We study, in the multipolar coupling scheme, a uniformly accelerated multilevel hydrogen atom in interaction with the quantum electromagnetic field near a conducting boundary and separately calculate the contributions of the vacuum fluctuation and radiation reaction to the rate of change of the mean atomic energy. It is found that the perfect balance between the contributions of vacuum fluctuations and radiation reaction that ensures the stability of ground-state atoms is disturbed, making spontaneous transition of ground-state atoms to excited states possible in a vacuum with a conducting boundary. The boundaryinduced contribution is effectively a nonthermal correction, which enhances or weakens the nonthermal effect already present in the unbounded case, thus possibly making the effect easier to observe. An interesting feature worth noting is that the nonthermal corrections may vanish for atoms on some particular trajectories.

We consider a multilevel hydrogen atom in interaction with the quantum electromagnetic 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 for uniform acceleration. It is found that the acceleration disturbs the vacuum fluctuations in such a way that the delicate balance between the contributions of vacuum fluctuation and radiation reaction that exists for inertial atoms is broken, so that the transitions to higher-lying states from ground state are possible even in vacuum. In contrast to the case of an atom interacting with a scalar field, the contributions of both electromagnetic vacuum fluctuations and radiation reaction to the spontaneous emission rate are affected by the acceleration, and furthermore the contribution of the vacuum fluctuations contains a nonthermal acceleration-dependent correction, which is possibly observable.

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.

We study a two-level atom in interaction with a real massless scalar quantum field in a spacetime with a reflecting boundary. The presence of the boundary modifies the quantum fluctuations of the scalar field, which in turn modifies the radiative properties of atoms. We calculate the rate of change of the mean atomic energy of the atom for both inertial motion and uniform acceleration. It is found that the modifications induced by the presence of a boundary make the spontaneous radiation rate of an excited inertial atom oscillate near the boundary and this oscillatory behavior may offer a possible opportunity for experimental tests for geometrical (boundary) effects in flat spacetime. While for accelerated atoms, the transitions from ground states to excited states are found to be possible even in a vacuum due to changes in the vacuum fluctuations induced by both the presence of the boundary and the acceleration of atoms, and this can be regarded as an actual physical process underlying the Unruh effect.

The effects of the gravitational back reaction of cosmological perturbations are investigated in a cosmological model where the universe is dominated by phantom energy. We assume a COBE normalized spectrum of cosmological fluctuations at the present time and calculate the effective energy–momentum tensor of the gravitational back-reactions of cosmological perturbations whose wavelengths at the time when the back-reactions are evaluated are larger than the Hubble radius. Our results reveal that the effects of gravitational back-reactions will counteract that of phantom energy sooner or later and can become large enough to terminate the phantom dominated phase before the big rip as the universe evolves. This arises because the phase space of infrared modes grows very rapidly as we come close to the big rip.