Gravitational Waves Reveal How Supermassive Black Holes Gain Weight

Supermassive black holes are located in the centers of large galaxies. For years, though, astronomers have wondered exactly how these black holes grow to be so big. Now, scientists may have discovered the answer behind their astounding size by using CSIRO's Parkes radio telescope in eastern Australia.

"This is the first time we've been able to use information about gravitational waves to study another aspect of the Universe--the growth of massive black holes," said Ramesh Bhat, co-author of the new study, in a news release. "Black holes are almost impossible to observe directly, but armed with this powerful new tool we're in from some exciting times in astronomy. One model for how black holes grow has already been discounted, and now we're going to start looking at others."

Gravitational waves, which are ripples in space-time that are generated by massive bodies changing speed or direction, can be cause by pairs of black holes orbiting each other. When galaxies merge, their central black holes are doomed to meet; first they swirl around one another before merging, emitting gravitational waves at a frequency that astronomers should be able to detect. This particular phenomenon plays out again and again across the universe.

In this case, though, astronomers have been searching for gravitational waves by examining a set of 20 small, spinning stars called pulsars. Pulsars act as extremely precise clocks in space. The arrival time of their pulses on Earth are measured with exquisite precision, to within a tenth of a microsecond. When gravitational waves roll through an area of space-time, they temporarily swell or shrink the distances within that region, altering the arrival time of the pulses on Earth.

So what did the scientists find? They've been able to discover how low the background rate of gravitational waves is. In fact, they were able to test four models of black hole growth. In the end, they ruled out that black holes gained mass only through mergers; however, the other three models are still a possibility.

The findings reveal a little bit more about black hole growth. More specifically, they provide another step toward understanding exactly how these supermassive black holes grow so large.

The findings are published in the journal Science.