The results, unveiled Thursday, are the result of extensive collaboration between the world's largest radio telescopes. They managed to capture this vibration of the Universe with "the precision of a clock", enthuse the authors of the work published simultaneously in several scientific journals.

Predicted by Einstein in 1916 and detected a hundred years later, gravitational waves are tiny distortions of space-time, similar to ripples of water on the surface of a pond. These oscillations, which propagate at the speed of light, arise under the effect of violent cosmic events such as the collision of two black holes.

They may be linked to massive phenomena, but their signal is extremely tenuous. In 2015, the gravitational-wave detectors Ligo (United States) and Virgo (Europe) revolutionized astrophysics by detecting the ultra-brief quiver - less than a second - of collisions between stellar black holes, about ten times the mass of the Sun.

This time, a much more time-long signal betrays a larger-scale phenomenon, picked up by a network of radio telescopes (from Europe, North America, India, Australia and China) of the International Puslar Timing Array (IPTA) consortium.

We are talking here about gravitational waves generated by black holes of "several million to several billion times the mass of the Sun," said Gilles Theureau, an astronomer at the Paris-PSL Observatory, who coordinated the work on the French side.

The "ticking" of pulsars

To detect these waves, scientists used a new tool: pulsars from the Milky Way. These stars have the particularity of having a mass of one to two Suns, compressed into a sphere about ten km in diameter.

Ultra-compact, these stars rotate on themselves at high speed, -up to 700 revolutions per second-, says the CNRS researcher. A crazy rotation that produces magnetic radiation at the poles, like the beams of a lighthouse, detectable thanks to radio waves emitted at low frequencies.

At each turn, pulsars send ultra-regular "beeps", which make them "remarkable natural clocks", explains Lucas Guillemot, of the laboratory of physics and chemistry of the environment and space (LPC2E) of Orleans.

Scientists have catalogued groups of pulsars, to obtain a "celestial mesh" in the meanders of space-time.

The Nançay radio telescope in Sologne, in the center of France, October 3, 2019 © GUILLAUME SOUVANT / AFP/Archives

And were able to measure a tiny disruption in their ticking, with "changes of less than a millionth of a second over more than 20 years," according to Antoine Petiteau, of the Atomic Energy Commission (CEA).

These delays were correlated, a mark of a "disturbance common to all pulsars", according to Gilles Theureau: the signature characteristic of gravitational waves. "It was a magical moment," Maura McLaughlin of the Pulsar Search Collaboratory told a news conference.

Like in a noisy restaurant

What is the source of these waves? The preferred hypothesis points to pairs of supermassive black holes, each larger than that of our solar system, "ready to collide", develops Gilles Theureau.

Antoine Petiteau describes two colossi that "turn around before merging", a dance that causes gravitational waves of "a period of several months to several years".

A continuous background noise that Michael Keith, of the European Pulsing Timing Array (EPTA), compares to a "noisy restaurant with lots of people talking around you".

The measurements do not yet make it possible to say whether this noise betrays the presence of a few pairs of black holes, or an entire population. Another hypothesis suggests a source in the earliest ages of the Universe, when it experienced a period of so-called inflation.

"We are opening a new window on the Universe," says Gilles Theureau. "We add a new range of information vectors", complementary to the research of Ligo and Virgo, which operate on different wavelengths, abounds Antoine Petiteau. In particular, this could shed light on the mystery of the formation of supermassive black holes.

However, studies will have to be further developed to claim a fully robust detection, hoped for within a year. The absolute criterion is "that there is less than one chance in a million that this happens by chance," say the Paris Observatory, the CNRS, the CEA and the universities of Orleans and Paris Cité, in a statement.

© 2023 AFP