How do supermassive black holes grow to be millions or billions of times bigger than their more ordinary cousins? No star grows that huge, so they aren't created from the same kinds of supernovae that make stellar-mass black holes. The earliest known galaxies indicate that these monsters were present from nearly the start, meaning that they were unlikely to be normal black holes that slowly gorged their way to supermassiveness. One potential explanation that has been considered is that they were born large and became truly gigantic when two or more collided and merged.

New observations could place stringent limits on that merger rate, however. R. M. Shannon and colleagues used the timing of light from pulsars as a means of measuring gravitational radiation. Gravity waves should be generated by pairs of black holes before and during their collisions. If there were a lot of mergers, the waves would create a noticeable fluctuation in the pulsar timing. But the new results are inconsistent with the merger rate predicted by the most widely accepted theoretical models, suggesting that either binary black holes don't collide as often as expected or that some other mechanisms for their growth are at work.

Supermassive black holes (SMBHs), which are hundreds of thousands to billions of times more massive than the Sun, lie at the center of nearly every large galaxy. These objects frequently drive powerful jets of matter, which (if the alignment is right) astronomers observe as quasars. The most distant quasars indicate that SMBHs have existed nearly as long as galaxies. Their large mass and early existence indicate that they could not have formed from the explosions of large stars, which is the mechanism by which stellar-mass black holes are born.

As a result, it's likely that SMBHs were born supermassive through one of several plausible scenarios involving the rapid collapse of matter during the early moments of galaxy formation. However, observations show that galaxies collide and merge after birth; the largest galaxies were likely produced by such processes. Based on that information, astronomers hypothesized that the galaxies' SMBHs also merge during galactic collisions.

Observations of binary SMBHs in several galaxies support that hypothesis. However, mergers are slow compared to the human life span, so the black holes we see locked in mutual orbit are a long way from collision. They will do so eventually, however, because energy is carried away from them as they churn space-time up in waves known as gravitational radiation. With every bit of energy radiated, the black holes grow closer until they finally merge. While astronomers have not observed orbital shrinkage (and have not detected gravitational waves directly), they have seen this process occur in binary pulsars.

However, binary SMBHs are rare in the astronomical data, partly because it's hard to distinguish a single black hole from a pair at large distances. So researchers resort to theory and computer simulations to predict how many black hole binaries there must be. Those models are primarily concerned with galaxy formation and mergers, but presumably many collisions will result in SMBH pairs. Estimates indicate that binaries should be common enough to create a gravitational wave background that fills the Universe with faint gravitational radiation.

The problem is that gravity is the weakest force, and the wavelengths of radiation from SMBH binaries are huge. This pushes them out of the realm of detection by ground-based gravitational observatories like LIGO (the Laser Interferometer Gravitational-wave Observatory).

However, astronomers realized that millisecond pulsars, which rotate hundreds of thousands of times each second, are extremely accurate timing devices: their flashes arrive at Earth with almost no variation. If gravitational waves were to pass through pairs of millisecond pulsars, they would produce a very slight variation in the relative timing we measure. That's the principle behind pulsar timing arrays: radio telescopes that compare the timing from multiple millisecond pulsars widely separated in space. While pulsar timing arrays aren't good enough yet to detect individual gravitational waves (from supernovae or colliding pulsars, for example), they should be adequate to measure the gravity wave background from binary black holes.

The authors of the present paper compared the predictions of the gravitational wave background to observations made using the Parkes Telescope in Australia. They found that the background was too weak to show up in the data, which implies that either supermassive black hole binaries are more rare than predicted or the gravitational waves they emit don't fit the expected profile.

If the gravity wave background profile is wrong, it could be because many black hole binaries orbit in eccentric paths. That would tend to produce more widely varied gravitational radiation as opposed to a steady flow, defeating detection using pulsar timing. Another possibility is that the environment of the black holes provides some friction, slowing the orbital speed and causing mergers before too many gravity waves are produced.

At this point, it's difficult to know which explanation is the correct one. Nevertheless, pulsar timing arrays could provide the best means of studying black hole binaries in distant galaxies, giving us a look at galaxy collisions in the Universe's early history.

Science, 2013. DOI: 10.1126/science.1238012 (About DOIs).