7,779 research outputs found
Necessary conditions for having wormholes in f(R) gravity
For a generic which admits a polynomial expansion of at least third
order (i.e. ) we find the near-throat wormhole
solution. Necessary conditions for the existence of wormholes in such
theories are derived for both zero and non-zero matter sources. A particular
choice of energy-momentum reveals that the wormhole geometry satisfies the weak
energy condition (WEC). For a range of parameters even the strong energy
condition (SEC) is shown to be satisfied.Comment: 7 pages, 1 figur
Cloud of strings as source in -dimensional gravity
We present three parameters exact solutions with possible black holes in dimensional modified gravity coupled minimally
to a cloud of strings. These three parameters are the cloud of string
coupling constant and an integration constant . Although in general
one has to consider each set of parameters separately; for an even integer
greater than one we give a unified picture providing black holes. For
we analyze null / timelike geodesic within the context of particle confinement.Comment: 5 pages, no figures. Revised versio
Black holes and the classical model of a particle in Einstein non-linear electrodynamics theory
Modified by a logarithmic term, the non-linear electrodynamics (NED) model of
the Born-Infeld (BI) action is reconsidered. Unlike the standard BI action,
this choice provides interesting integrals of the Einstein-NED equations. It is
found that the spherical matching process for a regular black hole entails
indispensable surface stresses that vanish only for a specific value of the BI
parameter. This solution represents a classical model of an elementary particle
whose radius coincides with the horizon. In flat space time, a charged particle
becomes a conducting shell with a radius proportional to the BI parameter.Comment: 11 pages, no figure, To appear in Phys. Lett.
2+1-dimensional traversable wormholes supported by positive energy
We revisit the shapes of the throats of wormholes, including thin-shell
wormholes (TSWs) in dimensions. In particular, in the case of TSWs this
is done in a flat dimensional bulk spacetime by using the standard method
of cut-and-paste. Upon departing from a pure time-dependent circular shape
i.e., for the throat, we employ a dependent
closed loop of the form and in terms of we find the surface energy density on the throat.
For the specific convex shapes we find that the total energy which supports the
wormhole is positive and finite. In addition to that we analyze the general
wormhole's throat. By considering a specific equation of instead of and upon certain choices of functions
for we find the total energy of the wormhole to be
positive.Comment: 8 pages, 9 figures, final version to appear in EPJ
Screening of the Reissner-Nordstr\"om charge by a thin-shell of dust matter
A concentric charged thin-shell encircling a Reissner-Nordstr\"{o}m black
hole screens the clectric / magnetic charge completely to match with an
external Schwarzschild black hole. The negative mass thin-shell is shown to be
stable against radial perturbations. It is shown further that by reversing the
roles of inside Reissner-Nordstr\"{o}m and outside Schwarzschild geometries the
mass of the appropriate shell becomes positive.Comment: 5 pages, one figure, final version published in EPJ
Charge screening by thin-shells in a 2+1-dimensional regular black hole
We consider a particular Bardeen black hole in 2+1-dimensions. The black hole
is sourced by a radial electric field in non-linear electrodynamics (NED). The
solution is obtained anew by the alternative Hamiltonian formalism. For
it asymptotes to the charged BTZ black hole. It is shown
that by inserting a charged, thin-shell (or ring) the charge of the regular
black hole can be screened from the external world.Comment: 8 pages, 2 figures, final version, accepted for publication in
European Physical Journal
- …