This is a first and it is extremely promising. A few days ago, a large international scientific collaboration involving several groups of researchers including Europeans (including many French), Americans, Australians, Indians and Chinese, announced that they had detected gravitational waves of a kind totally new… Remember these ripples in space-time due to cataclysms involving massive stars such as black holes or neutron stars.
Predicted by Albert Einstein for more than a century, these waves move at the speed of light, like ripples on the surface of a body of water. The announcement of their very first detection by the LIGO-Virgo collaborations caused a stir in February 2016. It must be said that it was the start of a real revolution in astronomy: we could no longer only perceive the stars thanks to the light emitted or reflected by them, but also by the effects of gravity resulting directly from their mass.
Since then, facilities designed to track these bursts of gravitational waves, called interferometers, have told us multiple cosmic stories: mergers involving stellar-mass black holes and/or neutron stars.
The signal that has just been extracted from the data accumulated over a quarter of a century by the most sensitive radio telescopes in the world is of an entirely different nature. These waves, captured at very low frequencies, would result from even more fundamental cosmic events since they would emanate from supermassive black holes located at the center of galaxies in the process of merging. Less anecdotal and more important episodes in the history and structuring of our Universe, even if they are not captured individually here but rather constitute a kind of background noise.
Moreover, the way in which these gravitational waves were detected is not trivial. Indeed, no interferometer traditionally used in this field was used to achieve this result – the signals in question would probably not have been within their reach. No, in addition to their radio telescopes, what astronomers have used are very specific stars.
When large stars end their lives in a supernova explosion, they leave behind tiny spheres of neutrons called pulsars. As their name suggests, these pulsars generate a sort of pulsation since two beams emanate from them which sweep through space like a beacon, generating regular radio beeps. These beeps are extremely regular for many so-called ultrastable pulsars. However, the astronomers who capture them with their radio telescope are also able to date each of them with an accuracy of about ten billionths of a second…
In practice, in the new study, the scientists therefore timed the beeps of a sample of these ultrastable pulsars spread across the entire sky. The idea is to identify tiny changes in the rate of their beeps caused by the passage of gravitational waves at the level of these pulsars and/or at the level of the Earth. Indeed, by temporarily deforming space-time, the passage of a gravitational wave also briefly modifies the distance which separates us from a given pulsar and then furtively alters its tempo as it is captured from the Earth.
