Black holes rotate in strange ways

GenevaThanks to signals from the merging of two pairs of black holes, scientists at various gravitational wave detectors have determined that these celestial bodies, from which not even light can escape, can rotate in unprecedented ways. The collaboration between the LIGO, Virgo, and KAGRA observatories recently presented these new results in the journal The Astrophysical Journal Letters. "These results demonstrate the extraordinary capabilities of gravitational wave observatories," explains Gianluca Gemme, spokesman for the Virgo collaboration.

The first event was detected on October 11 of last year as a result of the merger of two black holes with masses 17 and 7 times the mass of the Sun, located approximately 700 million light-years away. Scientists determined that the larger of these black holes was rotating at a speed never before observed. On November 10, a second event was detected, this time located at a distance of more than two billion light-years, involving the merger of two black holes with masses 16 and 8 times the mass of our star. The exceptional nature of this second signal lies in the fact that, for the first time, a black hole has been observed rotating at high speed in the opposite direction to its orbital rotation.

Both events share the common characteristic that one of the black holes in the respective binary systems is much more massive than the other and spins at high speed. This leads astrophysicists to believe that the largest black holes were not created by the collapse of a star. "These results are evidence that these black holes appeared through the previous merger of two other black holes," says Stephen Fairhurst, professor at Cardiff University and spokesperson for the LIGO collaboration. In this way, scientists believe that large numbers of black holes could fill vast regions of the universe. "Black holes not only exist in isolation, but can also form large, evolving clusters," says Gemme.

The music of black holes

Events such as the merger of two black holes release a vast amount of energy that generates ripples in the fabric of spacetime in the form of gravitational waves that travel at the speed of light. These waves were predicted by Einstein's equations of general relativity a century ago, although the first ones were detected experimentally only a decade ago. To date, more than 300 events originating from the merger of two black holes or from the merger of a black hole and a neutron star. The characteristics of these ripples allow scientists to extract detailed information about the properties of these celestial bodies, such as their mass, their speed and direction of rotation, or the distance at which they are located.

While black holes rotating at low speeds are approximately spherical, their shape changes when they spin at speeds approaching the speed of light. This deformation generates a distinctive signal in the gravitational waves emitted when they merge with another black hole. Furthermore, the large mass difference between the observed black holes also generates a characteristic acoustic pattern in the form of a higher harmonic, observed only a couple of times before. These two observations have allowed scientists to test the predictions of Einstein's theory of general relativity with a high degree of accuracy.

Discovering new types of particles

Beyond deepening our understanding of these fascinating and mysterious objects, black holes facilitate scientists' work in detecting new types of fundamental particles. These particles would complement the map of the subatomic world we have today. According to some hypotheses, a very light type of particle from the boson family, to which photons (the particles of light) belong, could "steal" energy from the rotation of black holes. This phenomenon would cause black holes to lose rotational speed at a rate that would depend on the mass of these particles, which is currently unknown. The fact that observed black holes continue to rotate at such high speeds after billions of years significantly constrains the mass that these hypothetical particles could have.

Shedding light on black holes

Ten years after the first discovery, gravitational wave detectors continue to reveal more details about the formation and evolution of black holes in our universe. "Thanks to improvements in our instruments, we will be able to increase the precision of our measurements, allowing us to learn more about these and other aspects of black holes," says Francesco Pannarale, professor at Sapienza University of Rome and co-chair of the observational science division of the LIGO-Virgo-KAGRA collaboration. Future upgrades to these detectors, as well as the upcoming Einstein Gravitational Wave Telescope, will significantly increase the number of such phenomena observed and provide essential information for understanding the workings of the cosmos.