When two black holes collide, the gravitational waves the titanic crash creates spread out across the universe, much like the ripple effect when dropping a pebble into a pond. Eventually, these waves wash over the Earth. Then, astronomical tools, like the Laser Interferometer Gravitational-Wave Observatory, or LIGO, pick them up.
But so far, the LIGO Observatory has only caught the gravitational waves from stellar-mass (less than 100 times the mass of the sun) black hole mergers. There is, however, a larger class of black holes that researchers have yet to hear smashing into each other. These are supermassive black holes. They clock in at millions or even billions of times the mass of the sun. Now, cosmologists believe that they have detected a pair of these giants locked in a death spiral, as Discover reports.
These particular supermassive black holes are about 800 million times the mass of the sun. They lurk in a galaxy 2.5 billion light-years away. The galaxy itself is the result of a collision between two galaxies. Each contained a supermassive black hole at their centers. Andy Goulding of Princeton University headed up a team that discovered the black holes using the Hubble Space Telescope. They published their findings in The Astrophysical Journal Letters on July 10.
Now that scientists have detected gravitational waves from the collisions of stellar-mass black holes, the “sound” of supermassive black holes colliding should echo in Earth’s observatories. “[Merging] supermassive black hole binaries produce the loudest gravitational waves in the universe,” study co-author, Chiara Mingarelli, from the Flatiron Institute’s Center for Computational Astrophysics, said in a press release.
According to Mingarelli, a supermassive black hole collision should send out gravitational waves “a million times louder” than that of stellar-mass black holes. So why aren’t astronomers hearing them?
A Parsec Problem
It all comes down to a parsec. Anyone who is a “Star Wars” fan is probably familiar with the term “parsec.” In “A New Hope,” Han Solo claims the Millennium Falcon made the famed Kessel Run in 12 parsecs. While this event is science fiction, the parsec is a science fact. Moreover, it plays a vital role in how supermassive black holes might merge or not.
Supermassive black holes exist in the center of most galaxies. So, for a smash-up to take place, two galaxies have to collide first. Then, black holes have to find each other in the chaos. But cosmologists theorize that when supermassive black holes get within one parsec (or about 3.2 light-years) of one another, they reach an impasse.
Scientists believe that the black holes stall because they no longer have the energy to merge. As the black holes travel through space, they eat up dust, gas, and even whole stars. But as they near each other, the black holes have swallowed all the matter around them. Thus, they don’t have the energy to keep going and collide. This issue is called the “final parsec problem.”
The recently discovered supermassive black holes hover at around 430 parsecs (or 1,400 light-years) from one another. In cosmological terms, this is very close, but it will still take 2.5 billion years for them to get to within a single parsec of each other. Because the galaxy is 2.5 billion light-years away, the black holes could be colliding right now. But even traveling at the speed of light, the gravitational waves won’t reach Earth for 2.5 million years.
Solving the Parsec Problem
Overall, the discovery has still offered up a lot of valuable information about how many supermassive black holes might be on a collision course. Goulding’s team estimates that just over 100 pairs of supermassive black holes orbit each other.
Furthermore, the team posits, if a merger doesn’t pop up within the next few years, the final parsec problem may be a real problem. But if a supermassive merger blasts across the universe to Earth, it will put the final parsec problem to rest.