Ring Wormholes in D-Dimensional Einstein and Dilaton Gravity

On the basis of exact solutions to the Einstein-Abelian gauge-dilaton equations in $D$-dimensional gravity, the properties of static axial configurations are discussed. Solutions free of curvature singularities are selected; they can be attributed to traversible wormholes with cosmic string-like singularities at their necks. In the presence of an electromagnetic field some of these wormholes are globally regular, the string-like singularity being replaced by a set of twofold branching points. Consequences of wormhole regularity and symmetry conditions are discussed. In particular, it is shown that (i) regular, symmetric wormholes have necessarily positive masses as viewed from both asymptotics and (ii) their characteristic length scale in the big charge limit ($GM^2 \ll Q^2$) is of the order of the ``classical radius" $Q^2/M$.Comment: Latex file, 15 page

Wormhole cosmic strings

We construct regular multi-wormhole solutions to a gravitating $\sigma$ model in three space-time dimensions, and extend these solutions to cylindrical traversable wormholes in four and five dimensions. We then discuss the possibility of identifying wormhole mouths in pairs to give rise to Wheeler wormholes. Such an identification is consistent with the original field equations only in the absence of the $\sigma$-model source, but with possible naked cosmic string sources. The resulting Wheeler wormhole space-times are flat outside the sources and may be asymptotically Minkowskian.Comment: 17 pages, LaTeX, 4 figures (hard copy available on request

Hidden symmetry of the three-dimensional Einstein-Maxwell equations

It is shown how to generate three-dimensional Einstein-Maxwell fields from known ones in the presence of a hypersurface-orthogonal non-null Killing vector field. The continuous symmetry group is isomorphic to the Heisenberg group including the Harrison-type transformation. The symmetry of the Einstein-Maxwell-dilaton system is also studied and it is shown that there is the $SL(2,{\bf R})$ transformation between the Maxwell and the dilaton fields. This $SL(2,{\bf R})$ transformation is identified with the Geroch transformation of the four-dimensional vacuum Einstein equation in terms of the Ka{\l}uza-Klein mechanism.Comment: 5 page

Density modulations in an elongated Bose-Einstein condensate released from a disordered potential

We observe large density modulations in time-of-flight images of elongated Bose-Einstein condensates, initially confined in a harmonic trap and in the presence of weak disorder. The development of these modulations during the time-of-flight and their dependence with the disorder are investigated. We render an account of this effect using numerical and analytical calculations. We conclude that the observed large density modulations originate from the weak initial density modulations induced by the disorder, and not from initial phase fluctuations (thermal or quantum).Comment: Published version; 4+ pages; 4 figure

Gravitating Chern-Simons vortices

The construction of self-dual vortex solutions to the Chern-Simons-Higgs model (with a suitable eighth-order potential) coupled to Einstein gravity in (2 + 1) dimensions is reconsidered. We show that the self-duality condition may be derived from the sole assumption $g_{00} = 1$. Next, we derive a family of exact, doubly self-dual vortex solutions, which interpolate between the symmetrical and asymmetrical vacua. The corresponding spacetimes have two regions at spatial infinity. The eighth-order Higgs potential is positive definite, and closed timelike curves are absent, if the gravitational constant is chosen to be negative.Comment: 11 pages, LaTe

Analytical treatment of critical collapse in 2+1 dimensional AdS spacetime: a toy model

We present an exact collapsing solution to 2+1 gravity with a negative cosmological constant minimally coupled to a massless scalar field, which exhibits physical properties making it a candidate critical solution. We discuss its global causal structure and its symmetries in relation with those of the corresponding continously self-similar solution derived in the $\Lambda=0$ case. Linear perturbations on this background lead to approximate black hole solutions. The critical exponent is found to be $\gamma = 2/5$.Comment: 22 pages, 6 figures. Major changes in the discussions of Sects. 2 and 5. The value of the critical exponent has been revised to \gamma = 2/

Multi-Black-Holes in Three Dimensions

We construct time-dependent multi-centre solutions to three-dimensional general relativity with zero or negative cosmological constant. These solutions correspond to dynamical systems of freely falling black holes and conical singularities, with a multiply connected spacetime topology. Stationary multi-black-hole solutions are possible only in the extreme black hole case.Comment: 8 pages, \LaTex, 4 figures (available on request), GCR 94/02/0

Fast production of Bose-Einstein condensates of metastable Helium

We report on the Bose-Einstein condensation of metastable Helium-4 atoms using a hybrid approach, consisting of a magnetic quadrupole and a crossed optical dipole trap. In our setup we cross the phase transition with 2x10^6 atoms, and we obtain pure condensates of 5x10^5 atoms in the optical trap. This novel approach to cooling Helium-4 provides enhanced cycle stability, large optical access to the atoms and results in production of a condensate every 6 seconds - a factor 3 faster than the state-of-the-art. This speed-up will dramatically reduce the data acquisition time needed for the measurement of many particle correlations, made possible by the ability of metastable Helium to be detected individually

Kaluza-Klein and Gauss-Bonnet cosmic strings

We make a systematic investigation of stationary cylindrically symmetric solutions to the five-dimensional Einstein and Einstein-Gauss-Bonnet equations. Apart from the five-dimensional neutral cosmic string metric, we find two new exact solutions which qualify as cosmic strings, one corresponding to an electrically charged cosmic string, the other to an extended superconducting cosmic string surrounding a charged core. In both cases, test particles are deflected away from the singular line source. We extend both kinds of solutions to exact multi-cosmic string solutions.Comment: 26 pages, LaTex, no figure