Astronomers were able to identify the background noise emitted by the whirlwind of giant black holes thanks to an unprecedented technique for detecting gravitational waves, which opens a “new window on the universe” after a quarter of a century of tracking this strange cosmic phenomenon.
These results, revealed Thursday, are the result of an extensive collaboration of the world’s largest radio telescopes. They managed to capture this vibration of the universe with “clockwise accuracy”, which excited the authors of the work published simultaneously in several scientific journals.
Predicted by Einstein in 1916 and his discovery a hundred years later, gravitational waves are tiny distortions in space-time, similar to ripples of water on the surface of a pond. These oscillations, which propagate at the speed of light, are generated under the influence of violent cosmic events such as the collision of two black holes.
Ligo gravitational waves
They may be associated with massive phenomena, but their signals are very weak. In 2015, the gravitational wave detectors Ligo (USA) and Virgo (Europe) revolutionized astrophysics by detecting very short flickers – less than a second – from collisions between stellar black holes, which have a mass ten times that of the Sun.
This time, the much longer-spanning signal indicates a large-scale phenomenon, captured by a network of radio telescopes (from Europe, North America, India, Australia, and China) of the International Pulsar Timing Array Association (IPTA).
We are talking here about gravitational waves generated by black holes of “several million to several billion solar masses,” says Gilles Thoreau, an astronomer at the Paris Observatory PSL, who coordinated the work on the French side.
To detect these waves, scientists used a new tool, the Milky Way’s pulsars. These stars have the peculiarity of having a mass of one to two suns, compressed into a ball about ten kilometers in diameter.
Extremely compact, these stars rotate in on themselves at a high speed – up to 700 revolutions per second -, the CNRS researcher determines. Crazy rotation generates magnetic radiation at the poles, like the rays of a beacon, and can be detected thanks to radio waves emitted at low frequencies.
At every turn, pulsars send out super-regular “beeps,” making them “fantastic natural clocks,” explains Lucas Guillemot of the Laboratory for Physics and Chemistry of the Environment and Space (LPC2e) in Orleans.
Scientists lined up clusters of pulsars to create a “celestial web” in the zigzags of space-time.
They were able to measure a slight perturbation in their beats, with “changes of less than a millionth of a second over more than 20 years,” according to Antoine Petito, of the Atomic Energy Commission (CEA).
These delays were correlated, a sign of “a disturbance common to all pulsars,” according to Gilles Thoreau: the characteristic signature of gravitational waves. “It was a magical moment,” said Maura McLaughlin of the US Pulsar Search Collaboratory Network during a news conference.
What is the source of these waves? The preferred hypothesis suggests pairs of supermassive black holes, each larger than our solar system, “ready to collide,” Gilles Thoreau put it.
Antoine Petito describes two giants “orbiting each other before merging”, a dance that causes gravitational waves of “a period of several months to several years”.
Constant background noise that Michael Keith of the European network EPTA (European Pulse Timing Array) likens to “a noisy restaurant with lots of people talking around you”.
The measurements do not yet make it possible to determine whether this noise is indicative of the presence of a few pairs of black holes, or the presence of an entire population. Another hypothesis indicates the existence of a source in the very early ages of the universe, when it experienced the so-called period of inflation.
“We are opening a new window on the universe,” says Jill Thoreau. “We’re adding a new set of information vectors,” complementing the Ligo and Virgo research, which operate at different wavelengths, abounding in Antoine Petiteau. In particular, this could explain the mystery of the formation of supermassive black holes.
However, the studies would have to be in-depth to claim a completely robust finding, hopefully within a year. The absolute criterion is that “there is less than a one-in-a-million chance of this happening by accident,” the Observatory of Paris, the National Center for Scientific Research, the CEA and the Universities of Orleans and Paris City confirmed in Connection.
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