Gravity, Astrophysics, and Strings @ the Black Sea

Plamen Fiziev, Sofia University

The first international conference Gravity, Astrophysics, and Strings @ the Black Sea took place from June 10 till June 16, in a small town on the Bulgarian sea shore, called Kiten. Somebody suggested that if we rename the place by replacing the ``K'' with ``W'', the conference will have much greater success. Actually, a brief account of the lectures presented at the meeting, suggests that the endeavor achieved our main goal - to bring together experienced researchers from the title subjects with younger physicists and students, mostly from the South Eastern corner of Europe. The meeting was organized by the Joint Group in Gravity and Astrophysics, including researchers from various institutions in Sofia. The participants were representing local researchers, students, and scientists from major institutions in roughly equal proportions. As the title suggests, our idea was to gather researchers from relatively distant although related areas and explore the consequences of this diversity.

Edward Seidel (MPI) opened the Conference by giving his first lecture on the Numerical Relativity and Its Applications to Gravitational Wave Astronomy. In total of 4 hour-long lectures the foundations of numerical relativity ware described. The theoretical formulations of Einstein's equations, tools and techniques for analyzing black hole space-times, gravitational waves, boson stars, relativistic hydrodynamics, and other topics were covered. Gabrielle Allen (MPI) introduced the CACTUS Toolkit as a freely available, modular, portable and manageable environment for collaboratively developing parallel, high-performance multi-dimensional simulations. Jorge Pullin (Louisiana State U) presented some new ideas on consistent discretizations in classical and quantum gravity. A very interesting discussion on Supercomputing in Europe completed the first day program. No question about it: Indeed, there was a Welcome Party in the evening.

On the second day, D. Sasselov (Harvard U) acquainted the audience with the first atoms in the Early Universe and the cosmic wave background radiation. Before lunch, U. Sperhake (Aristotle U of Thessaloniki) presented a numerical approach, which enables us to evolve radial oscillations of neutron stars over large amplitude range with high accuracy. In the afternoon, K. Kokkotas (Aristotle U of Thessaloniki) started his series of lectures on the recent results on instabilities of different kind of relativistic stars, and especially of neutron stars. Many new numerical results of different scientific groups were reported. Also, R. Rashkov (Sofia U) gave a review for students on strings, brane worlds, and Ekpyrotic theory. In these lectures a brief but intense review was given of the progress in understanding String theory and its implications to gravity, cosmology and gauge theories.

The third day started with some of the follow-ups of the abovementioned series of lectures as well as a presentation by E. Guendelman (Ben Gurion U) on two measure gravity theories and cosmology, in which the possibility of using a measure of integration independent of the metric was studied. In the afternoon a relaxing excursion took place to the outfall of the nearby Ropotamo river. The remaining program of the day included an account of spherically symmetric braneworld solution given by E. Papantonopoulos (National Technical U of Athens). After reviewing the known black hole solutions on the brane, the induced gravity scenario was discussed, according to which an R(4) term is induced on the brane. To close the scientific program for the day, M. Vavoulidis (Aristotle U of Thessaloniki) gave a talk on rotating relativistic stars.

On the next day, right on time, M. Bander (U of California, Irvine) resurrected time in quantum gravity. By allowing for non zero vacuum expectation values for some of the fields that appear in the Hamiltonian constraint of canonical general relativity a time variable, with usual properties, can be identified; the constraint plays the role of the ordinary Hamiltonian. Before lunch time, E. Horozov (Sofia U) talked about the Weyl algebra and bispectral operators. In the afternoon, another entertaining event took place: The participants in the Conference visited the old towns of Nessebar and Sozopol. These are towns, settled for the first time by the ancient Greeks, with many churches from Byzantine times and old fisherman houses. The closure for the day was provided by the official dinner.

On Friday, M. Todorov (Technical U - Sofia) presented a joint work with T. Boyadjiev, P. Fiziev, and S. Yazadjiev (Sofia U), on a new numerical algorithm for solving a class of BVPs with internal free boundaries. An investigation of numerically models of the static spherically symmetric boson-fermion stars in the scalar-tensor theory of gravity with massive dilaton field was presented. The proper mathematical model of such stars is interpreted as a nonlinear two-parametric eigenvalue problem with an unknown internal boundary. After that, P. Fiziev (Sofia U) gave the first of his pair of lectures on the basic principles of 4D dilatonic gravity and some of their consequences for cosmology and astrophysics. E. Nissimov (Bulgarian Academy of Science) completed the morning session by presenting his work on strings and branes with dynamically generated tension. The properties of a new class of string and p-brane models free of any ad hoc dimensionful parameters were discussed.

P. Bozhilov (Shoumen U) started the afternoon session by presenting his work on probe brane dynamics in nonconstant background fields. A probe p-brane dynamics in string theory backgrounds of general type was considered, with an action, which interpolates between Nambu-Goto and Polyakov type actions. In his talk, B. Ivanov (Bulgarian Academy of Science) acquainted the audience with his work on the maximal bounds on the surface redshift of anisotropic stars. It was shown that for realistic anisotropic star models the surface redshift cannot exceed values, higher than 2, the bound in the perfect fluid case, when the tangential pressure satisfies the strong or the dominant energy condition, respectively. R. Rashkov (Sofia U) completed the program for the day, by presenting his talk on the physical states in vacuum string field theory.

In a spark of scientific enthusiasm, the Conference continued its work on Saturday. P. Fiziev (Sofia U) and R. Rashkov (Sofia U) completed their series of lectures and V. Gueorguiev discussed the subject of relativistic particle and its D-brane cousin. The properties of classical reparametrization-invariant matter systems, mainly the relativistic particle and its generalization to extended objects (D-branes) were presented.

All the organizers were very much satisfied with the overall results from the Conference. We concluded that there is a need for such meetings and started preparation for the next one. One of the main supporting arguments was the very convenient Sofia University Summer House, which hosted the Conference. In accordance with the suggestions from the participants, we are considering building a new Lecture Hall, which will be able to host bigger crowds. So, we hope to see you next year at the Black Sea.

Jorge Pullin 2002-09-23