Bjørn Jensen

Stability of black hole event horizons.

In this work we study a {\it gedanken} experiment constructed in order to test the cosmic censorship hypothesis and the second law of black hole thermo-dynamics. Matter with a negative gravitating energy is imagined added to a near extremal

Comment on Casimir energy for spherical boundaries

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Anisotropic domain walls

-charged static black hole in Einstein-Maxwell theory. The dynamics of a similar process is studied and the thermo-dynamical properties of the resulting black hole structure is discussed. A new mechanism which stabilizes black hole event horizons is shown to operate in such processes.

Thermodynamics of black holes with gauge cosmic strings and global monopoles

It is shown that recent criticism by C. R. Hagen (hep-th/9902057) questioning the validity of stress tensor treatments of the Casimir energy for space divided into two parts by a spherical boundary is without foundation.

Notes on spinning strings

We find a static, anisotropic, non-supersymmetric generalization of the extreme supersymmetric domain walls of simple non-dilatonic supergravity theory. As opposed to the time-dependent isotropic non- and ultra-extreme domain walls, the anisotropic non-extreme wall has the same spatial topology as the extreme wall. The solution has naked singularities which vanish in the extreme limit.

Aspects of the Thermo-Dynamics of a Black Hole with a Topological Defect

We show that the tree-level entropy of a black hole is unchanged when hair in the form of a gauge cosmic string or a global monopole appear on the black hole. When one-loop effects are taken into account the entropy of the black hole seems to increase in such a process. This treatment indicates that the origin of the tree-level entropy of a black hole is of a fundamentally different kind than the statistical mechanical nature of the higher-order contributions to the black hole entropy.

Classical interactions for tensionless strings

The metric exterior to a spinning string in four spacetime dimensions is derived. It is interpreted as the exterior to a spinning cosmic string. A mass-spin relation is shown to exist. The possibility that such strings might serve as time machines is investigated.

Constructing new rotating dilaton black holes.

Aspects of the thermo-dynamics of a black hole which is either pierced by a cosmic gauge string or contains a global monopole are investigated. We also make some comments on the physical significance of the fact that the gravitational mass carried by a global monopole is negative. We note in particular that the negative monopole mass implies a gravitational super-radiance effect.

A reinterpretation of the Taub singularity

Abstract Using an “action at a distance” formulation we probe the possible classical interactions for tensionless strings, (the T → 0 limit of the ordinary bosonic string). We find Gμν and Bμν type interactions but no dilaton interactions.

Vanishing of gravitational particle production in the formation of cosmic strings

We consider the axially symmetric coupled system of gravitation, electromagnetism and a dilaton field. Reducing from four to three dimensions, the system is described by gravity coupled to a non-linear