Black hole collisions evade detection

Supermassive black hole duos are awfully quiet. Searches for spacetime ripples radiating away from these spiraling monsters are coming up empty, a new study reports. The silence is providing hints of the complex interactions at play in the final stage of galaxy collisions.

When galaxies collide, the gargantuan black holes in their cores sidle up to one another. As the two spiral together over billions of years, they radiate gravitational waves. Efforts to detect the cacophony of waves from black hole couples throughout the universe should be turning something up by now. New observations, however, indicate that this “gravitational wave background” is quieter than most theories predict, Ryan Shannon, an astronomer at CSIRO Astronomy and Space Science in Marsfield, Australia, and colleagues report. 

The new data, from 11 years of pulsar observations and reported in the Sept. 25 Science, sharply disagree with most calculations of just how loud this black hole chatter should be. That suggests either that black holes aren’t colliding or there’s more than gravitational waves at work, says Maura McLaughlin, an astronomer at West Virginia University in Morgantown not involved with this project.  “Now, it’s much more interesting,” she says.

The results are surprising but “not necessarily baffling,” says study coauthor Vikram Ravi, an astronomer at Caltech. The nondetection doesn’t mean that the spacetime ripples don’t exist. Rather the environment in the centers of galaxies might not be conducive to generating gravitational waves at the predicted levels, he says.

To settle into orbit around one another, pairs of black holes shed energy by kicking gas and stars out of the neighborhood. If the environment is sparse, then some binaries could stall before snuggling up close enough to generate detectable waves, Ravi says. Or the region could be so packed with stars and gas that the black holes spiral together quickly and don’t spend much time emitting gravitational waves.

Ravi and colleagues are using the Parkes radio telescope in Australia to hunt for gravitational waves with pulsars, rapidly spinning cores of dead massive stars. Apparent changes in the steady beat of radio pulses from pulsars can be used to sense spacetime subtly stretching and squeezing as gravitational waves pass Earth. “The best pulsar we timed is so quiet that there’s really no other source of noise that we can’t account for,” Ravi says. “There’s not even a hint of gravitational wave noise. That part is surprising.”

The Parkes pulsar project echoes recent results from a parallel North American effort called NANOGrav, which is observing pulsars with the Green Bank and Arecibo radio telescopes. NANOGrav isn’t as sensitive as Parkes, but is seeing a similar absence of gravitational waves from spiraling supermassive black holes, researchers report online August 12 at arXiv.org.

“Up until very recently, everyone assumed that gravitational waves are all that matters for these guys,” says Sean McWilliams, an astronomer at West Virginia University who, along with McLaughlin, is part of the NANOGrav team. The NANOGrav data favor the idea that stars and gas around the black holes are mucking things up. “The lack of detection suggests we shouldn’t be ignoring those things,” he says.

The pulsar data also roughly agree with a recent analysis of x-shaped radio beacons that crisscross the cores of some galaxies and are thought to form as two black holes coalesce. The radio waves come from opposing fountains of plasma launched away from one of the black holes. Gravitational forces can flip the fountains as the two black holes approach one another. X marks the spot where the old geyser and the new one exist side-by-side.

A closer look at 52 of these intersecting geysers indicates that fewer than about 1 percent of them result from merging black holes, David Roberts, an astronomer at Brandeis University in Waltham, Mass., and colleagues report. That’s about one-fifth of what had been predicted, which suggests that supermassive black holes are colliding a lot less frequently than thought. Those findings appear in the Sept. 1 Astrophysical Journal Letters.

The hunt for signs of black hole smashups is far from over. In the coming year, Parkes and NANOGrav will join forces with a third project, the European Pulsar Timing Array. By combing data from all three experiments, researchers might start to see hints of the gravitational wave background, which is the only way that astronomers have of probing deep into the heart of a galaxy collision endgame.

“The gravitational wave background will enable us to better understand the last step in the evolution of galaxies,” Roberts says, “where large galaxies are made from smaller ones.”