Necessary conditions for having wormholes in f(R) gravity

For a generic $f(R)$ which admits a polynomial expansion of at least third order (i.e. $\frac{d^{3}f}{dR^{3}}\neq 0$) we find the near-throat wormhole solution. Necessary conditions for the existence of wormholes in such $f(R)$ 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 2+12+1-dimensional f(R)=Rnf( R) = R^n gravity

We present three parameters exact solutions with possible black holes in $2+1-$dimensional $f\left( R\right) =R^{n}$ modified gravity coupled minimally to a cloud of strings. These three parameters are $n,$ the cloud of string coupling constant $\xi$ and an integration constant $C$. Although in general one has to consider each set of parameters separately; for $n$ an even integer greater than one we give a unified picture providing black holes. For $n\geq 1$ 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.

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 $r\rightarrow \infty$ 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

2+1-dimensional traversable wormholes supported by positive energy

We revisit the shapes of the throats of wormholes, including thin-shell wormholes (TSWs) in $2+1-$dimensions. In particular, in the case of TSWs this is done in a flat $2+1-$dimensional bulk spacetime by using the standard method of cut-and-paste. Upon departing from a pure time-dependent circular shape i.e., $r=a\left( t\right)$ for the throat, we employ a $$dependent closed loop of the form $r=R\left( t,\theta \right) ,$ and in terms of $R\left( t,\theta \right)$ we find the surface energy density $\sigma$ 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 $r=R\left( \theta \right)$ instead of $r=r_{0}=const.,$ and upon certain choices of functions for $R\left( \theta \right)$ we find the total energy of the wormhole to be positive.Comment: 8 pages, 9 figures, final version to appear in EPJ