By now, merging black holes and the gravitational waves they produce are a scientific surety. Astronomers have watched several black hole mergers, between stellar-mass black holes less than 100 times the mass of our Sun. But no merger between supermassive black holes, those with masses or billions times that of our star, have ever been seen; and in fact, astronomers wonder how likely a smash-up would be. Now, the discovery of two supermassive black holes headed for each other could help.

This particular pair, each more massive than 800 million Suns, read 2.5 billion light-years away in a galaxy. The galaxy itself is a reminder – all that's left after two galaxies, each hosting a supermassive black hole, combined. A team led by Andy Goulding at Princeton University made the Hubble Space Telescope and published their discovery The Astrophysical Journal Letters,

Although astronomers often spot merging galaxies, they have never caught a pair of supermassive black holes actually clashing. And yet, "[merging] supermassive black hole binaries produce the loudest gravitational waves in the universe, "said study co-author Chiara Mingarelli, at the Flatiron Institute's Center for Computational Astrophysics, in a press release. Supermassive black hole pairs, Mingarelli said, should send out gravitational waves "a million times louder" than those LIGO and VIRGO have already heard.

The final parsec problem

Supermassive black holes merge a little differently than their lower-mass counterparts. Because they start out very far apart – in two separate galaxies – it takes a long time for the black holes to meet in the middle of a galaxy merger. Astronomers believe the supermassive black holes slowly fall to the center of the final remnant galaxy and begin orbiting each other.

But there's a catch. Some theories state that once the supermassive black holes reach a certain distance – 1 parsec, or about 3.2 light years apart, they stall out and stop. That's because to get closer to each other, the black holes give their energy to other objects, like gas, dust, and stars that get too close. But as the distance between the black holes shrinks, the available space for objects to swoop in and steal energy so shrinks. Within about 1 parsec, astronomers calculate there is not enough space for the amount of "stuff" required.

So from there, the black holes endlessly orbit each other, without coming any closer and merging. Astronomers call this the final parsec problem because it's difficult to get two supermassive black holes any closer than 1 parsec apart. It is possible that a supermassive black hole is present.

So, why not look for supermassive black holes less than 1 parsec apart to see if it can happen? Astronomers certainly would if they could, but at the great distances of other galaxies, current telescopes simply can not separate two objects that are so close together. They look like one object. To add insult to injury, merging into a superficial black hole. Even if it does happen, it's rare, so it's not unusual that it's gravitational waves from a supermassive black hole.

"It's a major embarrassment for astronomy that we do not know about supermassive black holes merge," said co-author Jenny Greene at Princeton. "For everyone in black hole physics, this is a long-standing puzzle that we need to solve."

What, then, can astronomers do to determine what the final parsec problem is? Supermassive black hole pairs should approach each other and merge frequently enough to create a background "hum" of gravitational waves. "This noise is called the gravitational wave background, and it's a bit like a chaotic chorus of crickets in the night," Goulding said. LIGO and VIRGO, though astronomers are looking forward to projects like LISA A space-based gravitational wave detector that "hears" at frequencies.

Alternatively, astronomers can use pulsars – the spilling remnants of massive stars that have collapsed into neutron stars instead of black holes – to pick up on the passage of gravitational waves. Passing gravitational waves stretch and shrink space-time as they go, which would disrupt very easily the regular blips we receive from pulsars. Supermassive black holes can approach each other and merge.

,