We report about our first tests and results in simulating the last phase of the coalescence and the merger of binary relativistic stars. The simulations were performed using our code Whisky and mesh refinement through the Carpet driver.

Relativistic jet phenomena are most often observed in the vicinityof black holes, where the surrounding plasma accretion plays animportant role in the formation of these jets. The presence of amagnetic field is crucial since it has a significant effect on theaccretive behaviour of a plasma. Primarily, the magnetic field linksthe central source with the ambient plasma and can be considered asa set of wires which can transport energy toward the black hole andaway by means of MHD waves. Moreover, the magnetic field is able tocollimate the plasma flow, which gives rise to a relativistic jetformation. To investigate the behaviour of a magnetized plasmaaccretion around a spinning black hole we use a string approach,which allows to depict the magnetized plasma as a set of magneticflux tubes/string. It turned out that the interaction of themagnetic flux tube with the spinning black hole leads to an energyextraction process, which is attended by a relativistic jetcreation. The influence of the reconnection process on the jetevolution leads to the formation of plasmoids, which move outwardfrom the central source and remove energy and angular momentum. Thisprocess can be repeated over and over and finally the jet structureis composed of a chain of plasmoids which propagate along the spinhole axis.

We discuss the general framework forexploring the cosmological dynamics of Higher Order Theories ofGravity presented in Carloni et al. (2007).

Under the assumption on the fundamental character of the Holographic Principle as a primary principle guiding the behavior of our universe the saturation of the holographic limit is reasonable. On the other hand the Fischler-Susskind holographic prescription seems to be incompatible with closed cosmological models due to the apparently unavoidable recontraction of the particle horizon area. However we will show that the saturation of the Fischler-Susskind holographic prescription over a closed (although almost flat) cosmological model enforces a cosmological evolution very similar to the observed universe.

Binary neutron star merger lead to the formation of amassive, rapidly and differentially rotating neutron star (or astrange star) or to the prompt collapse to a black hole. The maximummass of a differentially rotating remnant is crucial for distinguishingbetween these two final objects. We study the effect of differentialrotation on the maximum mass of neutron stars (strange stars). Wenumerically construct stellar models using a highly accuraterelativistic code based on a multi-domain spectral method. We findmuch larger mass increases for strange stars than for neutron starsfor the same degree of differential rotation.

In this paper we present the mathematical consistency of braneworld theory with a new proof of the fundamental theorem of submanifolds for case of semi-Riemannian manifolds. We find the integrability conditions for the existence of space-time submanifolds in a pseudo-Euclidean space.

We have made evaluations of the α parameter used in the metrictheory f(R) proposed by Sobouti (2007), adjusting the theory to X-rayobservations of 13 clusters of galaxies with gravitational lensing.

The ESA astrometric mission Gaia (launch in late 2011) will be able to put to test General Relativity (GR) also thanks to a differential experiment, GAREX, implemented in the form of repeated Eddington-like measurement, aiming at measuring, for the first time, the quadrupole light bending due to an oblate planet.

Magnetic reconnection plays an important role in space physics, for example, in Earth's magnetosphere, on the Sun, in the magnetospheres ofmagnetars, pulsars, black holes, etc. Reconnection starts with abrupt drop of plasma conductivity in a small part of a current sheet, so called,diffusion region. As a result electric field is generated and is transferred by relativistic MHD surface wave from the diffusion region to thecurrent sheet which leads to decay of the disturbed part of the current sheet into a system of slow shocks. Plasma is highly accelerated andheated at the shock fronts forming outflow region with relativistic plasma jets and weak magnetic field (Semenov & Bernikov1991). At some stage the reconnection process has to switch-off, then outflow regions must detach from the site where the electricfield was initiated, and propagate along the current sheet as solitary waves (Tolstykh et al. 2005). The energy balance ofrelativistic reconnection is investigated in details. It is shown that magnetic and thermal energy from the inflow region is spent foracceleration and heating of the plasma in jets. It is interesting that the temperature of the plasma in the wake of the propagating outflowregions drops after each pulse of reconnection. This differ from usual explosion which heats the plasma behind the shock front (Tolstykh etal. 2007).


We show under which circumstances the gravitationalwaves produced by a metric theory f(R) travel at the velocity oflight.

Energy, and more generally conserved charges, is a subtle issue in general relativity and it is subject to controversy. Indeed, there aremany different definitions in the literature and in some cases they do not all give the same charges. For example, as emphasized byGibbons, et al. (2005), not even in four dimensions do all authors obtain the same expression for the energy of Kerr-AdS black holes and some of these expressions are in disagreement with the first law of black hole thermodynamics. I adopted in [arXiv:0705.0484] the method formulated by Regge & Teitelboim (1974). Based on the Hamiltonian formulation of gravitation, it associates to each asymptotic Killing vector a conserved charge without having to make any arbitrary choice. The charge is then expressed as a surface integral over a 2-sphere at spatial infinity. I investigate here conserved charges and asymptotic symmetries in both asymptotically flat and AdS spaces in arbitrary spacetime dimensions. The study is presented in general coordinates, including the ellipsoidal coordinates that are well adapted to the Kerr solution.

Considering inflationary, ekpyrotic and “dust-like” models we have shown that second order vector perturbations provide new distinguishing observational features between thoseearly univese scenarios.

We are currently building a free (in the sense of a GNU GPL license) 2DRHD code in order to be used for different astrophysical situations. Our final target will be to include strong gravitational fields and magnetic fields. We intend to form a large group of developers as it is usually done for GPL codes.