The black holes behind the gravitational wave signal recently picked up by LIGO might have been flung out of a cosmic mosh pit.
Gravitational waves are ripples in the fabric of the universe produced by the motion of massive objects like black holes. In February the team behind LIGO, the Laser Interferometer Gravitational-Wave Observatory, announced they had made the first ever detection of a gravitational wave on 14 September last year. The signal, dubbed GW150914, suggested that a pair of black holes 1.3 billion light years away, one 29 times the mass of the sun and the other 36 times, had spiralled together and merged into one.
Lining up such objects is harder than you might think. Although black holes form when stars collapse under their own gravity, and pairs of stars are abundant in the universe, that doesn鈥檛 mean these pairs orbit close enough to turn into black hole binaries that eventually merge.
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Instead, researchers believe these pairs can form in dense star regions known as globular clusters, and now of Northwestern University in Evanston, Illinois, and his colleagues have simulated how this might have happened for the binary black hole picked up by LIGO.
Spit it out
Dense globular clusters naturally form hundreds, even thousands of black holes, the most massive of which gravitate towards the centre a few dozen at a time.
There, they thrash around in a kind of black hole mosh pit, forming pairs and swapping partners until eventually some get spat out and the process repeats. 鈥淥nce those kick each other out, either as single or binary black holes, the next-most massive black holes rush in and start a new mosh pit,鈥� says Rodriguez.
These ejected black holes are close enough to one day crash into each other and produce gravitational waves, says , also at Northwestern University. 鈥淭hey go through this complicated, chaotic dance, but then inevitably the system spits out these pairs of black holes that are exactly the right kind.鈥�
To study how this might have happened for GW150914, the team simulated the evolution of 48 globular clusters. These clusters all started off with around 1 million stars and after around 10 million years formed between 1500 and 3000 black holes, says Rodriguez. Of these black holes, a total of 262 were in binary formations with similar masses to the black holes responsible for GW150914. Only 14 of these binaries were ejected from the cluster around the right time for their merger to match when we saw GW150914.
None of the 14 were formed by pairs of stars, although 12 of the larger 262 set were. Instead, the majority of the 14 were created when a binary black hole encountered a solo one, or two binaries came together, while one was the result of three solo black holes dancing.
A different spin
The results suggest GW150914 could have been produced by such a star cluster, but aren鈥檛 definitive proof.
The simulations don鈥檛 rule out the possibility that a lone pair of stars created the black holes that gave us 聽GW150914, says LIGO team member of Cardiff University, UK. 鈥淲e need a lot more data to determine by which of these two channels binary black holes generally form,鈥� he says. 鈥淚n fact, both of these scenarios might be at work.鈥�
One way to distinguish between the two models is by the spin of the black holes, since binaries that formed in cluster will have different spins, while those that evolved together will be aligned.
Unfortunately the LIGO team wasn鈥檛 able to detect the spins from GW150914, but future signals might give them a better look. 鈥淚f you can tell the difference from the gravitational wave signal, that gives you a strong clue,鈥� says Rasio.
Journal reference: Astrophysical Journal Letters, DOI:
Article amended on 14 June 2016
Correction: Since this article was first published, the number of stars in each simulated globular cluster has been corrected.
