A simulation of the black hole merger named GW190521 © N. Fischer, H. Pfeiffer/A. Buonanno/Max Planck Institute for Gravitational Physics/AFP

Out of the ancient cosmic darkness swirled the impossible. That is one way to describe the biggest recorded collision of two black holes, a cataclysmic event detected in May last year but revealed by scientists only last week.

The distant merger, 17bn light years away, revealed itself through the gravitational waves that rippled outwards from this violent union, which happened around 7bn years ago and took just a fraction of a second. The waves — wobbles in the space-time fabric of the universe — were detected by the Ligo (laser interferometer gravitational wave) and Virgo observatories based in the US and Italy respectively. These detectors are designed to pick up gravitational ripples induced by the most dramatic events in the universe, such as black holes devouring each other or huge stars exploding.

The event, named GW190521, challenges our understanding of these extreme objects. So-called stellar black holes are the corpses of stars; when massive stars run out of fuel, they collapse in on themselves. In some cases, this contraction creates a core so dense that nothing, not even light, can escape its gravitational grasp. Our own sun, whose solar mass provides the benchmark for star sizes, is not massive enough to become a black hole in its dotage.

GW190521 captures the moment when a black hole the size of 85 suns barged into one the size of 66 suns. These are the biggest black holes that have ever been “observed” colliding. The swollen black hole left behind is the size of 142 suns, with the missing solar masses lost as energy. 

The thrill of the observation comes wrapped in mystery: the two original black holes should not even exist, according to some. Because of the competing forces that play out in dying stars of different masses, it has been long thought theoretically impossible for stellar black holes to form in a narrow mass gap of between about 65 and 120 solar masses. Both participants in this set-up defied that convention.

“We can reconcile the smaller one but even if we tweak the parameters we shouldn’t find one around 85 solar masses,” said Alberto Vecchio, director of the Institute of Gravitational Wave Astronomy at the University of Birmingham, UK, and a member of the Ligo team. The professor remembers the observations sparking excitement within an hour of being recorded. Future research will investigate how the universe engineers these “impossible” objects, perhaps through collisions of smaller ones.

Beyond that, the clash of these two light-slurping titans produced a black hole that, at 142 solar masses, is in a class of its own. Stellar black holes, up to a few tens of solar masses, lie at the bijou end of the size spectrum; supermassive black holes, found at the centre of galaxies and at least as big as 1m suns, lie at the other. Supermassives are thought to form in a completely different way from their puny stellar cousins, perhaps through gas collapse at the heart of galaxies. GW190521 provides the strongest evidence yet of a so-called intermediate mass black hole, lying in an observational desert stretching between roughly 100 and 100,000 suns.

The allure of black holes, Prof Vecchio thinks, comes from the fact that “they are monster objects but we can comprehend them on a human scale”. A black hole of 100 solar masses would have a radius of around 200 miles, he explains, about the distance between London and Manchester. “Imagine taking a sphere of that radius and packing 100 suns into it.”

Even the sharpest minds struggled with cosmic concepts. Albert Einstein first predicted gravitational waves in 1916 but later doubted his calculations. How gratified he would have been, after his own wobble, to see that the universe wobbles too.


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