
EU Network Project DescriptionThe project seeks study the most promising sources of gravitational waves expected to be seen by the next generation of gravitational wave detectors under construction. We plan to use large scale numerical codes to study neutron star (NS) and black hole (BH) interactions which generate gravitational waves (GW). We will also develop accompanying approximation approaches, through postNewtonian and perturbative methods, to confirm numerical results where appropriate, identify physical mechanisms operating in different regimes, and to extend fully nonlinear numerical results once they have evolved to a suitable perturbative regime. Full Scale Simulations. We will place simulation technology, developed as the Cactus Toolkit, into our EU Network. The basic methods developed in Cactus for the simulation of any set of partial differential equations (PDE) are: (a) Finite difference methods for solving the PDE. (b) MPI based parallelism, for efficient use of massively parallel supercomputers or large clusters of workstations. (c) Adaptive mesh refinement (AMR), provided in parallel by DAGH. (d) Remote and distributed computing and visualization. (e) Solution of elliptic equations provided by iterative and multigrid methods. (f) Specially designed collaborative framework, so that modules can be developed independently of each other by different combinations of groups. Perturbative Approaches. Perturbative approaches a use suitable background (spherical or axisymmetric) spacetime model, and introduce small threedimensional (3D) perturbations. Equations describing the perturbations will be applied in various ways: (a) By Fourier transforming the equations, quasinormal modes of the systems can be determined, and instabilities can be identified. (b) The numerical evolution of the perturbations as an initial value problem on a fixed background, such as a Kerr BH to compute the resulting GW. (c) Evolutions of perturbations on a time dependent background (e.g. for a collapsing star). In this case both the background and the perturbation equations are evolved simultaneously. The perturbative approach can also be carried out to second order, which can be used to extend the regimes in which perturbation theory is valid and to identify nonlinear effects. PostNewtonian Approximations. The dissipative 3.5N postNewtonian dynamics (radiation reaction at the first postNewtonian wave generation level) and the first postNewtonian GW field will be worked out in a form directly useful for numerical implementations. This results in a selfconsistent treatment of gravitationally damped motion to first postNewtonian order. This treatment can be developed into a full numerical code, that incorporates the most important relativistic effects at this level of approximation. PostNewtonian approaches are also useful to determine better initial data for fully relativistic evolution. The project also leverages other efforts in mathematical and numerical relativity, astrophyiscs, and computational science. It is coordinated through AEI which is the world's largest institue devoted to relativity, with strong programs in classical and mathematical relativity, quantum gravity, and astrophysical and numerical relativity. It is expected that a strong interaction between the Network research team and the AEI staff will develop. The Network research is closely related to the NASA Neutron Star Grand Challenge project, and close cooperations exist between the teams of these projects. The work of the Network will be supported by several very strong efforts in computational science as well: The Cactus Computational Toolkit, designed to facilitate collaborative simulation projects has been developed at AEI, through collaborations with NCSA, Washington University in St. Louis, Argonne National Lab, and a growing list of international collaborators. It is used by a number of relativity and astrophysics codes around the world. Cactus is also the central computational simulation toolkit used in a large scale NSFfunded project to develop the socalled "Astrophysical Simulation Collaboratory. This project aims to build advanced computational tools for astrophysics and relativity, and is a collaboration between AEI, Argonne, NCSA, Rutgers, and WashU. The German DFNVerein has funded a collaborative project, called TIKSL, between AEI, the KonradZuseInstitut, and the RechenzentrumGarching to develop remote and distributed computing and visualization techniques that will be useful for a distributed collaborative project like this EU Network.
