June University of Galway Astronomers Participate in International Gravitational Wave Milestone

First compelling evidence for low-frequency gravitational waves announced

The European Pulsar Timing Array joins teams worldwide in announcing compelling evidence for gravitational wave signals of cosmological origin

University of Galway astronomers and international collaborators have published the results of more than 25 years of astronomical observations using the world’s most powerful radio telescopes demonstrating evidence for the detection of ultra-low-frequency gravitational waves.

Such waves, first predicted by Einstein’s General Theory of Relativity, are believed to be produced by supermassive black hole binaries - pairs of black holes with masses billions of times that of the sun – in the centres of distant galaxies, stirring up gravitational waves in space-time as they orbit one another. These waves spread away - like ripples across a pond - changing the properties of space and time as they pass by. 

The way in which these gravitational waves affect time itself forms the basis for work of Sai Chaitanya Susarla, astronomy PhD researcher in University of Galway, School of Natural Sciences.

Along with collaborators in the European Pulsar Timing Array (EPTA) and colleagues in Japan and Indian, they studied the way in which these gravitational waves – the ripples on the universe - affect the most accurate clocks known, extinguished stars called pulsars. 

These pulsars – collapsed stars the size of Galway Bay – spin tens to hundreds of times a second, with their lighthouse beams of radio emission yielding ultra-regular pulses, easily detectable with radio telescopes.

A remarkable prediction of Einstein’s Theory of Relativity is that these regular pulses will be subtly distorted as ultra-low-frequency gravitational waves wash over these pulsars, and astronomers realised that by studying and identifying the way in which these cosmic clocks change their tempo, we could confirm the existence of these bizarre gravitational waves for the first time, and furthermore start to study the Universe anew by in effect constructing a Galaxy-sized gravitational wave detector. 

In a series of papers published today, Sai Chaitanya Susarla and his colleagues show for the first time that their data are indeed consistent with a ‘background hum’ of low-frequency gravitational waves.

Sai Chaitanya Susarla said: “The evidence for nano-hertz gravitational waves represents a scientific milestone in our understanding of the universe. They offer a remarkable avenue for unraveling the mysteries of our universe, enabling us to explore its history, study black holes and dark matter, and probe the cosmological structure. The insights gained from these discoveries have the potential to reshape our understanding of the cosmos, deepening our knowledge of fundamental physics and providing invaluable insights into the workings of the universe at its grandest scales.”

Dr Aaron Golden, School of Natural Sciences, College of Science and Engineering, University of Galway and Sai Chaitanya Susarla’s co-PhD supervisor, said: “This is a fabulous outcome for all the relentless hard work Sai has put into his doctoral studies. It also speaks to the continuing tradition of research excellence in the physical sciences at University of Galway, where the cold hard practicalities of instrumentation and data analytics combined with boundless human curiosity are training the next generation of outstanding scientists and engineers.”

University of Galway alumnus Professor Evan Keane, Associate Professor of Radio Astronomy in Trinity College Dublin’s School of Physics, Head of the Irish LOFAR Telescope and co-PhD supervisor, described the results as “the start of something big”. 

Professor Keane added: “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 making the pulsar signals arrive earlier or later.” 

The European Pulsar Timing Array (EPTA) announcement is coordinated with similar publications from other teams across the world, namely the Australian (PPTA), Chinese (CPTA), Indian Pulsar Timing Array (InPTA), 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 analysis is in line with what astrophysicists expect. 

Professor Alberto Vecchio from the University of Birmingham, points out: “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 indeed to achieve further understanding of the history of the universe using gravitational waves.”

One of Sai Chaitanya Susarla’s collaborators, Dr Caterina Tiburzi, researcher at the Osservatorio Astronomico di Cagliari, Italy, puts it: “We are opening a new window in the gravitational wave universe”. 

Ends