GENERAL RELATIVITY AND GRAVITATIONAL PHYSICS: 16th SIGRAV Conference on General Relativity and Gravitational Physics

Open Strings and Supersymmetry Breaking
View Description Hide DescriptionWe review several mechanisms for supersymmetry breaking in orientifold models. In particular, we focus on non‐supersymmetric open‐string realisations that correspond to consistent flat‐space solutions of the classical equations of motion. In these models, the one‐loop vacuum energy is typically fixed by the size of the compact extra dimensions, and can thus be tuned to extremely small values if enough extra dimensions are large.

Flux and energy modulation of iron emission due to relativistic effects in NGC 3516
View Description Hide DescriptionWe report results from X‐ray observations of the Seyfert galaxy NGC 3516. We detect an emission feature at 6.1 keV in the time‐averaged spectrum, confirming previous results. By applying a new analysis technique, we find that the feature varies systematically in flux at intervals of 25 ks. The peak moves in energy between 5.7 keV and 6.5 keV. The spectral evolution of the feature agrees with Fe K emission arising from a spot on the accretion disc, illuminated by a corotating flare located between 7 and 16 gravitational radii from the central black hole, and modulated by Doppler and gravitational effects as the flare orbits around in the accretion disc. Combining the orbital time and the location of the orbiting flare, the mass of the central hole is estimated to be (1–5) × 10^{7} M _{⊙}, which is in agreement with values obtained from other techniques.

Gravitational Energies and Generalized Geometric Entropy
View Description Hide DescriptionA geometrical covariant definition of the variation of conserved quantities is introduced for Lagrangian field theories, suitable for both metric and affine gravitational theories. When this formalism is applied to the Hilbert Lagrangian we obtain a covariant definition of the Hamiltonian (and consequently a definition of the variation of Energy) for a gravitational system. The definition of the variation of Energy depends on boundary conditions one imposes. Different boundary conditions are introduced to define different energies: the gravitational heat (corresponding to Neumann boundary conditions) and the Brown‐York quasilocal energy (corresponding to Dirichlet boundary conditions) for a gravitational system. An analogy between the behavior of a gravitational system and a macroscopical thermodynamical system naturally arises and relates control modes for the thermodynamical system with boundary conditions for the gravitational system. This geometrical and covariant framework enables one to define entropy of gravitational systems, which turns out to be a geometric quantity with well‐defined cohomological properties arising from the obstruction to foliate spacetimes into spacelike hypersurfaces. This definition of gravitational entropy is found to be very general: it can be generalized to causal horizons and multiple‐horizon spacetimes and applied to define entropy for more exotic singular solutions of the Einstein field equations. The same definition is also well‐suited in higher dimensions and in the case of alternative gravitational theories (e.g. Chern‐Simons theories, Lovelock Gravity).

Inertial effects of an accelerating black hole
View Description Hide DescriptionWe consider the static vacuum C metric that represents the gravitational field of a black hole of mass m undergoing uniform translational acceleration A such that mA < 1/(3∛). The influence of the inertial acceleration on the exterior perturbations of this background are investigated. In particular, we find no evidence for a direct spin‐acceleration coupling.

Particle Dark Matter
View Description Hide DescriptionExtensions of the standard model of particle physics offer a number of interesting candidates for dark matter. We discuss some of these candidates, focussing in particular on the ones predicted by supersymmetric theories. We discuss their cosmological properties and strategies for detection.

Higher Order Curvature Theories of Gravity Matched with Observations: a Bridge Between Dark Energy and Dark Matter Problems
View Description Hide DescriptionHigher order curvature gravity has recently received a lot of attention since it gives rise to cosmological models which seem capable of solving dark energy and quintessence issues without using “ad hoc” scalar fields. Such an approach is naturally related to fundamental theories of quantum gravity which predict higher order terms for loop expansions of quantum fields in curved spacetimes. In this framework, we obtain a class of cosmological solutions which are fitted against cosmological data. We reproduce reliable models able to fit high redshift supernovae and WMAP observations. The age of the universe and other cosmological parameters are recovered in this context. Furthermore, in the weak field limit, we obtain gravitational potentials which differ from the Newtonian one because of repulsive corrections increasing with distance. We evaluate the rotation curve of our Galaxy and compare it with the observed data in order to test the viability of these theories and to estimate the scale‐length of the correction. It is remarkable that the Milky Way rotation curve is well fitted without the need of any dark matter halo, and similar results hold also for other galaxies.

Gravity and Yang‐Mills Fields: Geometrical Approaches
View Description Hide DescriptionA new geometrical framework for a tetrad‐affine formulation of gravity coupled with Yang‐Mills fields is proposed.

Interferometric readout for acoustic gravitational wave detectors
View Description Hide DescriptionA review is given of the optical readout for acoustic gravitational wave detectors, ranging from the working principle, to the experimental data and to the future developments. A summary is also given of the scientific results obtained while developing the optical readout for a bar detector, of interest also for the broader interferometer community.

Propagation of Neutrinos and Photons in Gravitational Fields
View Description Hide DescriptionWe discuss the propagation of bosons and fermions in gravitational fields. The geometrical background is provided by a rotating gravitational source. Use is made of neutrino and photon wave functions that are exact to first order in the metric deviation. These are applied to the lensing of photons and neutrinos and to neutrino helicity transitions and flavor oscillations.

Virgo and the worldwide search for gravitational waves
View Description Hide DescriptionLarge interferometric detectors of gravitational waves are approaching their design sensitivities and the plans for second generation detectors, expected to start the gravitational wave astronomy, are already under way. The goal of this paper is to give an overview of the status and perspectives of gravitational wave research with ground‐based interferometric detectors in the world, with special attention to the French‐Italian detector Virgo. The main design features and the status of largest detectors are reviewed, the main upgrades to the first generation detectors foreseen in the next years are outlined.

Analytical modelling of gravitational collapse
View Description Hide DescriptionAnalytical modelling of gravitational collapse is an open problem in General Relativity. No satisfactory non‐spherical model is known, and even in spherical symmetry our knowledge is still fragmentary. In the present paper status and perspectives in this field of research are briefly reviewed, putting special emphasis on the companion problem of obtaining exact solutions of the Einstein field equations in matter.

The Chaplygin gas, a model for dark energy in cosmology
View Description Hide DescriptionWe review the essential features of the Chaplygin gas cosmological models and provide some examples of appearance of the Chaplygin gas equation of state in modern physics. A possible theoretical basis for the Chaplygin gas in cosmology is discussed. The relation with scalar field and tachyon cosmological models is also considered.

The Double Pulsar binary J0737‐3039: a two‐clocks relativistic system
View Description Hide DescriptionThe double‐pulsar system J0737‐3039 is the most fascinating pulsar discovery of last decade. Its orbital parameters and close distance allow to perform unprecedented tests of theories of Gravity and of the physics of highly condensed matter. The discovery of this system enhances of about an order of magnitude the estimate of the merger rate of double neutron‐star systems, opening new possibilities for the current generation of gravitational‐wave detectors. The high orbital inclination offers the opportunity of using the radio beams from one of the two sources as a probe for studying the magnetosphere of the other. We report the status of the observations and discuss short‐ and long‐term perspectives.

Rotation Effects and The Gravito‐Magnetic Approach
View Description Hide DescriptionThe paper summarizes a review of the current state of theoretical and experimental research on gravito‐magnetic theory and effects. Three slightly different approaches to the problem are described. The observation of the orbital precession both of satellites around the Earth and of pulsars in binary systems is mentioned. On the experimental side GPB is considered. Last, a number of other possibilities are listed.

The EXPLORER and NAUTILUS Gravitational Wave Detectors and Beyond
View Description Hide DescriptionExplorer and Nautilus are the two detectors for gravitational waves that during the last few years have gathered data with the highest duty cycle. In this paper the present status, performances and recent results, obtained with these experiments, will be presented. Furthermore, one perspective of resonant antennas development, when the large‐scale interferometric detectors are starting to operate, will be discussed.

Minisuperspace, WKB and Quantum States of General Relativistic Extended Objects
View Description Hide DescriptionThe dynamics of relativistic thin shells is a recurrent topic in the literature about the classical theory of gravitating systems and the still ongoing attempts to obtain a coherent description of their quantum behavior. Certainly, a good reason to make this system a preferred one for many models is the clear, synthetic description given by the Israel junction conditions. On using some results from an effective Lagrangian approach to the dynamics of spherically symmetric shells, we show a general way to obtain WKB states for the system; a simple example is also analyzed.

Proper‐time regulators and RG flow in QEG
View Description Hide DescriptionA proper time renormalization group equation for Quantum Einstein Gravity is studied in the Einstein‐Hilbert truncation and its predictions are compared to those of the conceptually different exact renormalization group equation of the effective average action. A smooth infrared regulator of a special type is known to give rise to extremely precise critical exponents in scalar theories. In particular the proper time equation, too, predicts the existence of a non‐Gaussian fixed point as is necessary for the conjectured nonperturbative renormalizability of Quantum Einstein Gravity.

Three‐Dimensional Chern‐Simons and BF Theories
View Description Hide DescriptionOur aim in this note is to clarify a relationship between covariant Chern‐Simons 3‐dimensional theory and Schwartz type topological field theory known also as BF theory.

Singular PP waves, Junction Conditions and BPS States
View Description Hide DescriptionA simple model to study the collision of PP waves via the Israel junction conditions is proposed. The junction conditions are interpreted as topological conservation laws, and the relation with BPS states is shortly described.