Three’s a party. The LIGO collaboration has made its third observation of gravitational waves emanating from a pair of merging black holes – giving us more insight into how these pairs form and building up our catalogue of them.
“The first one was a novelty. The second one was confirmation that the novelty of the first one was not a fluke. The third one is astrophysics,” says LIGO spokesperson David Shoemaker at Massachusetts Institute of Technology (MIT). “We’re making the transition to talking about a population of these objects.”
LIGO detects waveforms, which are readouts of the ripples in the fabric of the universe caused by masses moving through it. The spins of merging black holes can warp those waveforms, which are mostly produced by their orbits and eventual collision.
For the first event, we did not have enough information to determine the direction in which each black hole was spinning. For the second, we had slightly more information, indicating that each black hole was probably spinning in the same direction as they were orbiting one another.
But this third pair of black holes, discovered on 4 January, is tilted differently with respect to Earth than the other two, says Shoemaker, allowing LIGO to see more about how each black hole is spinning.
This view has revealed that the black holes in this new event are not spinning in the same direction as they orbit each other. That means they’re probably doing so in different directions or – far less likely – not spinning at all.
Siblings, not twins
“Spins, and particularly misaligned spins, will help us figure out how these things are formed,” says Carl Rodriguez at MIT. The examination of these objects’ properties rather than just their detection turns this into a “new branch of astronomy”, he adds.
Black hole binaries form in one of two main ways: the two black holes are either born together from a pair of orbiting stars, or they form separately in a dense stellar cluster and later drift together at its centre. In the first case, the pair should rotate in the same direction as they orbit, as binary stars do; in the second, says Rodriguez, “they’re pointing in whatever directions they please”.
Although the second black hole binary, which LIGO detected in December 2015, appeared, from what limited data we had, to come from black holes born orbiting together, these new ones may have grown independently.
At least one of them seems to be spinning in a different direction from its orbit. The fact that this differs from the previous case indicates that both scenarios are possible, though we will need more observations to determine if either is more prevalent.
Because this new black hole binary is about 3 billion light years away – twice as far as the others we’ve seen – the gravitational wave has to ripple through more space before it reaches Earth. That distance allows us to get more insight into potential deviations from Einstein’s theory of general relativity.
General relativity states that all gravitational waves should travel at the same speed – the speed of light. Because the waves seemed to do that in this case, even over such a huge distance, they backed up Einstein’s cosmic rule.
Beyond the specifics of what this particular black hole binary can tell us, it marks a step towards using gravitational waves to study the general population of such binaries and potentially other massive objects.
Eventually, LIGO will also observe other types of cosmic event, but many detections of the same type of event – in this case binary black holes – are crucial to obtaining detailed scientific results.
With just three detections, researchers have already found that there is a population of binary black holes with masses over 25 times that of our sun – a group that we knew nothing about before the LIGO experiment began.
It may seem like LIGO is set in its gravitational-wave-finding ways now, but the charm hasn’t yet worn off, says Laura Cadonati at the Georgia Institute of Technology in Atlanta and the LIGO Scientific Collaboration. “I know it’s not the first time any more, but I’m thrilled that we were able to do this again,” she says.
Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.118.221101