Black holes, cosmic monsters that science is trying to tame

Stunning “immersive” monsters, by definition invisible … The mysteries of the universe and physics, black holes have long been detected by scientists who in a few years have managed to uncover them before the eyes of the world.

The first “black stars”

Their existence was assumed in the late 18th century by the Frenchman Pierre Simon de Laplace and the British John Michell, who at the time imagined “black stars”. Objects so massive in a volume so small – as if the Earth were compressed into a thimble – that gravity would prevent everything, matter like light, from escaping.

The hypothesis only reappears two centuries later, with Albert Einstein’s general theory of relativity (1915), which gives them a formal framework. Thanks to this, astrophysicist Karl Schwarzschild defines an area around the black hole, called the “event horizon”, where matter and light are swallowed up forever.

“The rough characteristics of black holes were there,” Guy Perrin, an astronomer at the Paris-PSL Observatory, told AFP.

Space and time intertwined

The model was refined in the 1930s, when physicists predicted that when a massive star (larger than the Sun) reaches the end of its life, it explodes like a supernova. Before his heart collapses over itself, under its own weight.

The mathematician Roger Penrose proves in the 1960s that this “death” of a star can actually create a black hole. Together with his colleague Stephen Hawking, he theorizes the existence of a “singularity” in which density and mass become infinite. A dizzying concept where time and space merge and defy the laws of physics.

“Waste trucks”

But all this was still only theory. Black holes only came out of the woods with the observation of quasars, Guy Perrin remembers: light sources that are very distant and so “incredibly bright that they must have been caused by a supermassive black hole monster”. As by swallowing gas emits intense radiation, visible in the disk of matter that surrounds it.

These supermassive objects would place themselves at the center of galaxies and act as “cosmic garbage cars”, like trolls whose appetite varies by age, as described by the German astrophysicist Heino Falcke in his book “Light in the Dark” (2022).

And why not with us? In the 1990s, the German Reinhard Genzel and the American Andrea Ghez (Nobel Prize in Physics 2020) discovered the existence of a supermassive compact object in the heart of the Milky Way with several million solar masses thanks to the study of the movements of the stars in orbit around it. “Sagittarius A *” was found. But the ultimate proof that it was actually a supermassive black hole was missing.

“The Beast” opposite

Black holes, cosmic monsters that science is trying to tame

Producing an image of these giants is truly a challenge, while evidence is easier to gather for their “little brothers” of the mass of stars, completely invisible but detectable via supernova explosions or gravitational waves.

A historic turning point came in April 2019, with the first image of a supermassive black hole in the center of the huge galaxy Messier 87, like a dark circle in the middle of a blazing halo.

What had hitherto been speculation suddenly became visible. But the black hole M87 * is very far away, unusually gigantic; Sagittarius A * is “ours, in our galaxy”, Professor Falcke told AFP. After five years of work, it came out of the shadows this Thursday, and finally let us see “the beast from the front”.

For physicists, this double proof of the existence of supermassive black holes is a huge step. It will allow them to test the solidity of general relativity in the “most extreme, chaotic and turbulent environment possible”, delights the German scientist.

For the time being, Einstein’s theory fails to explain what is at stake in the black hole on the scale of the infinitely small – this singularity that works.

The study of black holes will also make it possible to better understand the evolution of the universe, given their “fundamental role in the evolution of galaxies”, concludes Guy Perrin.

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