Scientists have discovered two supermassive black holes, locked together in a final, terrible spiral. They’re about to collide. And when they do, it will shake the fabric of spacetime itself.
Combing through decades of radio telescope observation data, a team of Caltech astronomers discovered a radio pattern from the deep sky unlike anything ever observed before. It was a flickering point of light, a blazar some nine billion light-years away. Every five years, it waxed and waned in brightness in a perfect sine wave, like clockwork. But that’s not what made it special. What made it special is where the signal diverged from the pattern. Over nearly fifty years, this point of light had obeyed a clockwork cycle of five-year pulses — except for the twenty years where it didn’t.
Five other observatories confirmed the readings, including the University of Michigan Radio Astronomy Observatory, MIT’s Haystack Observatory, the National Radio Astronomy Observatory (NRAO), Metsähovi Radio Observatory in Finland, and NASA’s Wide-field Infrared Survey Explorer (WISE) space satellite. This was no error.
While pooling data showed that the oscillation of the light formed a clear and distinct pattern, the mystery remained. In the middle of that elegant sine wave, there’s a big wobble, a dimming that lasted two decades. The fluctuation didn’t match any known astronomical phenomenon. What could explain this departure from the pattern?
“When we realized that the peaks and troughs of the light curve detected from recent times matched the peaks and troughs observed between 1975 and 1983, we knew something very special was going on,” says Sandra O’Neill, lead author of a new study describing the astronomers’ observations. But its identity would come from a surprising direction: not astrophysics, but simple orbital dynamics.
When black holes collide
Galaxies often have black holes at their centers. For example, our own Milky Way has a supermassive black hole named Sagittarius A* at its core. When black holes have cleared their stellar environs, they’re invisible, almost undetectable. Sometimes, they put on a light show: A quasar is the active core of a galaxy, powered by a supermassive black hole and illuminated by glowing-hot material falling in from its accretion disk. Certain quasars eject a jet of plasma or raw neutrons — when their beams point straight at Earth, we call them blazars.
There are internal phenomena that can cause a blazar to change in brightness, but none of them was a match. But study coauthor Roger Blandford had a different idea. He proposed a much simpler explanation: instead of an internal phenomenon, it was orbital dynamics. The fluctuations made sense if the point of light was not one entity, but two galaxies, about to collide. One had a “regular” supermassive black hole at its core; the other, a blazar. Instead of being pointed dead-on at us along our line of sight, the blazar’s jet was perhaps five degrees off-axis. As the black hole and blazar whip around their common gravitational center, the blazar moves in a circle. Like a flashlight, sometimes it’s in our eyes more than others.
At the top center of the image, you can see one supermassive black hole of the binary pair. Below it, the blazar, shown here with its relativistic jet.
“We knew this beautiful sine wave had to be telling us something important about the system,” said project mentor Tony Readhead. “Roger’s model shows us that it is simply the orbital motion that does this. Before Roger worked it out, nobody had figured out that a binary with a relativistic jet would have a light curve that looked like this.”
Making waves
When two galaxies merge, the supermassive black holes at their centers tend to “sink” toward the gravitational center of the new, combined galaxy. As the black holes spiral in toward one another, their enormous masses disturb space and time, kicking up gravitational waves. Eventually, the black holes collide. When they do, they each contribute their already mind-boggling mass, creating a monstrous new black hole at the new galaxy’s core. If this blazar really does have a companion, the next step is to try to detect the other supermassive black hole, using gravitational waves.
The National Science Foundation’s LIGO (Laser Interferometer Gravitational-Wave Observatory), jointly operated by Caltech and MIT, can detect gravitational waves created by colliding black holes of perhaps a few dozen solar masses. When we detected gravitational waves the first time around, LIGO was instrumental to the project. But LIGO can’t see this collision– simply because of the enormous mass of these celestial behemoths. The supermassive black holes at the centers of galaxies have millions to billions of times the mass of our sun. These huge collisions give off lower frequencies of gravitational waves than LIGO can detect. However, the upcoming Laser Interferometer Space Antenna (LISA) mission would detect merging black holes in the range of a thousand to ten million solar masses.
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