2023 Balzan Prize for High resolution images: from planetary to cosmic objects
Black holes form the most interesting and fascinating manifestation of gravity. In these cosmic objects, matter is compressed to such an extent that it concentrates a huge amount of mass at a point without size, which is called a density singularity. This point is surrounded by an almost spherical rim called the event horizon, within which gravity is very intense. To a distant observer, light from objects near the event horizon appears redshifted and time appears to run slower. Gravity within the horizon is so strong that not even light can escape, which means we cannot get any information from inside black holes, and the event horizon is literally in the dark.
Despite their darkness, black holes are the source of a variety of impressive astrophysical phenomena, from massive active galactic nuclei (giant galaxies that are home to a supermassive black hole with a bright accretion disk and that emit intense radiation ranging from radio to gamma rays) to quiet, unobservable isolated pairs of black holes that orbit around each other and eventually merge, emitting gravitational waves.
Black holes are very far away from us, which is why they appear very small. To observe their horizon from our position, the telescope must be extremely high-resolution. Taking a picture of the event horizon surrounding the black hole in our galaxy is comparable to photographing a mite from a distance of 2,000 km.
What do we expect to see and how can we build such a telescope? Professor Heino Falcke’s work began with these questions. He devoted many years to tirelessly refining the theory, and predicted that the image should look like a ring of light, with a “shadow of the black hole” caused by light deflection and absorption in the region of the event horizon. Thanks in part to his pioneering work and charisma, he was able to convince a large group of astronomers to combine their efforts and instruments to build a worldwide network of millimetre-wave telescopes that would function like a very large radio interferometer. The result of this joint effort was the Event Horizon Telescope (EHT), specially developed with the ambitious goal of taking the first image of the event horizon of a black hole, in which Heino Falcke played a leading role.
The image of the black hole shadow in M87, released in 2019, was a sensation, both for astrophysicists and the general public. This image provided the sharpest view to date of a black hole’s environment, clearly showing the presence of an event horizon as a dark spot surrounded by a characteristic ring of bright emission, as predicted by Falcke. Analysis of this image allowed scientists to test several predictions of Einstein’s general theory of relativity and confirm its validity in situations where gravity is much stronger than we are accustomed to.
The image confirmed the prediction of general relativity for a rotating black hole (Kerr metric). The results obtained constraint potential deviations from the no-hair theorem. The mass of the black hole inside the photon orbit was estimated at 6 billion solar masses.
Later, the EHT also provided an image of the black hole at the centre of our Milky Way, known as Sgr A*. This was more challenging because our black hole is a thousand times less massive than the one in M87, and both matter and light orbit the event horizon in a short time (typically between 15 minutes to 1.5 hours), causing rapid changes in the appearance of the bright ring. Moreover, the presence of gas along the line of sight between us and the black hole blurs the image.
These results are recognized as extremely important for astrophysics and general relativity. They are just the beginning of more advanced research that will develop in the coming years and allow scientists to gain a deeper understanding of the nature of these unique cosmic objects and the underlying laws of physics. The new insights will be made possible by the addition of more radio telescopes, both on the ground and in space.