The search for gravitational waves
Gravitational waves are ripples in space-time created by astronomical objects, similar to surface waves generated when a stone is thrown into a body of water. While Einstein's theory of general relativity predicts the existence of these unusual waves, they have not as yet been directly detected. However, the ability to 'see' gravitational waves generated by black holes merging can open up a new window into the most energetic events in the universe. When galaxies merge, the supermassive black holes thought to be at their centre will inevitably meet. They first 'dance' together, then enter a desperate 'embrace' before they merge. General relativity predictions suggest that towards the end of their dance, black holes emit gravitational waves. Scientists working on the EU-funded project CBHEO (Connecting numerical simulations of black holes with experiment and observations) have generated 'template' waveforms that should match such astrophysical signals. The observed waveforms are expected to include the signals from the two black holes spiralling towards each other, as well as their merger and the resulting ringdown. The search templates generated within the CBHEO project using numerical relativity and post-Newtonian techniques include all these features. Specifically, the inspiral portion of the waveform was modelled using analytic post-Newtonian calculations. On the other hand, numerical solutions of the general relativity field equations were required to accurately model the final orbits and black holes' merger. These theoretically predicted waveforms have been added to data recoloured to predictions of the sensitivity curves for the Virgo interferometer and the Laser Interferometer Gravitational-Wave Observatory (LIGO). The resulting data were analysed by gravitational wave detection algorithms. These efforts were part of the Numerical Injection Analysis (NINJA) project, whose purpose is to study the ability to detect gravitational waves emitted from merging black holes. CBHEO scientists have also extended the numerical infrastructure for modelling collisions in a space-time with more than four dimensions. Einstein equations in higher dimensions were applied to model parton-parton collisions produced by merging black holes. This study was motivated by the possibility of black holes forming in the particle collision experiments as predicted by new theories of gravity. Numerical relativity in higher-dimensional space-times offered a powerful tool to produce information of relevance for gravity scenarios in which particle collisions could produce black holes. The simulations' main output was the amount of energy and angular momentum lost in gravitational waves. Armed with CBHEO project results, scientists hope to identify in the data collected by the LIGO and Virgo observatories gravitational wave signatures emitted from merging black holes arriving from the nearby universe. In turn, information about gravitational waves could prove to be a powerful new tool to unlock the secrets of black holes that are otherwise hard to observe.
Keywords
Gravitational waves, waveforms, black holes, CHBEO, numerical relativity, LIGO
