Gravitational waves are helping us crack the mystery of how pairs of black holes form

Black hole collision and merger releasing gravitational waves.

A tiny disturbance in space became an enormous scientific discovery when LIGO amazingly managed to register it early on the morning of September 14, 2015. This was the first ever observation of a “gravitational wave” – a minute ripple in the structure of spacetime itself – predicted by Albert Einstein a century ago. The signal came from two black holes merging more than a billion light years away, and reached our planet on that very morning.

The detection ushered in a whole new era of astronomy. Two more detections followed (and a third likely one), all from mergers of pairs of black holes. Already, these measurements are starting to help scientists unravel some of the universe’s best-kept secrets. Our new study, published in Nature, shows just how close we are to working out how pairs of black holes form.

The black holes studied by LIGO – each weighing in at between 10 and 30 times the mass of the sun – collide while moving at half the speed of light, twisting space and time as they do so. The merger of two black holes releases more energy in a fraction of a second than all of the stars in the visible universe combined.

LIGO in Louisina. Kanijoman/Flickr, CC BY-SA

However, by the time the spacetime distortions, travelling at the speed of light for more than a billion years, get to the Earth, the ripples are very weak indeed – stretching and squeezing space by less than one part in 1021. That means they make the mirrors in the LIGO detector move by less than a thousandth of the size of an atomic nucleus. No wonder gravitational waves have been so hard to detect.

The incomplete science of black holes

Black holes are infinitely dense remnants of massive stars. Studying them provides astrophysicists with a glimpse into the lives of these stars. And one of the key questions puzzling us since the first gravitational wave detection is: how did these heavy black hole pairs get close enough to merge?

Unravelling the history of how merging black holes formed is important – it can help us to understand the mysterious ageing of massive stars and interactions in dense stellar environments.

There are two broad classes of scenarios that have been proposed so far. The first view holds that two massive stars were born as a pair. They may have interacted by raising tides on each other’s surface, in the way that the moon raises ocean tides on the Earth. Or they may have exchanged gas, with one star blowing off material into space and the other capturing some of it.

Eventually, each star collapsed into a black hole. If the black holes were close enough, then the gradual loss of energy from their orbits in the form of gravitational waves would cause the two black holes to spiral in and eventually merge. This scenario is known as isolated binary evolution.

The other option is that the two black holes formed independently, but did so in an environment where there were many stars closely packed together. In this scenario, known as dynamical formation, a sequence of gravitational interactions with other stars could bring the two black holes to orbit each other.

Numerical simulations of the gravitational waves emitted by the merger of two black holes, including spins (green arrow). NASA/wikipedia

We do not yet know which scenario is correct, but nature has provided an exciting hint. Black holes rotate around their own axes. We know from a few observations of stars orbiting black holes in our own galaxy and its immediate neighbours that sometimes black holes appear to be rapidly spinning. We think that if the black holes seen by LIGO were formed from stars already orbiting each other, these spins should be aligned with the orbit. But if the black holes formed by the gravitational influence of several other stars, the spins would be randomly oriented relative to the orbit – meaning they formed independently in a dense environment.

In a new paper, our team of scientists from the University of Birmingham in the UK and the Universities of Maryland and Chicago in the US, analysed the alignment of the spins and orbits of the merging black hole pairs detected by LIGO. It turns out that the phase of the gravitational waves measured is influenced by the spin of the black holes. A certain component of this spin – known as effective spin – is therefore imprinted in the data.

If this effective spin is large and positive, the black holes are rapidly spinning and rotating in the same direction as the orbit. If it’s large and negative, the black holes are rapidly counter-rotating with respect to the orbit. If it is near zero, then either the black holes’ spins are significantly misaligned with the orbit, or both black holes are spinning slowly.

The LIGO observations of merging black holes so far have found that the effective spin is consistent with zero for all but one observation. Therefore, we concluded that if the black holes are rapidly spinning, the data point to a lack of alignment – and that the black holes were not born from pairs of stars. It does indeed seem likely that the black holes could be rapidly spinning – observations in our galaxy after all suggest this is the case.

We suggest that with as few as ten additional detections, it may be possible to know for sure the origin of black hole pairs. However, it is possible that the merging black holes had a different evolutionary history to the black holes we’ve observed in our own galaxy, and are rotating slowly. If they are, many more observations would be required. Either way, the research goes to show just how important the discovery of gravitational waves really is – opening an entirely new window on the universe.