Scale invariance and critical gravitational collapse

We examine ways to write the Choptuik critical solution as the evolution of scale invariant variables. It is shown that a system of scale invariant variables proposed by one of the authors does not evolve periodically in the Choptuik critical solution. We find a different system, based on maximal slicing. This system does evolve periodically, and may generalize to the case of axisymmetry or of no symmetry at all.Comment: 7 pages, 3 figures, Revtex, discussion modified to clarify presentatio

Critical collapse of a massive vector field

We perform numerical simulations of the critical gravitational collapse of a massive vector field. The result is that there are two critical solutions. One is equivalent to the Choptuik critical solution for a massless scalar field. The other is periodic.Comment: 7 pages, 4 figure

Choptuik scaling in six dimensions

We perform numerical simulations of the critical gravitational collapse of a spherically symmetric scalar field in 6 dimensions. The critical solution has discrete self-similarity. We find the critical exponent \gamma and the self-similarity period \Delta.Comment: 8 pages, 3 figures RevTe

Exact static solutions in four dimensional Einstein-Maxwell-Dilaton gravity

Classes of exact static solutions in four-dimensional Einstein-Maxwell-Dilaton gravity are found. Besides of the well-known solutions previously found in the literature, new solutions are presented.It's shown that spherically symmetric solutions, except the case of charged dilaton black hole, represent globally naked strong curvature singularities.Comment: 8 pages, late

Janis-Newman-Winicour and Wyman solutions are the same

We show that the well-known most general static and spherically symmetric exact solution to the Einstein-massless scalar equations given by Wyman is the same as one found by Janis, Newman and Winicour several years ago. We obtain the energy associated with this spacetime and find that the total energy for the case of the purely scalar field is zero.Comment: 9 pages, LaTex, no figures, misprints corrected, to appear in Int. J. Mod. Phys.

Thick planar domain wall: its thin wall limit and dynamics

We consider a planar gravitating thick domain wall of the $\lambda \phi^4$ theory as a spacetime with finite thickness glued to two vacuum spacetimes on each side of it. Darmois junction conditions written on the boundaries of the thick wall with the embedding spacetimes reproduce the Israel junction condition across the wall in the limit of infinitesimal thickness. The thick planar domain wall located at a fixed position is then transformed to a new coordinate system in which its dynamics can be formulated. It is shown that the wall's core expands as if it were a thin wall. The thickness in the new coordinates is not constant anymore and its time dependence is given.Comment: 11 pages, to appear in IJMP

The need for dark matter in galaxies

Cooperstock and Tieu have proposed a model to account for galactic rotation curves without invoking dark matter. I argue that no model of this type can work

Scaling of curvature in sub-critical gravitational collapse

We perform numerical simulations of the gravitational collapse of a spherically symmetric scalar field. For those data that just barely do not form black holes we find the maximum curvature at the position of the central observer. We find a scaling relation between this maximum curvature and distance from the critical solution. The scaling relation is analogous to that found by Choptuik for black hole mass for those data that do collapse to form black holes. We also find a periodic wiggle in the scaling exponent.Comment: Revtex, 2 figures, Discussion modified, to appear in Phys. Rev.

Black Hole Remnants and the Information Puzzle

Magnetically charged dilatonic black holes have a perturbatively infinite ground state degeneracy associated with an infinite volume throat region of the geometry. A simple argument based on causality is given that these states do not have a description as ordinary massive particles in a low-energy effective field theory. Pair production of magnetic black holes in a weak magnetic field is estimated in a weakly-coupled semiclassical expansion about an instanton and found to be finite, despite the infinite degeneracy of states. This suggests that these states may store the information apparently lost in black hole scattering processes.Comment: 16 pages, revision has 5 figures uuencode

Pair creation of black holes joined by cosmic strings

We argue that production of charged black hole pairs joined by a cosmic string in the presence of a magnetic field can be analyzed using the Ernst metric. The effect of the cosmic string is to pull the black holes towards each other, opposing to the background field. An estimation of the production rate using the Euclidean action shows that the process is suppressed as compared to the formation of black holes without strings.Comment: 7 pages, LaTeX. Minor typos corrected