Supermassive black hole found thanks to a gravitational lens

A supermassive black hole has been tracked 2.5 billion years from Earth using a special technique called “gravitational lensing“.

In January 2023, the Hubble Space Telescope observed “live” the spaghettification of a star in a very distant galaxy, whose gaseous material occupied a region of space as large as the entire Solar System.

What is a black hole?

A black hole is a celestial object formed after the death of a massive star (at least 30 solar masses), following an implosion of matter caused by gravitational collapse, which concentrates the fabric of space-time towards a singularity. Because of this property, they have an extremely large and strong magnetic field, and nothing can escape from inside them, neither light nor any kind of information.

Direct observation of a black hole is virtually impossible, as it is an object that shines neither with its own light nor with reflected light, but its event horizon can be observed.

A star at the end of its life will use up all the hydrogen it has and then start burning helium. In the core, all the thermonuclear reactions typical of stars cease, and the star begins to expand or compress, depending on whether the force of gravity is inwards or the pressure of nuclear fusion is outwards.

Not all stars become black holes after their explosion. Small stars, such as the Sun, compress to become white dwarfs and then completely cool over millions of years.

Physical properties of a black hole

Event horizon: boundary surface beyond which no event can affect an external observer. Once you have crossed the outer boundary of the horizon, there is no going back.

Singularity: region of space where the curvature of space-time is infinite. The singularity may be a fixed point in space if it is in a non-rotating black hole, or it may be spread across the fabric of space-time and take the form of a ring if it is in a rotating black hole. In both cases, the singularity has a volume of 0, i.e. infinite density. While falling towards the singularity, an object or observer will be attracted by the ever-increasing gravitational force and will be transformed into a spaghetti due to the phenomenon of ‘spaghettification’.

Photon Sphere: a boundary zone with the event horizon in which photons of light that have fallen beyond the boundary of the horizon are trapped in a circular orbit. Note that some of the photons in this sphere may escape the black hole gravitationally, perhaps as a result of a collision with a particle of matter belonging to the sphere but not to the horizon!

Ergosphere: spheroid-shaped outer boundary zone with the event horizon, where the phenomenon of the ‘drag effect’ of the black hole is present, where all matter is sucked into the photonic sphere and then towards the singularity.

Gravitational lensing: the recent discovery

To discover this massive black hole, scientists used a very special technique called a gravitational lens.

The term ‘gravitational lens‘ refers to a distribution of matter that bends the light coming from distant stars or galaxies behind it, much like a real glass lens. In this case, however, it is the galaxy itself that bends the photons of light. 

Using data from the Hubble Space Telescope in early 2023, it was possible to simulate the journey of light from the galaxy. By including a supermassive black hole of 33 billion solar masses in the simulation, the data collected by Hubble was obtained.

Gravitational lensing, say the scientists, could also be used in the future to detect inactive black holes in distant galaxies, or even to find celestial objects that have never been detected before.