The clock stability properties of pulsars can be used as a detector of space-time perturbations at the scale of our galaxy, and hence of low-frequency gravitational waves. This idea was first proposed at the end of the 1970's, but it was only implemented in the mid-2000s, when the broadband processing capabilities of radio telescopes made it possible to achieve the required timing precision. 20 years later, the various programs that have been set up are beginning to bear fruit.
On June 29, 2023, the five continental consortia (in US, Australia, India, China and Europe), constituant members of the International Pulsar Timing Array collaboration, published simultaneaously and independently the first evidences of the presence of gravitational waves with frequencies in the nanoHz range. The emission is likely to come primarily from pairs of supermassive black holes located at the center of galaxies and formed during merging processes throughout the history of the universe, but there are many other possible sources from the primordial universe.
France is one of the most important contributors, providing nearly 70% of European data thanks to the Nançay Radio Telescope, with a dedicated real-time instrumentation and an unrivalled cadence in the monitoring of 80 millisecond pulsars over the last two decades. I propose to look back at this series of results as well as the detection technique and analysis methods employed in this long-term program.
