Supermassive black holes are thought to be at the center 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’re 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 going all smashy smashy 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’ve observed stellar mass black holes merging, but we’re not sure if their supermassive
counterparts can join forces megazord-style too.
It seems odd, I mean, these things each have an incredible gravitational pull, why wouldn’t
they ram 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,
like you and your crush at an 8th grade dance.
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 I said this is the closest we’ve seen two this big and they’re 1,400 light-years
away from each other?
1 parsec is way too close for us to distinguish two supermassive black holes apart.
And now that we’ve found these two, it’s not like we can wait around 2.5 billion years
to see if they merge.
I’ll probably be dead by then.
But since we’ve 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’re 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.
Thanks for watching, don’t forget to subscribe for more videos, like Amanda’s here on what
a black hole actually looks like.
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.
Well that’s all for now, I’ll see you next time on Seeker.