Gravitational wave detectors capture their most massive black hole merger to date © Getty Images

Gravitational wave detectors capture their most massive black hole merger to date

The collision was more of a 'bang' than a 'chirp'.

An international team of astronomers working at the LIGO and Virgo gravitational wave detectors have made their biggest discovery yet: the collision of two black holes that merged together to create a black hole around 142 times the mass of the Sun – the biggest ever detected using gravitational waves.

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The merger has several unusual features and could lead to the discovery of new physics, they say.

The collision occurred 7 billion years ago when the two black holes began spiralling into one another before crashing together and sending gravitational waves rippling through the fabric of space-time.

The resulting signal, dubbed GW190521, was first detected by scientists running the Advanced Virgo detector at the European Gravitational Observatory (EGO) in Italy and the two Advanced LIGOs, in the US on 21 May 2019.

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“This doesn’t look much like a chirp, which is what we typically detect: it is more like something that goes ‘bang’ and the system that generated it is the most massive that LIGO and Virgo have detected until now,” said Nelson Christensen, director at ARTEMIS in Nice, France and member of the Virgo Collaboration.

The resulting black hole belongs to the class of so-called ‘intermediate-mass black holes’ which includes those from 100 up to 100,000 solar masses. The discovery marks the first time that a black hole in this mass range has been observed.

Intermediate-mass black holes are of particular interest to researchers as they may hold the key to one of the big puzzles in astrophysics and cosmology: the origin of supermassive black holes. Although the question remains unanswered, one explanation for the formation of these cosmic monsters, millions or even billions of times the mass of the Sun, is the repeated mergers of intermediate-mass black holes.

Also, based on our current knowledge, the gravitational collapse of a star cannot form black holes in the approximate range of 60 to 120 solar masses, since they are completely blown apart by the supernova explosion due to a process called “pair instability”, and leave behind only gas and cosmic dust.

Therefore, astrophysicists would not expect to observe any black hole in this mass range – but the heavier component of GW190521 weighed in at 85 solar masses.

“Several scenarios predict the formation of black holes in the so-called pair instability mass gap: they might result from the merger of smaller black holes or from the collision of massive stars or even from more exotic processes,” said Michela Mapelli, professor at Padova University, member of INFN Padova and of the Virgo Collaboration.

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“However, it is also possible that we have to revise our present understanding of the final stages of the star’s life and the resulting mass constraints on black hole formation. Either way, GW190521 is a major contribution to the study of the formation of black holes.”

Reader Q&A: What would happen if a very strong gravitational wave passed through us?

Asked by: Thomas S Marcotte, USA

Gravitational waves spread out from any violent event involving matter – such as, say, the collision of two black holes. Like gravity, however, they’re incredibly weak, so you’d have to be extremely close to their source in order to feel their effects.

It would definitely feel weird, as they’d create a rhythmic stretching and squashing sensation on the body. But you’d have to be so close to the cataclysm itself that you’d never live to describe it.

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