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.

Absence of Buckling in Nerve Fiber

In this study we give a geometrical model which employs the smoothness of nerve fibers as differentiable curves. We show that a nerve fiber may encounter large curvature due to the possible helicial bending and hence it could cause the fiber to buckle. However, its membrane structure provides a mechanism, entirely geometrical to avoid it. To overcome the challenge of emerging helix we project it into a plane.Comment: 7 pages no figure. Final version presented at The 10th International Physics Conference of the Balkan Physical Union (BPU10), 26-30 August 2018. To be published in AIP Conference Proceeding

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

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

A topological metric in 2+1-dimensions

Real-valued triplet of scalar fields as source gives rise to a metric which tilts the scalar, not the light cone, in 2+1-dimensions. The topological metric is static, regular and it is characterized by an integer $\kappa =\pm 1,\pm 2,...$. The problem is formulated as a harmonic map of Riemannian manifolds in which the integer $\kappa$ equals to the degree of the map.Comment: 4 pages no figure, final version accepted for publication in EPJ

Black holes from multiplets of scalar fields in 2+1- and 3+1-dimensions

We obtain classes of black hole solutions constructed from multiplets of scalar fields in 2+1 / 3+1 dimensions. The multi-component scalars don't undergo a symmetry breaking so that only the isotropic modulus is effective. The Lagrangian is supplemented by a self-interacting potential which plays significant role in obtaining the exact solutions. In 2+1 / 3+1 dimensions doublet / triplet of scalars is effective which enriches the available black hole spacetimes and creates useful Liouville weighted field theoretic models.Comment: 10 pages, 10 figures, final version accepted for publication in EPJ