Never before seen fusion of giant black holes!


© Ingrid Bourgault

Huge black holes that aren't supposed to merge have ... merged! Thus forming a monster 142 times the mass of the sun. It’s a totally unexpected observation that calls into question the models of black hole formation. UCLouvain’s Sébastien Clesse and Giacomo Bruno contributed to the discovery, which suggests the existence of primordial black holes, formed a millisecond after the Big Bang.

The fusion of two giants – 85 and 66 times the sun’s mass, respectively – has formed a new monster of 142 solar masses. The gravitational waves emitted during the last tenth of a second of their infernal spiral were detected on 21 May 2019 by LIGO’s detectors, in the United States, and Virgo’s detectors, in Italy. This is the first time that the fusion of such massive black holes has been observed. Their existence was hitherto unsuspected, because theoretical models exclude the formation of black holes with a mass of between 65 and 120 solar masses when the most massive stars explode in supernovae, at the end of their lives. They could, however, come from previous fusions. But in this case, to have a chance of running into each other, they would have to inhabit an environment very dense in black holes, such as black hole and/or star clusters. An exotic alternative: these black holes could come from a cluster of primordial black holes. Rather than from the explosion of stars at the end of their lives, these hypothetical black holes could have formed a fraction of a millisecond after the Big Bang, from inhomogeneities in the plasma of the early universe. They could explain some characteristics of the black hole mergers observed to date. The idea was proposed in 1971 by Stephen Hawking and is now back in the forefront of scientific news!

The study was published in the journal Physical Review Letters. In Belgium, groups from seven universities (UCLouvain, the University of Liège, ULB, VUB, Ghent University, KULeuven, University of Antwerp) are part of the LIGO/Virgo collaboration. Our researchers’ activities involve primordial black holes and their possible contribution to dark matter, another great mystery of the universe. Future LIGO/Virgo observations should make it possible to discern different models of black hole formation. Belgian LIGO/Virgo researchers are also involved in the field of gravitational cosmology (cosmic strings, universe expansion, dark matter), data analysis, particularly through artificial intelligence, and certain aspects of instrumentation (noise controls, lasers, etc.).

In the more distant future, the Einstein Telescope, a third-generation underground gravitational wave detector project, will detect hundreds of thousands of black hole fusions per year. Its construction is planned at the end of the decade and one of the possible sites is on the border between Germany, the Netherlands and ... Belgium!

UCLouvain and Virgo

Several researchers at the UCLouvain Research Institute in Mathematics and Physics (IRMP) are involved in gravitational wave research as members of the Virgo collaboration.

Gravitational waves are distortions of space and time that propagate at the speed of light. Virgo is a giant laser interferometer capable of detecting variations of 10-18 m within its three km-length arm length; this is the effect observable on earth, i.e. 17 billion light years away, of gravitational waves emitted by the fusion of two black holes that were the subject of the discovery announced on 2 September by the LIGO and Virgo collaborations.

The UCLouvain Virgo group’s activities are data analysis, including interpretation of observations in the context of primordial black holes, and the development of numerical computing infrastructure and instrumentation.

Giacomo Bruno has been a professor at UCLouvain since 2004. After his studies in physics in Italy, and having worked for several years at the CERN International Laboratory, he and many UCLouvain IRMP colleagues contributed to the construction and analysis of data from the CMS experiment at CERN, which discovered the Higgs boson in 2012. Giacomo Bruno is now the spokesperson for the Virgo collaboration’s Belgian group, which currently includes some 20 researchers from KU Leuven, the University of Antwerp, UCLouvain, Ghent University, the University of Liège, and VUB.

Sébastien Clesse is an FNRS researcher at UCLouvain and the University of Namur. His research interests in the field of gravitational waves focus on primordial black holes, their possible contribution to dark matter and their signatures in observations. He has just been appointed lecturer at ULB, which will therefore be added to the list of Belgian universities involved in the Virgo collaboration.

Virgo collaboration UCLouvain members:

  • Giacomo Bruno
  • Sébastien Clesse
  • Federico De Lillo
  • Antoine Depasse
  • Andrew Miller
  • Krzysztof Piotrzkowski
  • Christophe Ringeval
  • Andres Jorge Tanasijczuk
  • Joris van Heijningen

See also: Detecting dark matter: seven hints point to primordial black holes


Bio of Sébastien Clesse

‘As an adolescent, I hesitated between history, maths, physics and computer science.’ Any outliers? None in Sébastien Clesse’s eyes. ‘The only way to combine the four was in physics, more specifically cosmology. It covers mathematics and computer science and even history – the longest, the universes’s!’ Thus he studied physics at the University of Namur, where he obtained his bachelor’s degree in 2006. He then began a joint PhD at ULB and UCLouvain (under the supervision of Michel Tytgat and Christophe Ringeval) and defended his thesis in 2011. Next, a long postdoctoral research journey in Cambridge, Munich, Namur (thanks to a BESPO return grant), and Aachen. He returned to UCLouvain and UNamur in 2017, as an FNRS postdoctoral researcher at the new Cosmology, Universe and Relativity Research Group. ‘In cosmology, we study the ultimate; no other sciences can explain the object of our research in other ways. Cosmology, just like particle physics, is at the very end of the science chain, and at the forefront of understanding the world.’» 

Published on September 09, 2020