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Background and Motivation

The primary motivation for solving the binary black hole problem in numerical relativity is to supply waveforms for gravitational wave detectors. Cross-validation of waveforms between different groups (and codes) and comparison with post-Newtonian predictions will be essential for numerical waveforms to be used in the computationally expensive searches conducted by the international gravitational wave community. The importance of cross-validation of numerical relativity results as a community effort has prompted the formation of the Apples with Apples Alliance, whose purpose is to coordinate and bolster the efforts of the various groups developing numerical relativity codes. The major activities of the Alliance are to formulate standardized tests for code comparison, document and archive the test results and analyze their implications for the design of improved codes.

Establishing a paradigm for standardized testbeds for numerical relativity is a formidable task in itself. First of all, it is important to realize that the numerical relativity community is small, with very limited available manpower. In contrast to the size of the field, we are trying to solve many difficult problems at the same time. Numerical methods are being developed in parallel with the formulation of the continuum problem, with the construction of physically relevant initial data sets, and with the unraveling ofthe physical processes involved in the systems under investigation. All of this is, so far, without the help of comparison with experiments. Groups working in the field are faced with many fundamental questions in designing their approaches, and codes are in a state of flux that makes careful documentation easy to postpone.

We propose to build up a suite of standardized testbeds for comparing approaches to the numerical evolution of Einstein's equations, that are designed to both probe their strengths and weaknesses, and to separate out different effects, and their causes, seen in the results. We distinguish two fundamentally different types of testbed: The first type compares different codes and methods in the treatment of a physically interesting set of solutions. In the context of the binary black hole problem, a detailed comparison of nonspinning equal-mass inspiral would be a natural example. The second type are idealized situations, such as the shock tube test in computational fluid dynamics.

The ideas for this program originated in informal discussions at the numerical relativity workshop at Krugersdorp in South Africa, 2001. An initial meeting was held at UNAM, Mexico City, in May 2002.

These web pages are a resource for a number of numerical relativists from different groups, who intend to participate in this project of code-comparison. The intent of the web site is to facilitate the exchange of information and results. It also forms a central repository of useful notes, links, and tips, for working relativists, and we welcome everyone in the community to make use of these pages and hopefully even contribute their own results and ideas.

Please contribute to these pages!

Image taken from Universe Today