The Universe should be teeming with gravity waves. As near as we can tell, just about every galaxy has at least one supermassive black hole at its core. Most large galaxies were formed by multiple mergers, which would put more than one of these supermassive black holes in close proximity. As they get close enough to start spiraling in towards a merger, their orbital interactions should produce gravity waves. As long as this process doesn't end in a merger too quickly, the Universe's population of merging black holes should fill space with a gravity wave background.
Our Earth-bound detectors aren't sensitive enough to pick this background up. Conveniently, however, nature has provided us with its own detector: pulsars. Unfortunately, a detailed study of a handful of pulsars has failed to turn up any sign of gravity waves, suggesting it might be time to revisit some of our models.
A pulsar is a rapidly spinning neutron star. Each revolution, it sends flashes of light towards Earth, often separated by a handful of milliseconds. The timing of these pulses can sometimes be tracked with a precision of 20 nanoseconds, providing an extremely tight constraint on their expected behavior. If a gravity wave happened to ripple through the right patch of space-time as the light pulse was on its way to Earth, it could distort the timing enough to be detectable.
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