Greek Relativity Conference, NEB-X

Kostas Kokkotas and Nick Stergioulas, Aristotle University of Thessaloniki, Greece mailto:kokkotas@astro.auth.gr, niksterg@astro.auth.gr

The 10th Greek Relativity meeting has been hosted by the Thessaloniki group in Kallithea, Chalkidiki, an idyllic sea resort near Thessaloniki, from May 29 till June 2, 2002. The series of meetings of the Greek relativity community, named NEB (New Developments in Gravity), was initiated by Basilis Xanthopoulos in 1984. Since then, every second year Greek Relativists working either in Greek universities or abroad meet to present their research work and to discuss recent developments in gravity.

This year's meeting (http://www.astro.auth.gr/gravity/)attracted increased international participation with respect to the previous ones, in an attempt to promote NEB into a regional meeting for South-East Europe. Several of the world's top relativists were invited and delivered plenary talks in Cosmology, Mathematical Relativity, Relativistic Astrophysics, Gravitational-Wave Detection and Quantum Gravity

In the first session, the focus was on Cosmology and Brane Gravity. Roy Maartens gave a review talk on the geometry, dynamics and perturbations of brane-world models. The simplest of these models are able to reproduce the predictions of general relativity at low energies, while introducing interesting new features at high energies - for example in the very early universe, or during gravitational collapse to a black hole, or in cosmic ray showers. A new method for constructing branes of any dimensionality was presented by Nikolaos Batakis while Georgios Kofinas showed an analysis of the induced brane dynamics, when the intrinsic curvature term is included in the bulk action. The invariant description of Bianchi-homogeneous 3-spaces, by considering the action of the automorphism group in the configuration space of real, symmetric and positive definite $3 \times 3$ matrices, was the subject of the talk of Theodosios Christodoulakis. The session continued with Nicolaos Spyrou, who presented work on the conformal-invariance approach of hydrodynamical flows in cosmological models. Leandros Perivolaropoulos in his talk showed that the redshift of pressureless matter density due to the expansion of the universe, generically induces small oscillations in the stabilized radius of extra dimensions (the radion field); low-frequency oscillations lead to oscillations in the expansion rate of the universe, which could naturally resolve the coincidence problem. A dynamical systems approach to scalar field cosmologies has been presented by John Miritzis, who analyzed general mathematical properties of the differential equations describing the evolution of FRW models. Argyris Nicolaidis described how experiments may yield proof of the existence of extra dimensions of space, and Christos Eleftheriadis concluded the session with a presentation of the CAST experiment at CERN, aiming at the detection of axions.

The second session, devoted to Mathematical Relativity, opened with Jiri Bicak's in-depth review of exact models of radiative spacetimes. Sayan Kar argued that the Kalb-Raymond field, coupled gravitationally to the Maxwell field, can lead to a wavelength-independent optical activity in synchrotron radiation from cosmologically distant radio sources. Work aimed at finding interior, anisotropic fluid solutions, matched to the Kerr metric, was reported by Taxiarchis Papakostas, while the conditions under which the nonradial stresses might prevent spherical gravitational collapse were discussed by Petros Florides. Dimitris Tsoubelis presented a new family of integrable nonlinear systems that include the Ernst equation for colliding gravitational waves. Anastasios Tongas discussed geometrical aspects of integrable nonlinear equations of the Schwarzian type. Spyros Cotsakis reported on work with Y. Choquet-Bruhat in which they prove completeness theorems in general relativity under generic geometrical assumptions, while George Papadopoulos presented a talk on an alterative proof of the generality of the Kantowski-Sachs vacuum, based on general coordinate transformations that preserve spatial homogeneity. The definition of conserved quantities in general theories of gravity was discussed by Andreas Zoupas. The session ended with a presentation by Christos Tsagas on how gravitational waves can produce and sustain large-scale magnetic fields, strong enough to seed the galactic dynamo.

The next session was devoted to Astrophysical Relativity and the Detection of gravitational waves. Bernard Schutz gave a thorough introduction on the most promising astrophysical sources of gravitational waves for the new-generation detectors, while on the experimental side Gabriela Gonzalez updated us on the current status of the LIGO detectors. In the same session, Remo Ruffini gave a historical review of the gamma-ray burst puzzle and summarized the current theoretical understanding. 3-D numerical simulations of rotating relativistic stars and the first computation of their quasi-radial modes of pulsation in rapid rotation were shown by Nick Stergioulas, followed by a report by Theocharis Apostolatos on computations of differentially rotating relativistic stars and their pulsations. The effect of quasi-normal mode excitation on the detection of gravitational waves from neutron star binaries was discussed by Emanuele Berti. Kostas Kokkotas argued in his talk that, due to the $r$-mode instability, strange stars can be a good and persistent source of gravitational waves. Uli Sperhake talked about his studies on nonlinear radial oscillations of relativistic stars viewed as deviations from an equilibrium state, and Adamantios Stavridis reported on recent computations of $r$-modes in relativistic stars, showing that discrete modes exist in most cases, due to the coupling of axial and polar terms. Dimitrios Papadopoulos talked on acceleration and cyclotron radiation induced by gravitational waves, Sotirios Bonanos ended this session with a description of the capabilities of his ``Riemannian Geometry and Tensor Calculus'' package for Mathematica, which is freely available at http://www.inp.demokritos.gr/~sbonano/RGTC/RiemannTensorCalculus.html.

The conference concluded with a special review by Jorge Pullin on the stability properties of various discrete versions of Einstein's equations, and their use in the canonical quantization of general relativity.

The next (11th) Greek Relativity meeting will take place in Lesvos, in the summer of 2004 http://www.astro.auth.gr/gravity/NEB.html.