Black hole mystery: The spinning giants that could rewrite astrophysics

When scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) recently announced they had seen a bizarre pair of rapidly spinning black holes, the astrophysics community agreed it defied expectations – and may rewrite our understanding of the universe. 

The announcement – the “binary black hole merger GW231123” – describes unusually large black holes essentially smashing into each other. Each weighs more than 100 times more than our sun, and are spinning, where they would be expected to rotate more slowly, taking them to near the limits of what scientists understand to be physically possible. 

The questions now are: How did these black holes form? Why are they so massive? And why are they spinning so fast?

The 100-year journey to discovery

The journey to all this began when Albert Einstein published his landmark work on the theory of general relativity in 1915. One hundred years later in 2015, scientists observed the merging of two black holes via their emission in gravitational waves.

Much like a stone causes a ripple in a pond, merging black holes create ripples in the fabric of spacetime. Gravitational waves were theorised to be a direct result of Einstein’s theory, but even he had doubts about their physical reality.

Despite the immense masses and energies involved in the collisions that produce these waves (roughly the equivalent of converting all of the sun to pure energy), detecting them here on Earth is a considerable engineering feat.

Incredibly sensitive instruments allow astronomers to probe the stretching and squeezing of the fabric of the universe caused by these merging black holes, which, despite their incredible size, only squeeze space-time by less than the width of an atomic nucleus.

The LIGO Collaboration (along with its partners Virgo and KAGRA) has now observed hundreds of these events, giving astronomers important clues about the nature of the stars in our universe. We think of it as a stellar graveyard of cosmic fossils that tells astronomers how stars lived and died.

The black holes, despite their vast size, the most massive ever observed by LIGO, are still incredibly compact, just 800 kilometres across, roughly the distance from Melbourne to Sydney.

These black holes shouldn’t exist. And yet, here they are.

A delicate balance

What we know is that black holes form from massive stars. While stars are alive, the extreme gravity of stars is balanced by the pressure from intense nuclear reactions. However, as the stars near the end of their lives, they collapse and explode in violent reactions called supernovae, leaving behind a black hole.

This is a delicate balance. If the stars are too light, a supernova will lead to the formation of “neutron stars” – the corpses of dead stars. If the stars are too massive, like the stars that would be needed to form the black holes in GW231123, the physics gets weird.

As the massive star dies, instead of collapsing to a black hole, the star turns itself into a giant nuclear bomb. The result is an incredibly bright giant supernovae, an explosion that is 100 to 1000 times brighter than a regular supernovae.

Crucially, these gigantic explosions leave behind no black hole. 

A secret history of stars?

The presence of massive black holes like those in GW231123 suggests a secret history of stars and black holes. Perhaps they’re formed from the mergers of previous black holes; astronomers aren’t sure.

Moreover, the black holes in GW231123 were also both measured to be spinning close to the speed of light. Highly-spinning black holes aren’t uncommon in the universe, with the supermassive black hole at the centre of our galaxy, Sagittarius A*, estimated to be moving at 60% of its top speed.

However, astronomers predict that black holes in binaries, like those in GW231123, should have little to no spin, and at most, one of the black holes should be spinning. Measurements of both black holes spinning close to the maximum value are highly surprising.

GW231123 forces astronomers to confront a puzzle. Is everything we thought we knew about the lives and deaths of stars wrong?

Future observations of black holes by LIGO and its partners will hold the key.

This article was co-authored by Spencer Magnall, from the School of Physics and Astronomy.