Enigmatic black holes revealed by gravitational waves


This is the biggest catch to date for the LIGO and Virgo gravitational wave detectors: a black hole with the mass of 142 suns, resulting from the fusion of two black holes of 85 and 65 times the mass of Sun. The final black hole is the heaviest ever observed with gravitational waves and it could give indications on the formation of the supermassive black holes which sit at the center of some galaxies. The mass of one of the merged black holes, 85 times the mass of the Sun, provides evidence that could improve our understanding of the final stages in the evolution of massive stars. The discovery, to which several CNRS teams contributed within the Virgo collaboration, including the Optomechanics and Quantum Measurements team at the LKB, was published on September 2, 2020 in the journals Physical Review Letters and Astrophysical Journal Letters.

Detecting the birth of a black hole resulting from the merger of two others, accompanied by the emission of an enormous quantity of energy: this new episode could seem trivial as such detections have been repeatedly performed since 2015, when the gravitational waves resulting from such a phenomenon were observed for the first time. However, GW190521, the signal recorded on May 21st, 2019 by the LIGO and Virgo instruments, not only stands out because it is the most distant and therefore the oldest (the gravitational wave took 7 billion years to reach us) , but also because the resulting black hole is the heaviest ever seen. Above all, this observation is the first direct proof of the existence of so-called “intermediate mass” black holes (between 100 and 100,000 times more massive than the Sun). Such black holes are heavier than those created by the collapse of massive stars, but much lighter than the supermassive black holes lodged in the center of some galaxies.

The merged black holes, with their mass about 65 and 85 times that of the Sun, also intrigue astrophysicists. According to current knowledge, the gravitational collapse of a star cannot form black holes between about 60 and 120 solar masses because the most massive stars are completely blown away by the supernova explosion that comes with this collapse, leaving behind only gas and dust. So how did the heavier black hole form? Is there something misunderstood about the end of life of massive stars? If it does not have a stellar origin, could it itself be the result of an earlier merger of less massive black holes? On the contrary, is it a hypothetical primordial black hole, formed during the Big Bang? GW190521’s observation raises new questions about the formation of the enigmatic stars that are black holes.


Extract from a simulation of the merger of the two black holes. A pair of black holes orbiting each other loses energy in the form of gravitational waves. The two stars are slowly approaching one another, a phenomenon that can last for billions of years before suddenly accelerating. In a fraction of a second, the two black holes then collide at roughly half the speed of light and merge into a single black hole. The mass of the resulting black hole issmaller than the sum of the two initial masses because part of their mass (here, the equivalent of 8 suns, or a colossal energy) has been converted into gravitational waves according to Einstein’s famous formula E = mc2.

It was this burst of gravitational waves that the two LIGO detectors (in the United States) and the Virgo detector (in Italy) observed. As it passes, this wave expands then contracts spacetime. Thus, any object that is in the path of a gravitational wave sees its length vary: it is these tiny variations that are detected in the LIGO and Virgo detectors.

© N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes (SXS) Collaboration