Question of the week: can one black hole absorb another?

“Can a black hole absorb another black hole? And if so, what does that give?”, asks Nathanaël Zreik on our Facebook page. Thank you all for participating in our “question of the week”.

Yes, a black hole can indeed absorb another black hole! The collision between these two very dense and very compact bodies causes some eddies in the Universe … like circles in the water of a pond after a stone has been thrown into it. “In a fraction of a second, black holes collide at half the speed of light and merge into a single black hole. This is lighter than the sum of the two original black holes. because part of their mass (here, the equivalent of 3 suns, or a colossal energy) has been converted into gravitational waves according to Einstein’s famous formula E = mc2 “, detailed the CNRS in a previous article by Sciences and the Future.

And this burst of gravitational waves was first observed in 2016 by scientists, according to an announcement that had made a lot of noise and initiated by the scientific collaboration LIGO (which includes the collaboration GEO and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo collaboration, the Franco-Italian detector located in Italy.

What is a gravitational wave?

The hypothesis of the existence of this cosmic thrill dates back to the first decades of the 20th century. In 1915 precisely, a time when the general relativity theorized by Einstein jostled our understanding of the world. Now it was necessary to envision space, endowed with elasticity and shaped by matter, exactly as if the content (planets, stars and galaxies) curved the container, that is to say the entire space. So, Einstein tells us, the shape of space depends on the matter that lodges in it. Therefore, any acceleration of mass should change it and this will manifest itself as a wave that runs through the entire Cosmos. The famous pebble thrown into a lake! On its passage, this wave would expand then contract space.

Thus, in theory, any object which is on the path of a gravitational wave sees its length vary: everything happens as if the space between the atoms of its molecules is stretched and then tightened. This surprising observation indicates that in the distance two massive stars approach each other to collide or that a star explodes, ejecting its envelope. Indeed, in the 2016 announcement, were involved two black holes with respective masses of 29 and 36 times the mass of the Sun which approached and ended up merging 1.3 billion years ago. The source was therefore more than a billion light years away. The merger gave rise to a gigantic black hole with a final mass of 62 solar masses. Now, 29 + 36 are supposed to be 65. Which means that the equivalent of 3 solar masses was expelled in the form of gravitational waves.

And it is this event of great intensity that caused this vibration of space-time, titillating the LIGO and Virgo detectors as it passes through Earth. Better still, the source of these gravitational waves would be located in the southern hemisphere, a hypothesis made possible thanks to the comparison of the arrival times of the gravitational waves in the two detectors located across the United States ( 7 milliseconds difference) and the study of the characteristics of the signals measured by LIGO and Virgo.

The fusion of two very different black holes

This first detection of a gravitational wave, resulting from the merger of two black holes, was followed by a few dozen events of this type, all the stars at the origin of these waves had approximately the same mass. But in 2020, LIGO and VIRGO made another great discovery, surprising the merger of two very different black holes, one almost four times as massive as the other.

By Azar Khalatbari and Erwan Lecomte

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