2023-06-29 00:37:00
Astronomers have been tracking it for a quarter of a century: the background noise emitted by the whirlwind of gigantic black holes has been identified thanks to an unprecedented technique for detecting gravitational waves, which opens “a new window on the Universe”.
These results, unveiled on Thursday, are the result of a vast collaboration of the largest radio telescopes in the world. They succeeded in capturing 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 in water on the surface of a pond. These oscillations, which propagate at the speed of light, are born 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-short quiver – less than a second – of collisions between stellar black holes, of a ten times the mass of the Sun.
This time, a much more time-stretched signal betrays a larger-scale phenomenon, captured by a network of radio telescopes (from Europe, North America, India, Australia and China) of the International Consortium Pulsar Timing Array (IPTA).
We are talking here about gravitational waves generated by black holes of “several million to several billion times the mass of the Sun”, told AFP Gilles Theureau, astronomer at the Paris-PSL Observatory, who coordinated the work French side.
– The “tick-tock” of the pulsars –
To detect these waves, the scientists used a novel tool: Milky Way pulsars. These stars have the particularity of having a mass of one to two Suns, compressed in a sphere about ten km in diameter.
Ultra-compact, these stars turn on themselves at high speed, -up to 700 revolutions per second-, specifies the CNRS researcher. A crazy rotation that produces magnetic radiation at the poles, like the beams of a lighthouse, detectable thanks to the radio waves emitted at low frequencies.
At each turn, the pulsars send out ultra-regular “beeps”, which make them “remarkable natural clocks”, explains Lucas Guillemot, from the laboratory of physics and chemistry of the environment and space (LPC2E) of Orleans.
Scientists have listed groups of pulsars, to obtain a “celestial mesh” in the meanders of space-time.
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 characteristic signature of gravitational waves. “It was a magical moment,” said Maura McLaughlin of the American network Pulsar Search Collaboratory during a press 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 our solar system, “ready to collide”, develops Gilles Theureau.
Antoine Petiteau describes two giants who “turn around before merging”, a dance which causes gravitational waves of “a period of several months to several years”.
A continuous background noise that Michael Keith, of the European network EPTA (European Pulsing Timing Array), compares to a “noisy restaurant with many 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 of an entire population. Another hypothesis suggests a source at the very earliest ages of the Universe, when it experienced a so-called period of inflation.
“We are opening a new window on the Universe”, welcomes Gilles Theureau. “We are adding a new range of information vectors”, complementary to the research of Ligo and Virgo, which operate on different wavelengths, abounds Antoine Petiteau. This could in particular clear up the mystery of the formation of supermassive black holes.
The studies will however have to be deepened to claim a fully robust detection, hoped for within a year. The absolute criterion being “that there is less than one chance in a million that this happens by chance”, underline the Paris Observatory, the CNRS, the CEA and the universities of Orléans and Paris Cité, in a communicated.
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