Most galaxies contain a supermassive black hole (SMBH) at their center. When two galaxies merge, their SMBHs start a cosmic waltz, emitting gravitational waves while orbiting each other. This happens again and again across the whole universe. These encounters create a background of gravitational waves, like the noise from a restless crowd.

The astronomers have been searching for this gravitational-wave background by observing a set of small, spinning stars called pulsars for several years. Pulsars act as extremely precise clocks in space. Scientists measure when their pulses arrive on Earth to within a tenth of a microsecond.

As gravitational waves roll through an area of space-time, they temporarily stretch or shrink the relative distances between objects in that region, causing minute variations in the pulse arrival times.

There was no direct detection of a gravitational-wave background. But the obervational data were so good, that the astronomers could set upper limits on the strength of the gravitational-wave background. It depends on how often SMBHs spiral together and merge, how massive they are, and how far away they are. So if the background is low, that puts a limit on one or more of those factors. The paper submitted to Science with these results can be found here: http://www.sciencemag.org/content/342/6156/334.abstract.

The LISA mission will be able to detect the SMBH binaries with total masses of 10,000 to 10 million solar masses throughout almost the entire universe. This will allow us to trace the evolution of the SMBH population and have a close look at how they grow by hierarchical merging, forming bigger and bigger black holes. LISA will also be able to precisely measure their masses and spins, opening a new window for the study of this dar side of the universe.

Source: CSRIO press release, 2013 Oct 17