2023-06-29 00:43:12
The Nançay radio telescope in the Cher region of France. CNRS
Nestled at the heart of merging galaxies, invisible pairs of supermassive black holes are making space-time vibrate. European astronomers, working with Japanese and Indian researchers, have just demonstrated this by detecting, alongside teams from the US, Australia and China, the gravitational waves produced by these pairs of colossal objects. Scientists from the EPTA (European Pulsar Timing Array) network, which brings together the continent’s six largest radio telescopes, including the one in France at Nançay (Cher), published a series of articles on June 29 in the journal Astronomy and Astrophysics. In these articles, they explain how they succeeded in creating a detector the size of the Milky Way, sensitive to these signals, by assiduously observing a handful of dead stars!
The source of this tour de force, which opens the way to a better understanding of the origins of black holes and galaxies, is the father of the theory of general relativity, Albert Einstein himself. Einstein predicted the existence of gravitational waves in 1915. According to his thesis, any acceleration of a mass in the Universe can generate – if conducted asymmetrically – “waves” that propagate through the cosmos at the speed of light. These vibrations distort space in their path, first stretching it, then contracting it, before returning it to its original dimensions.
These “ripples” in space-time are no figment of the imagination. They have been detected more than 100 times since their discovery in 2015 by the French-Italian and American Virgo and LIGO facilities. This has given astronomers the opportunity to confirm the nature of some of the phenomena that produce them. These phenomena include the merger of two stellar black holes or two neutron stars, or the absorption of a neutron star by a stellar black hole.
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Precise timing
The results were hailed by a Nobel Prize in Physics in 2017. But while the LIGO and Virgo interferometers have validated a concept for accessing events unobservable by other means – a black hole emits neither light nor radiation, and no matter escapes from it! – they were not designed to go any further. “Only sensitive to high-frequency gravitational waves, these instruments are blind to others,” explained Ismael Cognard, CNRS research director at the Laboratoire de Physique et de Chimie de l’Environnement et de l’Espace (LPC2E) in Orléans. And they are blind, in particular, to gravitational waves created throughout the universe by the encounter of pairs of other black holes, millions of times more massive than the first. Those are the black holes occupying the center of galaxies.
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