Abstract:
The late-time tail behaviors of massive scalar fields are examined analytically in the background of a black hole with a global monopole. It is found that the presence of a solid deficit angle in the background metric makes the massive scalar fields decay faster in the intermediate times. However, the asymptotically late-time tail is not affected and it has the same decay rate of $t^{-5/6}$ as in the Schwarzschild and nearly extreme Reissner-Nordstr\"{o}m backgrounds.

Abstract:
Using Visser's semi-analytical model for the stress-energy tensor corresponding to the conformally coupled massless scalar field in the Unruh vacuum, we examine, by explicitly evaluating the relevant integrals over half-complete geodesics, the averaged weak (AWEC) and averaged null (ANEC) energy conditions along with Ford-Roman quantum inequality-type restrictions on negative energy in the context of four dimensional evaporating black hole backgrounds. We find that in all cases where the averaged energy conditions fail, there exist quantum inequality bounds on the magnitude and duration of negative energy densities.

Abstract:
We analyze, in the paradigm of open quantum systems, the reduced dynamics of a freely-falling two-level detector in de Sitter space-time in weak interaction with a reservoir of fluctuating quantized conformal scalar fields in the de Sitter invariant vacuum. We find that the detector is asymptotically driven to a thermal state at the Gibbons-Hawking temperature, regardless of its initial state. Our discussion therefore shows that the Gibbons-Hawking effect of de Sitter space-time can be understood as a manifestation of thermalization phenomena that involves decoherence and dissipation in open quantum systems.

Abstract:
The generalized Chaplygin gas (GCG), is studied in this paper by using the latest observational data including 182 gold sample type Ia supernovae (Sne Ia) data, the ESSENCE Sne Ia data, the distance ratio from $z=0.35$ to $z=1089$ (the redshift of decoupling), the CMB shift parameter and the Hubble parameter data. Our results rule out the standard Chaplygin gas model ($\alpha=1$) at the 99.7% confidence level, but allow for the $\lambda CDM$ model ($\alpha=0$) at the 68.3% confidence level. At a 95.4% confidence level, we obtain $w=-0.74_{-0.09}^{+0.10}$ and $\alpha=-0.14_{-0.19}^{+0.30}$. In addition, we find that the phase transition from deceleration to acceleration occurs at redshift $z_{q=0}\sim 0.78-0.89$ at a $1\sigma$ confidence level for the GCG model.

Abstract:
We study the modification by the presence of a plane wall of energy level shifts of two-level atoms which are in multipolar coupling with quantized electromagnetic fields in a thermal bath in a formalism which separates the contributions of thermal fluctuations and radiation reaction and allows a distinct treatment to atoms in the ground and excited states. The position dependent energy shifts give rise to an induced force acting on the atoms. We are able to identify three different regimes where the force shows distinct features and examine, in all regimes, the behaviors of this force in both the low temperature limit and the high temperature limit for both the ground state and excited state atoms, thus providing some physical insights into the atom-wall interaction at finite temperature. In particular, we show that both the magnitude and the direction of the force acting on an atom may have a clear dependence on atomic the polarization directions. In certain cases, a change of relative ratio of polarizations in different directions may result in a change of direction of the force.

Abstract:
We examine the entanglement creation between two mutually independent two-level atoms immersed in a thermal bath of quantum scalar fields in the presence of a perfectly reflecting plane boundary. With the help of the master equation that describes the evolution in time of the atom subsystem obtained, in the weak-coupling limit, by tracing over environment (scalar fields) degrees of freedom, we find that the presence of the boundary may play a significant role in the entanglement creation in some circumstances and the new parameter, the distance of the atoms from the boundary, besides the bath temperature and the separation between the atoms, gives us more freedom in manipulating entanglement generation. Remarkably, the final remaining entanglement in the equilibrium state is independent of the presence of the boundary.

Abstract:
We study the spontaneous excitation of an accelerated multilevel atom in dipole coupling to the derivative of a massless quantum scalar field and separately calculate the contributions of the vacuum fluctuation and radiation reaction to the rate of change of the mean atomic energy of the atom. It is found that, in contrast to the case where a monopole like interaction between the atom and the field is assumed, there appear extra corrections proportional to the acceleration squared, in addition to corrections which can be viewed as a result of an ambient thermal bath at the Unruh temperature, as compared with the inertial case, and the acceleration induced correction terms show anisotropy with the contribution from longitudinal polarization being four times that from the transverse polarization for isotropically polarized accelerated atoms. Our results suggest that the effect of acceleration on the rate of change of the mean atomic energy is dependent not only on the quantum field to which the atom is coupled, but also on the type of the interaction even if the same quantum scalar field is considered.

Abstract:
We investigate, from the point of view of a coaccelerated frame, the spontaneous excitation of a uniformly accelerated two-level atom interacting with a scalar field in a thermal state at a finite temperature $T$ and show that the same spontaneous excitation rate for the uniformly accelerated atom in the Minkowski vacuum obtained in the inertial frame can only be recovered in the coaccelerated frame assuming a thermal bath at the Fulling-Davies-Unruh temperature $T_{FDU}=a/2\pi$ for what appears to be the Minkowski vacuum to the inertial observer. Our discussion provides another example of a physical process different from those examined before in the literature to better understand the Fulling-Davies-Unruh effect.

Abstract:
We place, by the maximum likelihood method, constraints on a variable dark energy model with the equation of state $w=w_0/[1+b\ln (1+z)]^2$ using some recent observational data, including the new Sne Ia data from the SNLS, the size of baryonic acoustic oscillation peak from SDSS and the CMB data from WMAP3. We find that the SNLS data favor models with $w_0$ around -1, in contrast to the Gold data set which favors a more negative $w_0$. By combining these three databases, we obtain that $\Omega_m=0.27_{-0.038}^{+0.036}$, $w_0=-1.11_{-0.30}^{+0.21}$ and $b=0.31^{+ 0.71}_{-0.31}$ with $\chi^2=110.4$ at the 95% confidence level. Our result suggests that a varying dark energy model and a crossing of the $w = -1$ line are favored, and the present value of the equation of state of dark energy is very likely less than -1.

Abstract:
The generalized Chaplygin gas (GCG), is studied in this paper by using the latest observational data including 182 gold sample type Ia supernovae (Sne Ia) data, the ESSENCE Sne Ia data, the distance ratio from $z=0.35$ to $z=1089$ (the redshift of decoupling), the CMB shift parameter and the Hubble parameter data. Our results rule out the standard Chaplygin gas model ($\alpha=1$) at the 99.7% confidence level, but allow for the $\lambda CDM$ model ($\alpha=0$) at the 68.3% confidence level. At a 95.4% confidence level, we obtain $w=-0.74_{-0.09}^{+0.10}$ and $\alpha=-0.14_{-0.19}^{+0.30}$. In addition, we find that the phase transition from deceleration to acceleration occurs at redshift $z_{q=0}\sim 0.78-0.89$ at a $1\sigma$ confidence level for the GCG model.