6,419 research outputs found
Casimir Densities for a Massive Fermionic Quantum Field in a Global Monopole Background with Spherical Boundary
We investigate the vacuum expectation value of the energy-momentum tensor
associated with a massive fermionic field obeying the MIT bag boundary
condition on a spherical shell in the global monopole spacetime. The asymptotic
behavior of the vacuum densities is investigated near the sphere center and
surface, and at large distances from the sphere. In the limit of strong
gravitational field corresponding to small values of the parameter describing
the solid angle deficit in global monopole geometry, the sphere-induced
expectation values are exponentially suppressed.Comment: 8 pages, 4 figures, 6th Alexander Friedmann International Seminar on
Gravitation and Cosmolog
Scalar self-energy for a charged particle in global monopole spacetime with a spherical boundary
We analyze combined effects of the geometry produced by global monopole and a
concentric spherical boundary on the self-energy of a point-like scalar charged
test particle at rest. We assume that the boundary is outside the monopole's
core with a general spherically symmetric inner structure. An important
quantity to this analysis is the three-dimensional Green function associated
with this system. For both Dirichlet and Neumann boundary conditions obeyed by
the scalar field on the sphere, the Green function presents a structure that
contains contributions due to the background geometry of the spacetime and the
boundary. Consequently the corresponding induced scalar self-energy present
also similar structure. For points near the sphere the boundary-induced part
dominates and the self-force is repulsive/attractive with respect to the
boundary for Dirichlet/Neumann boundary condition. In the region outside the
sphere at large distances from it, the boundary-free part in the self-energy
dominates and the corresponding self-force can be either attractive or
repulsive with dependence of the curvature coupling parameter for scalar field.
In particular, for the minimal coupling we show the presence of a stable
equilibrium point for Dirichlet boundary condition. In the region inside the
sphere the nature of the self-force depends on the specific model for the
monopole's core. As illustrations of the general procedure adopted we shall
consider two distinct models, namely flower-pot and the ballpoint-pen ones.Comment: 26 pages, 7 figures. Paper accepted for publication in CQG with minor
revision. arXiv admin note: text overlap with arXiv:1009.019
Casimir effect in hemisphere capped tubes
In this paper we investigate the vacuum densities for a massive scalar field
with general curvature coupling in background of a (2+1)-dimensional spacetime
corresponding to a cylindrical tube with a hemispherical cap. A complete set of
mode functions is constructed and the positive-frequency Wightman function is
evaluated for both the cylindrical and hemispherical subspaces. On the base of
this, the vacuum expectation values of the field squared and energy-momentum
tensor are investigated. The mean field squared and the normal stress are
finite on the boundary separating two subspaces, whereas the energy density and
the parallel stress diverge as the inverse power of the distance from the
boundary. For a conformally coupled field, the vacuum energy density is
negative on the cylindrical part of the space. On the hemisphere, it is
negative near the top and positive close to the boundary. In the case of
minimal coupling the energy density on the cup is negative. On the tube it is
positive near the boundary and negative at large distances. Though the
geometries of the subspaces are different, the Casimir pressures on the
separate sides of the boundary are equal and the net Casimir force vanishes.
The results obtained may be applied to capped carbon nanotubes described by an
effective field theory in the long-wavelength approximation.Comment: 24 pages, 5 figure
Vacuum polarization by a cosmic string in de Sitter spacetime
In this paper we investigate the vacuum polarization effect associated with a
quantum massive scalar field in a higher dimensional de Sitter spacetime in the
presence of a cosmic string. Because this investigation has been developed in a
pure de Sitter space, here we are mainly interested on the effects induced by
the presence of the string. So this analysis is developed by expressing the
complete Wightman function as the sum of two terms: The first one corresponds
to the bulk where the cosmic string is absent and the second one is induced by
the presence of the string. By using the Abel-Plana summation formula, we show
that for points away from the string the latter is finite at the coincidence
limit and it is used to evaluate the vacuum averages of the square of the field
and the energy-momentum tensor induced by the cosmic string. Simple asymptotic
formulae are obtained for these expectation values for points near the string
and at large distances from it. It is shown that, depending on the curvature
radius of de Sitter spacetime, two regimes are realized with monotonic and
oscillatory behavior of the vacuum expectation values at large distances.Comment: 18 pages, 2 figures, discussion on string metric in static
coordinates is adde
- …