Paired pulsars put upper limit on background gravitational radiation

Ars Technica : How supermassive black holes form at the centers of galaxies is unknown. One theory is that they result from the collision and merger of two or more large black holes. Observations of merging galaxies have shown supermassive black holes in orbit around each other. Because of the release of gravitational radiation that slowly shrinks their orbits, those supermassive black holes will eventually collide. Estimates suggest that there should be enough of those binary systems to fill the universe with a certain level of background gravitational radiation. An international team of astronomers attempted to detect that radiation by looking for variations in the timing of light from pairs of millisecond pulsars. However, their observations found no evidence of gravitational radiation in the timing variations. That means the amount of gravitational radiation is much less than predicted. While several possible explanations have been proposed, further study is necessary to determine which is correct.