As far as we know, supermassive black holes are the largest singular objects in the universe. Comprising millions, sometimes even billions of times the mass of our Sun, they are believed to lurk at the centres of most galaxies, including our own Milky Way. Yet despite their immense size, the fact that they don’t emit any light themselves means that we can’t observe them directly.

Instead, astronomers identify supermassive black holes through the effects of their colossal gravity on nearby matter. Two of these effects are especially important.

Firstly, accretion disks of gas and dust orbit around the equators of black holes. This material is heated to extreme temperatures, causing it to emit intense radiation.

Secondly, they emit energetic, cone-shaped outflows of plasma. Fed by their accretion disks, pairs of these jets are ejected along each pole of a rotating black hole, and can extend for thousands of light-years into space. In both cases, accretion disks and plasma jets can be identified visually through images taken in radio wavelengths, providing astronomers with key clues about their speed, temperature, densities, and so on.  

In 2009, a new project was launched to take the first detailed pictures of these monster black holes. It was named the Event Horizon Telescope, or EHT. The scale of the project was monumental. It comprised a global network of radio telescopes, overseen by a collaboration of hundreds of astronomers from around the world.

To produce images, each telescope in the EHT observes the same object simultaneously, with the timing of their data collection synchronised to within just a trillionth of a second. The data are then loaded on thousands of terabyte-hard-disks, which are transported to central processing facilities, where specialised algorithms are used to reconstruct images. Essentially, this creates a single, Earth-sized telescope with an extremely high resolution.

In April 2019, the EHT produced its most iconic observation to date: the supermassive black hole at the centre of the galaxy Messier 87, or M87. In these now-famous images, astronomers could clearly see a bright accretion disk surrounding the central black hole, distorted by the effects of general relativity. However, this incredible achievement would soon be followed by another, far stranger observation.    

In July 2021, the EHT released new images of the centre of the galaxy Centaurus, or Cen A. At just 12 million light-years away, the galaxy is very close to us by cosmic standards. The result was hailed as another triumph for the EHT’s astronomers, and even appeared on the cover page of the journal Nature.

This time, however, the EHT image seemed to lack some of the key features of the radio signals characteristic of supermassive black holes. Not only did its plasma jets have a strange, curved shape, the black-hole and accretion disk system seemed to be missing altogether, whereas they should have been shining very brightly on the image.

To make matters worse, both plasma jets appeared to come out of nowhere, starting some distance from where the black hole and accretion disk should have been. Even worse, the jets seemed to be just empty envelopes of plasma, which heated up away from the central black hole. None of it made any sense.      

For Dr David Cosandey, a Swiss astrophysicist at the Zurich Higher Education Centre and an enthusiastic fan of the EHT project, the image was bitterly disappointing. After the EHT had taken such a perfect image of the core of M87, how could the new image of Centaurus A so clearly lack the distinguishing features of a supermassive black hole, accretion disk, or plasma jets? Had some problem emerged within the deep complexity of the EHT’s reconstruction algorithms, or had astronomers missed something incredibly important?    

Following the image’s publication, a wide array of theories emerged to explain why Cen A’s supermassive black hole appeared to be so strange: perhaps its accretion disk had been hidden by an as-yet undetected gas cloud, or the appearance of its plasma jets had been distorted by complex absorption effects.

But as he considered the problem, Cosandey hit upon a far simpler idea: that Centaurus A doesn’t contain just a single supermassive black hole, but a pair of the massive objects, orbiting around each other.

According to Cosandey’s theory, the pair would have formed around 2 billion years ago, as two earlier galaxies merged to form Centaurus A. Each of these initial galaxies could have carried a supermassive black hole in its core. Eventually, the pair will merge to create a single, even larger black hole – but until then, both black holes will continue to spiral around each other.  

In the EHT’s image, Cosandey noticed two bright dots which couldn’t be explained by any previous theories. Yet according to his theory, these dots had clearly been created by the two separate black holes, and their brightly shining accretion disks. The two individual supermassive black holes are far smaller than that at the centre of M87, explaining why both objects only appear as simple dots.   

In addition, Cosandey’s theory shows that there is no longer just one pair of plasma jets on the picture. Instead, we see two pairs of jets, each connected to a different supermassive black hole.

All of this gives a clear explanation as to why the centre of Cen A appears to be empty: that there really is nothing there at all. The emptiness inside the jets can also be explained as the actual empty space that separates both pairs of jets.

One particularly striking consequence of Cosandey’s theory is that the two supermassive black holes would be closer together than any other binary pair ever observed: spiralling at a distance comparable to the orbit of Neptune around the Sun. If this is the case, the eventual merger between the two objects is far more imminent than for any other known pair.

When the supermassive black holes eventually merge, they will release – in a fraction of a second – a tremendous amount of energy in the form of gravitational waves: unleashing more energy than all stars of the Universe combined during that instant. This explosion will be cataclysmic for Cen A: releasing what Cosandey describes as a “tsunami of spacetime”. Although our own galaxy is far enough away that the merger won’t pose any threat to us, the tsunami could tear apart every star and planet close to the galaxy’s centre.    

For now, Cosandey is confident that his theory will be consistent with subsequent EHT observations of Centaurus A, and is hoping that the EHT project team will release further images of this galaxy in the future. He is also confident that his discovery may be confirmed by future observations relying on other methods: such as detections of gravitational waves coming from other galaxies.

As for naming the pair, Cosandey suggests the names ‘Fenrir’ for the larger supermassive black hole, and ‘Odin’ for the smaller body. In Scandinavian mythology, Odin was swallowed by a giant wolf, Fenrir, at the end of times, during the great battle of Ragnarok. Just like the merger between these two supermassive black holes, this battle would wreak havoc upon the world.

If Cosandey’s theory is confirmed, it could spark a gold rush for astrophysics: inspiring astronomers to gain deeper insights into the enigmatic nature of black holes, and the stretching and squeezing of spacetime as these two monster spheres weighing 40 million times the sun rush around each other at 5% of the speed of light.