Paris (AFP)

It took 7 billion light years to reach us: a black hole of unprecedented mass, resulting from the fusion of two black holes, was directly observed for the first time thanks to gravitational waves, a major discovery for the understanding of the universe.

"It is a door which opens on a new cosmic landscape. A whole new world!", Welcomed during a press conference Stavros Katsanevas, the director of Virgo, one of the two detectors of gravitational waves that picked up the signals from this new black hole.

This is the first direct proof of the existence of black holes of intermediate mass (between 100 and 100,000 times more massive than the Sun), which could explain one of the puzzles of cosmology: the formation of supermassive black holes. , these cosmic monsters lurk in the heart of certain galaxies, including the Milky Way.

The mysterious object, described in Physical Review Letters and Astrophysical Journal Letters by an international team of more than 1,500 scientists, is called "GW190521".

Most likely the result of the merger of two black holes, it is 142 times the mass of the sun and forms the most massive black hole ever detected by gravitational waves (supermassives, billions of times larger, are detected otherwise).

Predicted by Albert Einstein in 1915 in his theory of general relativity and observed directly a century later, gravitational waves are tiny deformations of space-time, similar to ripples of water on the surface of a pond.

They are born under the effect of violent cosmic phenomena, such as the collision of two black holes which emit a phenomenal amount of energy.

The gravitational wave of GW190521 took 7 billion years to reach us: it is the most distant black hole, and therefore the oldest, ever discovered.

- A primordial black hole?

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This signal was ultra-short (a tenth of a second) and very low frequency (the more we go back in the past, the more the frequencies decrease): "A challenge to analyze it", underlined Nelson Christensen, for the Ligo collaboration.

Until now, only indirect evidence, by electromagnetic observations, suggested the existence of this population of intermediate black holes.

Heavier than black holes resulting from collapsing stars, but much lighter than supermassive monsters, they could be "the key to one of the enigmas of astrophysics and cosmology: the origin of black holes supermassives ", according to the CNRS.

One of the hypotheses explaining the birth of the latter would be, precisely, the repeated fusion of black holes of intermediate mass.

Another intriguing phenomenon: where did the two black holes that merged come from?

According to current theories, the collapse of a star cannot give rise to black holes 60 to 120 times the mass of the Sun, which is precisely the size of the two objects that have merged.

Could there be a primordial black hole formed during the Big Bang, 13.8 billion years ago?

Or are they themselves the result of a merger?

The detection of GW190521 raises new questions.

Confirming "that there is a large part of the universe which has remained invisible to us," commented astrophysicist Karan Jani, for Ligo.

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