“The start of something big” – first compelling evidence of low-frequency gravitational waves in the cosmos

Posted on: 29 June 2023

The international collaboration of European astronomers, together with Indian and Japanese colleagues, have today published the results of more than 25 years of diligent observations using six of the world's most sensitive radio telescopes – reporting the first evidence of ultra-low-frequency gravitational waves in the cosmos.

In a series of papers published today in Astronomy & Astrophysics and on ArXiv.org, the scientists show that their data are consistent with a “background hum” of low-frequency gravitational waves.

As these black holes orbit each other they lose energy through gravitational wave emission causing them to in-spiral and ultimately merge. 

The European Pulsar Timing Array (EPTA) is a collaboration of scientists from more than ten institutions across Europe, which brings together astronomers and theoretical physicists in order to use observations of ultra-regular pulses from extinguished stars called “pulsars” to construct a Galaxy-sized gravitational wave detector. 

Prof. Evan Keane, Associate Professor of Radio Astronomy in Trinity College Dublin’s School of Physics, and Head of the Irish LOFAR Telescope, describes the results as “the start of something big.”

“Pulsars are basically super clocks in space. By monitoring the ‘ticks’ from these clocks, which are spread throughout our Galaxy, we can see the impact of passing gravitational waves in making the pulsar signals arrive earlier or later,” he says. 

The EPTA announcement is coordinated with similar publications from other teams across the world, namely the Australian (PPTA), Chinese (CPTA) and North-American (NANOGrav) collaborations. This same evidence for gravitational waves is seen by NANOGrav and is consistent with the results reported by the CPTA and PPTA. 

The Jodrell Lovell Telescope used by the EPTA looks up at the sky, which is blue with pale clouds. Image credit: Ant Holloway.

The analysis is in line with what astrophysicists expect, although Prof. Alberto Vecchio from the University of Birmingham, UK, says: “The gold-standard in physics to claim the detection of a new phenomenon is that the result of the experiment has a probability of occurring by chance less than one time in a million.” 

The results reported by EPTA – as well as by the other international collaborations – do not yet meet this criterion.

However, combining all of the world-wide datasets, as part of what is termed the International Pulsar Timing Array, should allow the astronomers to obtain irreproachable proof and achieve further understanding of the history of the Universe using gravitational waves.

The aim is to expand the current datasets by exploiting an array consisting of over 100 pulsars, observed with 13 radio telescopes, and agglomerating more than 10,000 observations for each pulsar. As Dr Caterina Tiburzi, researcher at NAF, Osservatorio Astronomico di Cagliari, Italy, puts it: “We are opening a new window in the gravitational wave universe.”

And Trinity’s Prof. Keane is understandably excited for the future. He says: “As we collect more data, and as more telescopes join in and we refine our analysis techniques, we can expect an ever more precise view of what the Universe looks like in gravitational waves. I can’t wait to add more layers to the painting.”

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