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Gravitational waves “know” how black holes grow

Scientists are pitting the front-running ideas about the growth of supermassive black holes against observational data.
RELATED TOPICS: BLACK HOLES | GRAVITATIONAL WAVES
two black holes orbiting each other
An artist's impression of two black holes orbiting each other, generating gravitational waves.
Swinburne Astronomy Productions
Supermassive black holes: Every large galaxy’s got one. But how did they grow so big?

Scientists pit the front-running ideas about the growth of supermassive black holes against observational data — a limit on the strength of gravitational waves from pairs of black holes, obtained with the Commonwealth Scientific and Industrial Research Oganisation’s (CSIRO) 64-meter Parkes radio telescope in eastern Australia.

“For the first time, we’ve used information about gravitational waves as a tool in astrophysics,” said Ryan Shannon with CSIRO. “It’s a powerful new tool. These black holes are very hard to observe directly, so this is a new chapter in astronomy.”

“One model for black-hole growth has failed our test, and we’re painting the others into a corner,” said Vikram Ravi from the University of Melbourne. “They may not break, but they’ll have to bend.”

Einstein predicted gravitational waves — ripples in space-time generated by bodies, such as pairs of black holes orbiting each other, changing speed or direction.

When galaxies merge, their resident central black holes are doomed to meet. They first waltz together, then enter a desperate embrace and merge. “Theorists predict that towards the end of this dance, they’re growling out gravitational waves at a frequency we’re set up to detect,” said Shannon.

Played out again and again across the universe, such encounters create a background of gravitational waves, like the noise from a restless crowd.

Astronomers have been searching for gravitational waves with the Parkes radio telescope and a set of 20 small spinning stars called pulsars. 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 swell or shrink the distances between objects in that region. “That can alter the arrival time of the pulses on Earth,” said Michael Keith of the University of Manchester in the United Kingdom.

The Parkes Pulsar Timing Array (PPTA) project and an earlier collaboration between CSIRO and Swinburne University together provide nearly 20 years’ worth of timing data. “We haven’t yet detected gravitational waves outright, but we’re now into the right ballpark to do so,” said George Hobbs from CSIRO. Combining pulsar-timing data from Parkes with that from other telescopes in Europe and the United States — a total of about 50 pulsars — should give astronomers the accuracy to detect gravitational waves “within 10 years,” Hobbs said. Meanwhile, the PPTA results are showing scientists how low the background rate of gravitational waves is.

The strength of the gravitational wave background depends on how often supermassive black holes 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.

Armed with the PPTA data, the researchers tested four models of black-hole growth. They effectively ruled out black holes gaining mass only through mergers, but the other three models “are still in the game,” said Sarah Burke-Spolaor from the California Institute of Technology in Pasadena.
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