Gravitational waves and supermassive black holes with the Earth at the centre
Gravitational waves by supermassive black holes - Danielle Futselaar, MPIfR

Tantalising evidence of elusive low-frequency gravitational waves has been revealed by a European team of scientists studying data collected over two decades.

The results, which include analysis carried out by astronomers at the University of Birmingham and Manchester, are published today in Astronomy and Astrophysics.

Ultra-low frequency gravitational waves carry information about some of the best kept secrets of the Universe including the formation of binary systems of super-massive black holes and events that may have taken place when the Universe was in its infancy, just a few seconds old.

Scientists from the European Pulsar Timing Array, in collaboration with Indo-Japanese astronomers from the Indian Pulsar Timing Array have been searching for these signals using six of the world’s most powerful radio telescopes. The telescopes monitor radio signals from rotating neutron stars, called pulsars.

The results presented today mark the beginning of a radically new journey into the Universe to unveil some of its mysteries.

Professor Alberto Vecchio - University of Birmingham Institute for Gravitational Wave Astronomy

By harnessing the combined power of 25 of these pulsars, the team have been able to form a galactic sized gravitational-wave detector which measures the exact arrival time of the radio signals from pulsars. As the signals travel through space and time, the presence of gravitational waves affects their path, creating characteristic irregularities.

The analysis of this unprecedented data set has revealed irregularities which are consistent with the effect produced by gravitational waves at ultra-low frequency, waves that oscillate with periods between one and ten years.

Professor Alberto Vecchio, of the University of Birmingham’s Institute for Gravitational Wave Astronomy, commented: “The results presented today mark the beginning of a radically new journey into the Universe to unveil some of its mysteries. Our analysis reveals a common signal which has been persistent throughout the many years of monitoring these pulsars, as if these cosmic clocks are pitching and rolling in the waves of space time.

“Although we are not able yet to confirm definitively the presence of ultra-low frequency gravitational waves, the presence of this common signal across all the pulsars in the array is consistent with what astrophysicists expect.”

The European Pulsar Timing Array (EPTA) has combined the observations of 25 millisecond pulsars, spanning over 25 years of regular observations taken with five of the largest radio telescopes in Europe: the Lovell Telescope of the Jodrell Bank Observatory in the United Kingdom, the 100-m Effelsberg Radio in Germany, the Nancay Radio Telescope in France, the Sardinia Radio Telescope in Italy and the Westerbork Radio Synthesis Telescope in the Netherlands. To complement this data set, also 10 years of data acquired at the Giant Metrewave Radio Telescope by the Indian Pulsar Timing Array have been included in the analysis.

Dr Paul Brook, a post-doctoral researcher at the University of Birmingham’s Institute for Gravitational Wave Astronomy, who has been heavily involved in the analysis of the EPTA data commented: “It's taken the EPTA 25 years of incredibly meticulous pulsar observations to finally see some evidence of the gravitational wave background. Though predicted by his general theory of relativity, the fact that Einstein thought gravitational waves were too quiet to ever be observed conveys the magnitude of the undertaking.“

The results from the EPTA are also consistent with the results of analyses using data sets collected at different radio telescope monitoring other pulsars by the Australian (PPTA), North-American (NANOGrav) and Chinese pulsar timing array collaborations that are also released today.

The gold-standard in physics to claim the detection of a new phenomenon is that the result of the experiment has a probability of less than one time in a million of occurring by chance. The results reported by EPTA do not yet meet this gold-standard, but work is already in progress to combine data sets from the four collaborations – EPTA, InPTA, PPTA and NANOGrav – under the auspices of the International Pulsar Timing Array to create an array consisting of over 100 pulsars.

This combined effort may allow this goal to be reached soon. “This new journey has just begun,” said Professor Vecchio. ”Listening to the gravitational broadcast of the Universe with even better telescopes monitoring a growing number of pulsars will surely reveal many surprises in the future.”