Black holes are among the most mysterious and powerful phenomena in the Universe, with the largest known example—TON 618—boasting a staggering 66 billion times the mass of our Sun. This ultramassive black hole dwarfs even the supermassive black hole at the centre of the Milky Way, which contains a relatively modest four million solar masses.
Most black holes grow through a process called accretion, where matter spirals inward due to gravitational forces. However, this process is far from straightforward. Matter must lose energy through collisions to fall past the ‘event horizon’—the point of no return. This is complicated by the fact that black holes, particularly the ultramassive ones, exert such extreme gravitational forces that they can stretch objects into long strands in a process aptly named ‘spaghettification’.
The growth of a black hole is limited by the Eddington limit, a balance between the gravitational pull inward and the radiation pressure pushing outward from the heated, glowing disc of matter encircling the black hole. This glow is what makes black holes visible to astronomers and is often brighter than entire galaxies, appearing as quasars.
Yet, there may be an ultimate cap on how massive black holes can become via accretion. In 2015, Professor Andrew King of the University of Leicester proposed that a non-spinning black hole cannot exceed 50 billion solar masses through this method. If a black hole spins in the same direction as its galaxy, this limit might extend to 270 billion solar masses. TON 618, already at 66 billion solar masses, challenges these theoretical boundaries.
As black holes grow, their innermost stable circular orbit (ISCO) moves outward. Once the ISCO extends beyond the self-gravitational radius—the point where a galaxy’s gas is more attracted to itself than to the black hole—accretion becomes nearly impossible. Without a disc of matter, these black holes become invisible, no longer glowing or growing.
This suggests we may be entering an era where black holes reach their growth limits. Quasars could fade as these cosmic giants age, and some ultramassive black holes might already be lurking undetected in the Universe, hidden in plain sight.
