Black holes are among the most mysterious objects in the cosmos. Although their existence is no longer debated within the scientific community, the impossibility of directly observing them sometimes makes their representation complicated. In particular, scientists have wondered about the true shape of black holes.
If the idea of a body whose escape velocity would be greater than that of light in a vacuum appeared in the 18th century, it was at the beginning of the 20th century, with Albert Einstein's general relativity, that it is really taking shape. After its first publication in 1915, some physicists began to find particular solutions to the equations of general relativity, such as Karl Schwarzschild and Roy Kerr. These solutions will quickly be interpreted as black hole type solutions (a term that only appeared in France in 1973.
It was from 1971 that the first black hole was detected thanks to the observation of the high-mass X binary star, Cygnus X-1. As a result, many other black holes will be identified. These objects do not let escape any light, it is impossible to be able to observe them directly. Observations are therefore indirect, and are based on the detection of electromagnetic radiation emitted from the accretion disk (disk of gas and dust rotating around the black hole) or during a polar relativistic jet, on gravitational disturbances and on gravitational waves.
The question of the shape of black holes naturally arises when considering the shape of other visible celestial bodies. In the Universe, and unless there is a collision or an origin other than gravitational collapse or accretion, bodies are globally spherical. Indeed, gravity acting with the same intensity in all directions, during its formation, an object naturally takes on a spherical shape.
However, this is not a perfect sphericity; other parameters such as mass, magnetic field or rotation, play a role and usually give the object a spheroid shape. This effect is all the more pronounced when the body is in rapid rotation, resulting in a flattening of the poles and thus tending towards an oblong spheroid geometry. Therefore, does a black hole respond to this situation in the same way?
The question is legitimate in the sense that the simplest formation mechanism of a black hole is that of the gravitational collapse of a massive body. So there could be a potential direct connection between the shape of the collapsed body and the shape of the black hole. Generally, black holes are considered spherical.
And if a massive non-spherical body were to collapse, what would be the result? For physicists, two possible answers. The first suggests that any non-spherical feature of the body is "lost" in gravitational collapse. The second suggests that the collapse of a non-spherical body would not result in a black hole but in a bare singularity or some other compact configuration.
It was in 1916 that Karl Schwarzschild demonstrated an exact solution to the equations of general relativity:the Schwarzschild metric. It is a value of the metric tensor for a distribution of matter that is motionless, spherical, of zero rotation and zero electric charge. This particular configuration of spacetime thus corresponds to the Schwarzschild black hole.
Three concepts therefore appear with this first solution:the event horizon — the region of the black hole from which the escape velocity becomes greater than the speed of light in a vacuum. The gravitational singularity — an area of spacetime at the center of the black hole near which the gravitational field diverges (tends to infinity). Schwarzschild radius — the radius of a black hole's horizon.
The latter is important because it establishes the first condition for the formation of black holes:for a black hole to form, the mass distribution radius of the object must be less than the Schwarzschild radius. Therefore, following the gravitational collapse, a spherical horizon of radius equal to the Schwarzschild radius is formed. It is therefore mainly the appearance of an event horizon that defines a black hole. This solution therefore confers a spherical geometry to the black holes.
In 1917, physicists Hans Reissner and Gunnar Nordström generalized the Schwarzschild metric for spherical, charged, zero-rotation black holes within the Reissner-Nordström metric. It wasn't until 1963 that physicist Roy Kerr generalized Schwarzschild's solution to quasi-spherical, uncharged, rotating black holes via the Kerr metric.
In 1965, with the physicist Ezra Newman, Kerr extends this solution to charged black holes in rotation; this is the Kerr Newman metric. Kerr's work is important because with a non-zero angular momentum (rotation), the structure of black holes is modified. All astrophysical black holes are said to be Kerr black holes because, to obtain a Schwarzschild black hole, the unrealistic assumption of a progenitor star of zero rotation and zero charge would be required.
Black holes can therefore be…(continued on next page)
