The Royal Astronomical Society has awarded the Group Achievement Award 2025 to a large-scale scientific project in which France has made a major contribution since its inception: the European Pulsar Timing Array (EPTA).

EPTA was founded in the mid-2000s with the aim of revealing very low-frequency gravitational waves, thanks to the joint observation of very precise "cosmic clocks": pulsars. These are tiny but very dense neutron stars whose apparent beat (corresponding to stellar rotation) can vary, depending on their nature, from a few seconds to the extreme rate of over 700 revolutions in a single second: these ultra-fast-rotating pulsars are called "millisecond pulsars". By correlating the minute variations in the beats of these stars, it is possible to measure the passage of very low-frequency gravitational waves such as those originating from binary systems of supermassive black holes present at the center of galaxies.
In order to measure the minute variations in space-time produced by these very long waves, the EPTA consortium has brought together a dozen research institutes and over 80 researchers (in 2025, but many others have followed in their footsteps) who have observed and continue to observe dozens of pulsars for almost 25 years.
The very first EPTA observations were made in the 1990s, with the program really taking off in 2006, the year the collaboration was founded.
The campaigns were carried out by Europe’s largest and most powerful radio telescopes: Effelsberg (Germany), Jodrell Bank (UK), Nançay (France) and Westerbork (Netherlands), soon joined by the Sardinia radio telescope (Italy).
The quality of chronometric data acquired with a radio telescope, and therefore the sensitivity of temporal measurement series to gravitational waves, depends above all on outstanding instrumentation, which enables successive pulses received from pulsars to be processed and stacked in real time, then corrected for the imprint left by crossing the interstellar medium.
Birth and composition of the PTA-France team}
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After more than twenty years of accumulated data, the first results in terms of the detection of a low-frequency gravitational wave signal were presented in 2023, obtained from the analysis of a set of 25 pulsars.
A new version is underway, with an extended sample of 60 pulsars, which will enable us to better characterize the emission spectrum and improve the measurement of its spatial distribution on the celestial sphere.
Gilles Theureau explains: "The 2023 results are the fruit of a large-scale effort spanning several decades, during which it was crucial to pool the best of European instrumentation (including the large decimetric radio telescope at Nançay) and research, both in pulsar studies and in the astrophysics of ultra-long-period gravitational waves. The union of technical resources and skills was the fundamental ingredient.
This way of working is precisely why the Royal Astronomical Society has decided to award a Group Achievement Award, as the British institute itself explains, ahead of the official ceremony in a few months’ time: "One of EPTA’s strengths is its broad, diverse and egalitarian structure. By involving collaborators of different nationalities and backgrounds, and in particular by encouraging and supporting early-career researchers, EPTA is a model of international and intergenerational collaboration."