Two black holes are heading for a collision that will shake up spacetime

Two black holes are heading for a collision that will shake up spacetime


A team of astronomers announces that they have identified two supermassive black holes on the way to an inevitable collision. The encounter, which is expected to take place in about ten thousand years, is expected to shake the very fabric of spacetime and generate gravitational waves. Details of the study are published in The Astrophysical Journal Letters.

In the Universe, everything is in motion. And if the distances between objects are often staggering, it sometimes happens that some of them meet. A team of researchers has just identified two black holes, which share the name PKS 2131-021, locked in one of these fatal dances about nine billion light years from Earth.

The two objects, which have been approaching each other for about a hundred million years, now share a binary orbit. According to the study, in about ten thousand years they will be close enough to merge. The event should then generate gravitational waves (ripples in the fabric of spacetime) that will surge through the Universe.

How were they discovered?

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PKS 2131-021 is a special type of black hole known as a blazar. Imagine a supermassive black hole pointing a jet of matter directly at Earth. This material comes from the hot gas rings formed around it. When one of these objects sucks in this gas, matter can indeed escape before being propelled into a jet of plasma moving at a speed close to light.

That said, the researchers here were monitoring the brightness of around 1,800 blazars. In the data, they then noticed that that of blazar PKS 2131-021 fluctuated at regular intervals, like the ticking of a clock. The researchers then suspected that these variations testify to the presence of a second black hole in orbit around the first. According to the analyses, the two objects would thus turn around every two years or so.

To confirm their suspicions, the researchers drew on data from five observatories, covering forty-five years of observations. These analyzes ultimately matched the team’s predictions. This work also made it possible to estimate the “date” of this now inevitable fusion between the two objects.

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Second binary discovered

The discovery is interesting in that it is only the second pair of binary black holes ever discovered. It is also the tightest. The first binary candidate was isolated in 2020 in a galaxy about 3.5 billion light-years from Earth. However, the two objects concerned rotate around every nine years. They are therefore more distant from each other than those of the PKS 2131-021 couple.

While none of us will witness this epic collision, analysis of this upcoming merger could also provide new insights into how supermassive black holes form. We know that these objects lie at the heart of most, if not all, large galaxies. But astronomers still don’t know exactly how these objects get so big. One possibility is that the largest black holes in the Universe result from at least one merger between smaller objects. Tracking this new binary pair could help confirm this hypothesis.