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We present new static spherically-symmetric solutions of Einstein equations with the quintessence-like matter surrounding a global monopole. These new solutions of the coupling scalar-Einstein equations are more complicated, which depend on the parameter of equation of state $-1 < w_{q} <-{1/3}$. A gravitating global monopole produces a gravitational field of de Sitter kind outside the core in addition to a solid angular deficit. In the $w_{q} = -{1/3}$ case, we have proved that the solution cannot exist since the density of quintessence-like tends to zero if $w_{q} \to -{1/3}$. As a new feature, these monopoles have the outer horizon depending on both Goldstone field and quintessence-like. Since current observations constrain $-1.14 < w_{q} < -0.93$, new global monopoles have interesting astrophysical applications., Comment: 8 pages, 8 figures, added discussion and some references, the form accepted for publication in Physics Letter B

The Casimir effect giving rise to an attractive or repulsive force between the configuration boundaries that confine the massless scalar field is reexamined for one to three-dimensional pistons in this paper. Especially, we consider Casimir pistons with hybrid boundary conditions, where the boundary condition on the piston is Neumann and those on other surfaces are Dirichlet. We show that the Casimir force on the piston is always repulsive, in contrast with the same problem where the boundary conditions are Dirichlet on all surfaces., Comment: 8 pages, 4 figures,references added, minor typos corrected

Viscous GCG(generalized Chaplygin gas) cosmology is discussed, assuming that there is bulk viscosity in the barotropic fluid and GCG. The dynamical analysis indicates that the phase $w_g=-1+\sqrt{3}\gamma\kappa\tau_{g}/(\gamma-\sqrt{3}\kappa\tau_{\gamma})$ 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. Obviously, the equation of state $w_g$ can cross the boundary $w_g=-1$. Also, from the point of view of dynamics, the parameters of viscous GCG should be in the range of $\gamma>\sqrt{3}\kappa\tau_{\gamma}/(1-\sqrt{3}\kappa\tau_{g})$ and $0<\alpha<1+\sqrt{3} \kappa\gamma\tau_{g}/(\gamma-\sqrt{3}\kappa\tau_{\gamma}-\sqrt{3}\kappa\gamma\t au_{g})$., Comment: 8 pages, 5 eps figures, typos corrected

In this paper, we study a realistic model of quintessential inflation with radiation and matter. By the analysis of the dynamical system and numerical work about the evolution of the equation of state and cosmic density parameter, we show that this model is a good match for the current astronomical observation. The conclusion we obtain is in favor of the model where the modular part of complex field plays the role of the inflaton whereas the argument part is the quintessence field. The numerical calculation shows that a heteroclinic orbit (solution of dynamical system) which interpolates between early-time de Sitter phase (an unstable critical point) and a late-time de Sitter attractor., Comment: 5 pages, 2 figures

In this paper, we wish to point out the possibility of using a complex scalar field to account for the inflationary stage and the current acceleration. By the analysis of the dynamical system and numerical work, we show that the amplitude of the complex scalar field plays the role of the inflaton whereas the phase is the quintessence field. The numerical solutions describe heteroclinic orbits, which interpolate between an unstable critical point and a late-time de Sitter attractor. Therefore, this model is more natural to explain the two stages of acceleration., Comment: 7 pages, 2 figure

The motion of light and a neutral test particle around the charged D-star has been studied. The difference of the deficit angle of light from the case in asymptotically flat spacetime is in a factor $(1-\epsilon^2)$. The motion of a test particle is affected by the deficit angle and the charge. Through the phase analysis, we prove the existence of the periodic solution to the equation of motion and the effect of the deficit angle and the charge to the critical point and its type. We also give the conditions under which the critical point is a stable center and an unstable saddle point., Comment: 9 pages, 2 figures

We study the perturbations on both "radial" and "angular" components of the quintessence with an internal abelian and non-abelian symmetry. The properties of the perturbation on the "radial" component depend on the specific potential of the model and is similiar for both abelian and non-abelian case. We show that the consine-type potential is very interesting for the O(\textit{N}) quintessence model and also give a critical condition of instability for the potential. While the properties of perturbations on "angular" components depend on whether the internal symmetry is abelian or non-abelian, which we have discussed respectively. In the non-abelian case, the fluctuation of the "angular" component will increase rapidly with time while in the abelian case it will not., Comment: 15 pages, to be published in Int. J. Mod. Phys. A

We show two analytical solutions of the tachyon inflation for which the spectrum of curvature (density) perturbations can be calculated exactly to linear order, ignoring both gravity and the self-interactions of the tachyon field . The main feature of these solutions is that the spectral indices are independent with scale., Comment: 5 pages, no figure, to appear in Phys. Rev. D

The Casimir effect giving rise to an attractive force between the configuration boundaries that confine the massless scalar field is rigorously proven for odd dimensional hypercube with the Dirichlet boundary conditions and different spacetime dimensions D by the Epstein zeta function regularization., Comment: 7 pages, no figure