology

We apply the statefinder diagnostic to the torsion cosmology, in which an accounting for the accelerated universe is considered in term of a Riemann-Cartan geometry: dynamic scalar torsion. We find that there are some typical characteristic of the evolution of statefinder parameters for the torsion cosmology that can be distinguished from the other cosmological models. Furthermore, we also show that statefinder diagnostic has a direct bearing on the critical points. The statefinder diagnostic divides the torsion parameter a1 into differential ranges, which is in keeping with the requirement of dynamical analysis. In addition, we fit the scalar torsion model to ESSENCE supernovae data and give the best fit values of the model parameters.

Analytical approach of late-time evolution in a torsion cosmology

Abstract In this Letter, we study the late-time evolution of a torsion cosmology only with the spin- 0 + mode. We find three kinds of analytical solutions with a constant affine scalar curvature. In the first case, it is not physical because the matter density will be negative. In the second case, it shows that the dark energy can be mimicked in the torsion cosmological model. In the third case, the characteristic of late-time evolution is similar to that of the universe of matter dominant. And we also find a kind of expression with the non-constant curvature that the periodic character of numerical calculation is only the reflection of solution in a specific period of evolution. Using these expressions, we shall be able to predict the evolution over the late-time. From this prediction, we know the fate of universe that the universe would expand forever, slowly asymptotically to a halt.

Rigorous proof of the attractive nature for the Casimir force of a p-odd hypercube

The Casimir effect giving rise to an attractive force between the configuration boundaries that confine the massless scalar field is rigorously proved for an odd dimensional hypercube with the Dirichlet boundary conditions and different spacetime dimensions D by the Epstein zeta function regularization.

Exponential Cardassian universe

Abstract The expectation of explaining cosmological observations without requiring new energy sources is forsooth worthy of investigation. In this Letter, a new kind of Cardassian models, called exponential Cardassian models, for the late-time universe are investigated in the context of the spatially flat FRW universe scenario. We fit the exponential Cardassian models to current type Ia supernovae data and find they are consistent with the observations. Furthermore, we point out that the equation-of-state parameter for the effective dark fluid component in exponential Cardassian models can naturally cross the cosmological constant divide w = − 1 that observations favor mildly without introducing exotic material that destroy the weak energy condition.

Quasinormal modes of stringy black holes

Abstract String theory is a promising candidate for a fundamental quantum theory of all interactions including Einstein gravity. Some solutions in string theory can be interpreted as black holes. Using the semi-analytic method and WKB method, the quasinormal modes (QNMs) of (1+1)-dimensional black hole in string theory are studied.The QNMs of (1+3)-dimensional black hole in string theory are also calculated through numerical approach.The numerical investigation has shown that the late time gravitational oscillation of the black hole under an external perturbation is dominated by certain QNMs.

de Sitter gauge theory of gravity: an alternative torsion cosmology

A new cosmological model based on the de Sitter gauge theory (dSGT) is studied in this paper. By some transformations, we find, in the dust universe, the cosmological equations of dSGT could form an autonomous system. We conduct dynamics analysis to this system, and find 9 critical points, among which there exist one positive attractor and one negative attractor. The positive attractor shows us that our universe will enter a exponential expansion phase in the end, which is similar to the conclusion of ΛCDM. We also carry out some numerical calculations, which confirms the conclusion of dynamics analysis. Finally, we fit the model parameter and initial values to the Union 2 SNIa dataset, present the confidence contour of parameters and obtain the best-fit values of parameters of dSGT.