Supermassive black holes are thought to be at the centre of most galaxies, and they are huge. The Milky Way’s own supermassive black hole, Sagittarius A*, is about 4 million times the mass of our sun. But scientists have just spotted two absolute behemoths, that dwarf Sagittarius A*, and they are on a collision course. It’s the first time such massive black holes have been spotted this close together, and it could help us detect a hum of gravitational background noise.
Of course “close” is a relative term and in this particular instance when scientists say close, they mean about 1,400 light-years apart. The black holes are located about 2.5 billion light-years from us, so since the light from them took 2.5 billion years to reach us, we are observing them as they were 2.5 billion years ago.
Coincidentally, the scientists who discovered them estimate that that’s about how long it will take before they collide. They could be merging with each other right now, unleashing huge gravitational waves millions of times more powerful than those previously detected by LIGO and Virgo. Of course, because of how far away they are, the waves won’t reach us for 2.5 billion years.
That is if they happen at all. We have observed stellar-mass black holes merging, but we are not sure if their supermassive counterparts can join forces by merging too. It seems odd, these things each have an incredible gravitational pull, why wouldn’t they run head-on into each other?
Right now the thinking is when galaxies merge, their supermassive black holes begin to orbit each other. As they do, dust and stars in between them sap some of their energy, causing their orbits to tighten. But as they get closer, that region of space between them shrinks, until theoretically there’s no way to lose more energy.
The two black holes find themselves stably orbiting each other but never getting closer. Some studies suggest that happens at about 1 parsec, or roughly 3.2 light-years distance, so it’s known as the final parsec problem. But all that is theoretical, and we’re lacking more observational data.
It’s possible our predictions are wrong and black holes of this size do merge instead of stalling out a parsec apart. Unfortunately, black hole pairs are very hard to spot. Remember how we mentioned earlier this is the closest we have seen two this big and they’re 1,400 light-years away from each other?
Because 1 parsec is way too close for us to distinguish two supermassive black holes apart. And now that we have found these two, it’s not like we can wait around 2.5 billion years to see if they merge. we will probably be dead by then. But since we have spotted these two, we can start to guess how common merging supermassive black holes would be.
Based on their findings the scientists estimate that optimistically there are 112 black holes whose gravitational waves we can detect from Earth. This would make a kind of constant hum, the scientists likened this gravitational background noise to a chorus of chirping crickets.
Normally it’d be impossible to distinguish one cricket from another. But if there’s no final parsec problem and they can merge, it should create a massive chirp at the moment they collide. When that happens, the waves will be at frequencies outside what LIGO and Virgo can detect. So instead, scientists will have to keep a close eye on pulsars, special stars that send out radio waves at regular intervals.
If a supermassive merger stretches or compresses the space between us and a pulsar, the rhythm will appear to be thrown off. These frequency changes are so small, just tens to hundreds of Nanohertz, it will require close to a decade of observation to spot the weak signal hiding in the noise.
They are searching for more pairs of black holes to refine their prediction further, but it’s possible we never detect a merger and the final parsec problem is insurmountable after all. And while LIGO can’t detect supermassive mergers, it was recently upgraded, making it 40% more sensitive as it continues its hunt for merging stellar-mass black holes.