Merging compact binaries, consisting of two neutron stars, black holes or a combination of the two, are the most likely sources of gravitational waves, a new study finds.

Gravitational waves, which have not yet been detected, are fluctuations in space-time caused by the motion of massive objects, and are predicted in Einstein’s general theory of relativity. They are incredibly weak and decay quickly, but it is possible that with the planned upgrade on the Laser Interferometry Gravitational-Wave Observatory (LIGO) to increase sensitivity, astronomers would be able to spot the waves, if they knew where to look.

New simulations, from Luke Zoltan Kelley, an undergraduate at University of California, Santa Cruz (UCSC) working with Enrico Ramirez-Ruiz, finds that these elusive gravitational waves may turn up in some quite unexpected places.

Compact binaries may hold a clue. The pair of stars could spiral toward each other until they collide, producing dramatic explosions which should produce the gravitational waves. As well as orbiting each other, they are also careering through space together at enormous velocities, which means that their collisions may occur far from the galaxy where they originated.

“Our predictions show that the proposed use of galaxy catalogues for follow-up from possible gravity-wave detections will need to account for the possibility of mergers away from the observed galaxies,” says Ramirez-Ruiz. The scientists hope to match a detection at a gravitational-wave observatory with telescope observations of the same merger event.

The binaries are given their 'kick' out of their home galaxies by asymmetries in the supernova explosions that give birth to the neutron stars and black holes in the first place – an asymmetry of just one percent could give a 'recoil velocity' of about a thousand kilometres per second. But this is the observed velocity for lone neutron stars and pulsars, and in binary systems, though still not exactly known, the velocity would be much less and thought to be around two hundred kilometres per second.

The simulation, which was run on a super-computer at UCSC, used standard cosmological simulations of dark matter and the formation of structure in the Universe to study how different 'kicks' could affect the distribution of colliding binary pairs. After running the program for a simulated 13.8 billion years (the current age of the Universe), Kelley found galaxies very similar to our own, in an area comparable to ours and showed the possible locations of compact binaries. The results showed that small variations in kick velocity can result in large fluctuations in the dispersal of compact binaries.

This research means that these compact binaries could be found away from the brightness of galaxies, and therefore could be detected by survey telescopes. “The operators of gravitational-wave observatories would then know where and when to look in there data for a gravitational-wave signal,” says Ramirez-Ruiz.

The research features in the December 10 issue of Astrophysical Journal Letters.