Black holes: Flash of light seen from possible collision (An artist’s impression of a supermassive black hole and its surrounding disc of gas © Robert Hurt/Caltech/PA)

Flash of light seen from possible black hole collision

The gravity associated with black holes is so great that nothing – not even light – usually escapes from them.

Flares of light that may have been created by two black holes colliding have been seen for the first time, astronomers say.

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Experts say the findings potentially create a new chapter within astrophysics because the merger of black holes was not expected to generate light waves.

The gravity associated with black holes is so great that nothing – not even light – usually escapes from them.

Previous observations have shown that when two black holes spiral around each other and ultimately collide and merge they generate ripples in space and time known as gravitational waves, the phenomena being a direct consequence of Einstein’s theory of gravity.

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In the latest study, published in Physical Review Letters, a black hole merger was spotted by the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo detector in May 2019.

As the black holes collided with each other, they sent out the expected gravitational waves, however shortly afterwards, the California Institute of Technology’s (Caltech) Zwicky Transient Facility (ZTF) captured a flare of light that was pinpointed to the same area as the gravitational wave event.

Black holes: Flash of light seen from possible collision (An artist’s impression of a supermassive black hole and its surrounding disc of gas © Robert Hurt/Caltech/PA)
An artist’s impression of a supermassive black hole and its surrounding disc of gas © Robert Hurt/Caltech/PA

Matthew Graham, lead author, research Professor of Astronomy at Caltech and project scientist at ZTF, said: “This supermassive black hole was burbling along for years before this more abrupt flare.

“The flare occurred on the right timescale, and in the right location, to be coincident with the gravitational-wave event.

“In our study, we conclude that the flare is likely the result of a black hole merger, but we cannot completely rule out other possibilities.”

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The study involved an international team of scientists, including physicists from the University of Edinburgh.

Scientists said that supermassive black holes lurk at the centre of most galaxies, including our own, the Milky Way.

These central supermassive black holes can be surrounded by a disc of flowing gas which contains swarms of stars and smaller black holes.

The flow of the gas helps to bring the smaller black holes together, enabling them to merge, and creates a larger black hole within the disc.

Upon creation, the new black hole has a large velocity and it is given what scientists described as “a kick” through the gas disc.

Experts said it is the reaction of the gas to the new speeding black hole that creates a bright light flare, visible with telescopes.

The newly-formed larger black hole should cause another burst of light in the next few years, according to the scientists.

Dr Nicholas Ross, project collaborator and STFC Ernest Rutherford Fellow at the Institute for Astronomy, University of Edinburgh, said: “This result, the optical flash resulting from two black holes colliding and crushing the gas around them, is so exciting.

“As a wee kid, I was hooked by the idea of black holes and now, as a big kid, the fact that we have ‘seen’ as well as ‘heard’ these black hole mergers, is an amazing discovery that has deep implications for astrophysics.”

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The research was funded by the National Science Foundation, NASA, the Heising-Simons Foundation, and the Growth (Global Relay of Observatories Watching Transients Happen) programme.

Reader Q&A: Do black holes collapse?

Asked by: Patricia Rodrigues, King’s Lynn

The Schwarzschild radius (event horizon) of a black hole is sometimes thought of as the black hole’s ‘size’. It is proportional to mass, which means that more massive black holes have bigger Schwarzschild radii.

Left alone, black holes lose mass due to ‘Hawking radiation’, so that their event horizons are slowly shrinking. A typical black hole would take many billions of times the age of the Universe to completely ‘evaporate’ and disappear.

But, the interior of the black hole, or its ‘singularity’ (the point at which all the black hole’s matter is concentrated) has already reached the limit of its density and cannot ‘collapse’ any further.

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