Know how a runner might ‘hit a wall’ towards the end of their race? Supermassive black holes can relate. When two of them are travelling towards each other, they slow to almost a complete stop and struggle to move over the final parsec.
Now, a new study reveals that the hero that helps them jump the last hurdle could be dark matter.
That’s because we’ve overlooked a crucial dark matter behaviour, the researchers behind the study say – that dark matter must be able to interact with itself.
“The possibility that dark matter particles interact with each other is an assumption that we made, an extra ingredient that not all dark matter models contain,” said paper co-author Dr Gonzalo Alonso-Álvarez, a postdoctoral fellow at the University of Toronto and McGill University. “Our argument is that only models with that ingredient can solve the final parsec problem.”
What’s the final parsec problem?
The ‘final parsec problem’ is, essentially, that last hurdle that makes the black holes slow almost to standstill before merging.
It was discovered after a study last year observed gravitational waves rippling through the Universe. The ‘hum’ of these was coming from millions of merging pairs of supermassive black holes – each one billions of times more massive than our Sun.
But when the researchers behind the new study, published in the journal Physical Review Letters, looked into it further, they noticed something strange. The merging of the spiralling black holes stalled when they were just one parsec (about three light years) away from each other.
The question is: if the black holes can’t join, where do the gravitational waves come from?
The answer could be that we’ve been missing something crucial in the behaviour of dark matter, they say. In fact, it’s this mysterious behaviour that may enable the supermassive black holes to push past that final parsec and coalesce with their partner.
But how? Firstly, supermassive black holes sit in the centre of most galaxies. When two galaxies collide, these black holes fall into orbit around each other – the gravitational pull tugging at them and slowing them down.
They spiral towards a point of merging – until they are just that one parsec away. Now, their orbit has shrunk down so much that it cannot support its own final collapse. In this way, the supermassive black hole is actually the opposite of a tired track runner: while the runner loses so much energy that they can’t cross the finish line, the black holes have too much energy left for it to go anywhere.
At this point, they start interacting with a ‘halo’ of dark matter. The new model reveals that this halo is so dense that the tiny dark matter particles absorb the remaining orbital energy – therefore degrading the final orbits. Now, the path is clear for the black holes to finally join.
As for gravitational waves, a ‘hum’ detected by the Pulsar Timing Array supports this new model. It’s made up of much longer wavelengths, but is still coming from a supermassive black hole merger – as predicted by Alonso-Álvarez and his team.
“The significance of this paper is that it demonstrates that the final parsec problem can only be resolved if the dark matter is self-interacting,” Pennsylvania State University postdoctoral scholar Dr Bingjie Wang, who was not involved in the study, told BBC Science Focus. “If true, this behaviour of dark matter would have remarkable implications for cosmology and galaxy formation models.”
She added: “The conventional narrative in galaxy formation is that structures form hierarchically – so supermassive black holes are expected to grow from the merger of less massive black holes. By resolving the final parsec problem with self-interacting dark matter, we also fill in a key missing piece here.”
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