IN THE PRESS

Two new articles accepted in the journal Physical Review D

18 September, 2015

Two new works written in collaboration with the relativity group at the Cardiff University have been accepted for publication by the journal Physical Review D (accessible for free at http://arxiv.org/abs/1508.07250 and http://arxiv.org/abs/1508.07253).

S. Husa, S. Khan, M. Hannam, M. Pürrer, F. Ohme, X. Jiménez Forteza, A. Bohé: Frequency-domain gravitational waves from non-precessing black-hole binaries. I. New numerical waveforms and anatomy of the signal.
S. Khan, S. Husa, M. Hannam, F. Ohme, M. Pürrer, X. Jiménez Forteza, A. Bohé: Frequency-domain gravitational waves from non-precessing black-hole binaries. II. A phenomenological model for the advanced detector era.

These two papers constitute the latest result of our long-term program to describe the gravitational wave (GW) signals emitted during the inspiral and coalescence of the BHs due to radiation reaction. Like BHs, GWs are predictions of Einstein’s general relativity, generated as massive compact bodies dynamically distort the surrounding spacetime. Since September 2015 a new generation of GW observatories based on kilometer-scale laser interferometers is listening to tremors of the space-time geometry, generated by cosmic catastrophies like the collision and merger of two black holes. However, the efficient detection and identification (e.g. the distinction of BHs from neutron stars) of such events relies on accurate waveform models computed within general relativity, which can then be used in a matched filter analysis. For massive BH-BH systems, for which the merger is in band of the detectors, and post-Newtonian perturbative methods are insufficient to provide waveform templates the full Einstein equations need to be solved numerically for selected cases, Waveform models need to be synthesised from numerical and perturbative results, and appropriately interpolated over the 7-dimensional parameter space of mass ratio and spins. This is the goal of our work. Without a detailed modelling of the late inspiral and merger of binary BHs, advanced detectors are unlikely to unfold their full scientific potential, and the possibility of learning from GW observations about stellar evolution, binary populations, and the validity of general relativity will be limited by insufficient theoretical knowledge. However, solving the Einstein equations numerically for the last ten or more orbits of a BH coalescence is numerically expensive, and requires up to several hundred thousand CPU hours for BHs with significant difference in masses or large spins.

The ultimate goal of this work is to construct a waveform model that is sufficiently accurate for detection and parameter estimation for design sensitivity advanced gravitational wave detectors before they come online, and to develop a detailed quantitative understanding of the parameter space and its degeneracies. To this end, over the past few years, we have been carrying out some of the first systematic parameter studies of the gravitational wave signal and other physical properties of coalescing black hole binaries in general relativity, and have established a program of “phenomenological waveform modeling” to construct an increasingly sophisticated series of models, exploring a hierarchy of “principal directions” in the parameter space. Following a model for non-spinning waveforms, we have produced the first analytical model for spinning inspiral-merger-ringdown waveforms, which has been calibrated to numerical solutions of the full Einstein equations [Ajith et al., Phys. Rev. Lett. 106:241101,2011]. A key idea of this model is that for detection and very rough parameter estimation purposes, a single effective spin parameter is sufficient, and the orthogonal direction (essentially the difference in spins) can be neglected. We have recently tested this assumption in more detail [Pürrer+, Phys.Rev.D 88 064007 (2013)], leading to the adoption of a modified effective spin as suggested by [Ajith, Phys. Rev. D 84, 084037 (2011)].