Astronomers have been tracking it for a quarter of a century: the background noise emitted by the hurricane of gigantic black holes has been identified thanks to an unprecedented technique for detecting gravitational waves, opening “a new window on the universe”.
These results, presented Thursday, are the result of extensive collaboration between the world’s largest radio telescopes. They succeeded in capturing this vibration of the universe with “the precision of a clock”, enthused the authors of the work, which was published in several scientific journals at the same time.
Gravitational waves were predicted by Einstein in 1916 and discovered a hundred years later. They are tiny distortions of space-time, similar to the ripples in the water on the surface of a pond. These oscillations, which propagate at the speed of light, arise under the action of violent cosmic events such as the collision of two black holes.
They may be associated with massive phenomena, but their signal is extremely tenuous. In 2015, the Ligo (USA) and Virgo (Europe) gravitational-wave detectors revolutionized astrophysics by detecting the ultra-short quiver — less than a second — of collisions between stellar black holes ten times the mass of our Sun.
This time, a much more time-stretched signal reveals a larger-scale phenomenon detected by a network of radio telescopes (from Europe, North America, India, Australia and China) of the International Consortium Pulsar Timing Array (IPTA).
We’re talking about gravitational waves produced by black holes with “several millions to several billion solar masses,” said AFP Gilles Theureau, an astronomer at the Paris PSL Observatory who coordinated the work from the French side.
Pulsar tick-tock
To detect these waves, the scientists used a novel tool: Milky Way pulsars. These stars have the peculiarity of having a mass of one to two suns and being compressed into a sphere about ten kilometers in diameter.
These ultra-compact stars rotate on their own at high speeds – up to 700 revolutions per second – the CNRS researcher specifies. A crazy rotation that produces magnetic radiation at the poles, like the rays of a lighthouse, detectable thanks to radio waves emitted at low frequencies.
With each rotation, the pulsars emit ultra-regular “beeps” that make them “remarkable natural clocks,” explains Lucas Guillemot of the Laboratory for Physics and Chemistry of the Environment and Space (LPC2E) in Orleans.
The Nançay radio telescope in Sologne, central France, on October 3, 2019
AFP/Archives
Scientists have mapped groups of pulsars to create a “celestial web” in the meanders of spacetime.
And have been able to measure a tiny perturbation 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 sign of “a perturbation 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 and “ready to collide,” writes Gilles Theureau.
Antoine Petiteau describes two giants “flipping before merging,” a dance that causes gravitational waves “lasting from several months to several years.”
A continuous background noise that Michael Keith of the European network EPTA (European Pulsing Timing Array) likens to “a noisy restaurant with lots of people talking around you”.
Whether this noise indicates the presence of a few pairs of black holes or an entire population cannot yet be said from the measurements. Another hypothesis suggests a source in the very first age of the universe when it was experiencing a so-called inflationary period.
“We are opening a new window on the universe,” welcomes Gilles Theureau. “We are adding a new set of information vectors” that complement the research of Ligo and Virgo, which work on different wavelengths, says Antoine Petiteau. In particular, this could clarify the mystery surrounding the formation of supermassive black holes.
However, studies need to be deepened to achieve fully reliable detection, which is hoped to be achieved within a year. The absolute criterion is “that the probability of this happening by chance is less than one in a million,” emphasize the Paris Observatory, the CNRS, the CEA and the Universities of Orléans and Paris Cité in a statement.