The Universe should be teeming with gravitational 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 gravitational 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 gravitational 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 gravitational 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 gravitational 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.

Unfortunately, the Universe's gravitational wave background is expected to be random, so there'd be no pattern to this distortion. As a result, it would be impossible to tell the difference between gravitational waves and some other form of erratic interference, like an intervening gas cloud.

To get around this problem, astronomers are tracking several pulsars. While the gravitational waves should be random, their influence on different pulsars should be correlated over periods up to 30 years—beyond 30 years, they're far enough apart that they don't emit gravitational waves. So, all it takes is a good telescope and a long period of observations.

That's precisely what the Parkes Pulsar Timing Array has provided for over a decade now. They've just updated their analysis to include 11 years of data from the monitoring of 24 different pulsars. For the present analysis, they've focused on the four pulsars that have the least variability in their timing. Adding any of the noisier ones actually decreased the sensitivity of their study. After creating a model that accounted for typical sources of timing variations ranging from intrinsic behavior of the pulsar to instrument noise, they looked for signs of correlations from gravitational waves.

They found absolutely nothing.

But they found nothing in a useful way. Because of the precision of their measurements, they can actually rule out most of the models of gravitational wave production by binary supermassive black holes. The lack of results from the study rule out these models with "between 91 and 99.7 percent probability."

(Technically, they found something. One of the pulsars had an intrinsic variability that was incompatible with the gravitational wave background and the strongest hint of a signal from a different pulsar. So, they're not sure what this might be, just what it's not.)

Why would the gravitational waves go missing in action? The authors suspect it's most likely due to a problem with our models of black hole mergers. Gravitational waves are only produced after the two black holes get within a critical distance, and they stop after the merger. If that period's shorter than we expect, then the gravitational wave background will be smaller. There are a number of things in the environment—gas clouds, stars, and so on—that could potentially accelerate the merger. So, the authors suggest we revisit the merger process in the presence of additional objects to see if it could account for their results.

But, the authors note one additional possibility: We have not yet tested [Gravitational Wave Backgrounds] expected from alternate theories of gravity."

Science, 2015. DOI: 10.1126/science.aab1910 (About DOIs).