Extraction of Gravitational Waves in Numerical Relativity Nigel T. Bishop Department of Mathematics, Rhodes University, Grahamstown 6140, South Africa email:
[email protected] http://www.ru.ac.za/mathematics/people/staff/nigelbishop Luciano Rezzolla Institute for Theoretical Physics, 60438 Frankfurt am Main, Germany Frankfurt Institute for Advanced Studies, 60438 Frankfurt am Main, Germany email:
[email protected] http://astro.uni-frankfurt.de/rezzolla July 5, 2016 Abstract A numerical-relativity calculation yields in general a solution of the Einstein equations including also a radiative part, which is in practice computed in a region of finite extent. Since gravitational radiation is properly defined only at null infinity and in an appropriate coordinate system, the accurate estimation of the emitted gravitational waves represents an old and non-trivial problem in numerical relativity. A number of methods have been developed over the years to \extract" the radiative part of the solution from a numerical simulation and these include: quadrupole formulas, gauge-invariant metric perturbations, Weyl scalars, and characteristic extraction. We review and discuss each method, in terms of both its theoretical background as well as its implementation. Finally, we provide a brief comparison of the various methods in terms of their inherent advantages and disadvantages. arXiv:1606.02532v2 [gr-qc] 4 Jul 2016 1 Contents 1 Introduction 4 2 A Quick Review of Gravitational Waves6 2.1 Linearized Einstein equations . .6 2.2 Making sense of the TT gauge . .9 2.3 The quadrupole formula . 12 2.3.1 Extensions of the quadrupole formula . 14 3 Basic Numerical Approaches 17 3.1 The 3+1 decomposition of spacetime .