Modeling gravitational waves from compact-object binaries

During its first observing run (O1), the Advanced Laser Interferometer Gravitational wave Observatory (LIGO) detected gravitational waves (GWs) emitted by the coalescence of two binary black holes (BBHs), GW150914 and GW151226 [1, 2]. A third candidate event, LVT151012, was also recorded [3], but with not high enough statistical significance to claim a detection. These discoveries opened the possibility of observing and probing the most extreme astrophysical objects in the Universe. These first detections and their detailed characterization represent the culmination of more than a decade of synergy between analytical relativity, numerical relativity (NR) and data analysis. The problem of describing the GW signal generated by a pair of BHs that (quasicircularly) orbit each other and eventually merge into a single BH is challenging because of the different dynamical regimes that this process spans. When the binary is wide —say, as compared to the BH horizons— the component objects move at orbital speeds (in the center-of-mass frame) that are small with respect to the speed of light. During this phase of the coalescence, the post-Newtonian (PN) (i.e., slow-motion and weak-field) approximation to general relativity can be employed to model the orbital dynamics and the associated GW emission (see, e.g., ref. [4] for an extensive review of the current status of PN theory applied to the two-body problem). As the BHs spiral in, plunge and eventually merge, their orbital motion becomes more relativistic and the GW energy flux is stronger. NR techniques are required to obtain highly-accurate waveforms during this stage of the process. State-of-the-art codes can now accurately evolve BBHs for several tens of orbits (∼ 40–60) in large regions of the parameter space [5-11]: i) at large