Stars rotate on their axis and this must also be the case for stellar black holes created by the gravitational collapse of these stars. The same is thought to be true of supermassive black holes at the heart of galaxies, and the theory of quasar radiation is crucially linked to the fact that these black holes rotate. In the case of our Milky Way, astrophysicists, by delving into the archives of X-ray observations from NASA’s Chandra satellite, have just deduced that our giant black hole was also spinning, at a significant fraction of the speed of light.

The nearest supermassive black hole is simply located at the heart of the Milky Way and contains just over 4 million solar masses. We know this after decades of research into the movements of certain stars close to the source of radio waves Sagittarius A*Sagittarius A*. Some of these stars experience such intense gravity that they move in response to a remarkable fraction of the speed of light.

Sgr A*, as our central black hole is also known, is a laboratory for understanding the PhysicallyPhysically of the supermassive black holessupermassive black holes behind the quasarsquasarsthis wonderful active galactic nucleiactive galactic nuclei. It also serves us to test alternatives to the relativistic theory gravitygravity byEinsteinEinstein and even to test the hypothesis of the existence of a fifth force.

You can study Sgr A*, in the field of x-raysx-raysespecially with the mythical and venerable satellite ChandraChandra of the NASANASA. A team of researchers has just made a new estimate of the rotation speedrotation speed of this black hole, as she explains in an article published in *Monthly notices of the Royal Astronomical Society*.

Mathematician Roy Kerr, who discovered the solution to Einstein’s equations describing rotating black holes, explains the story of black hole theory, its associated singularity problems, and his discovery. To get a fairly accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Auto translate”. Choose “French”. © Vetenskapsakademien

## Extreme black holes and cosmic censorship

If you assume, not only as is now very likely, especially after the images of the*Event Horizon telescope*that we are indeed in the presence of a Kerr black holeKerr black hole in rotation, Chandra data tells us that the parameter describing this rotation happens to have a value between 0.84 and 0.96. The maximum would be 1, and this would roughly amount to the following: if we had as an image of this black hole that of a sphere firmfirm — that the surface of this sphere is onequatorequator spins at the speed of light. Technically this would be what we call an extreme Kerr black hole and if we imagined the parameter to be greater than 1 we would no longer have a black hole because theevent horizonevent horizon would disappear, creating a singularity of thespace timespace timewhat we suspectsuspect are usually impossible.

This presumption, mentioned in English *Cosmic censorship hypothesis*what can be translated as the hypothesis of cosmic censorship was put forward in 1969 in an initially non-mathematically precise form by the Nobel laureate in Physics and Physics. mathematicianmathematician Roger Penrose.

Researchers are divided on its validity.

Let’s remember that general relativitygeneral relativitya black hole has a precise mathematical shape with a very extensive theory behind it. In short, a black hole is defined by the existence of a boundary surrounding an area from which, to escape, one would have to exceed the speed of light: the event horizon. We know all the solutions comparisonscomparisons of Einstein describing a black hole and in addition to having an event horizon, they have space-time singularities where the laws of physics, such as that of causality, are destroyed.

Penrose had suspected that the laws of nature nevertheless isolated singularities from the rest of the worldUniverseUniversethus preventing them from causing problems by surrounding them with an event horizon.

An overview of extreme black holes and cosmic censorship. To get a fairly accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Auto translate”. Choose “French”. © PBS Spacetime

However, there are situations involving rotating and/or charged black holes that make it possible to imagine the disappearance of the event horizon according to the parameters describing these black holes. When they are about to become pathologicalpathological in this way they are therefore commonly spoken of as extreme black holes. The video below provides some details about these black holes and cosmic censorship.

Similar studies with the black hole M87* show that the parameter describing its rotation is between 0.89 and 0.91