Clock-like precision of pulsars opens new gravitational-wave window

First evidence for ultra-low-frequency gravitational waves

An international collaboration of European astronomers including scientists from the Max Planck Institutes for Gravitational Physics and Radio Astronomy, together with Indian and Japanese colleagues, have published the results of more than 25 years of observations from six of the world's most sensitive radio telescopes. Within these data, the first evidence for ultra-low-frequency gravitational waves has been seen, expected to come from a background of pairs of supermassive black holes found in the centres of merging galaxies. These results are a crucial milestone in opening a new, astrophysically-rich window in the gravitational-wave spectrum.

In a series of papers published today in “Astronomy and Astrophysics”, scientists of the European Pulsar Timing Array (EPTA), in collaboration with Indian and Japanese colleagues of the Indian Pulsar Timing Array (InPTA), report on results from data collected over 25 years, which bring with them the promise of unprecedented discoveries in the study of the formation and evolution of our Universe and the galaxies that populate it.

The EPTA is a collaboration of scientists from more than ten institutions across Europe, and 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.

A giant gravitational-wave detector

Pulsars are excellent natural clocks. Scientists use the incredible regularity of their signals to search for minute changes in their ticking to detect the subtle stretching and squeezing of space-time by gravitational waves originating from the distant Universe. This gigantic gravitational-wave detector – spanning from the Earth to pulsars across the Galaxy – makes it possible to probe gravitational-wave frequencies much lower than those probed by other experiments. The observations are shedding light on the gravitational-wave Universe in the nanohertz regime, revealing unique sources and new phenomena.

“In the data we analysed we found evidence for a gravitational-wave background,” says Jonathan Gair, Group Leader in the “Astrophysical and Cosmological Relativity” department at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Potsdam and co-author of the study. The most likely origin of this background is the cosmic population of binary black holes with masses millions to billions of times that of the Sun which form when galaxies merge. This is an entirely new source of gravitational waves inaccessible to terrestrial gravitational-wave detectors.

“Analysing the data from pulsar timing arrays is complicated by the fact that PTAs use astrophysical objects as detectors,” adds Gair. “There are many different sources of noise intrinsic to the pulsars that must be accounted for while searching for the signature of gravitational waves. The signal itself is also stochastic, so it looks like noise.” Evidence for an astrophysical background comes from seeing noise with similar characteristics in all the pulsars in the array. The unique feature of the gravitational-wave origin appears in the way the amplitude of the signal is correlated between different pulsars. It is this correlation that is now beginning to be seen in the data.

Coordinated effort yields a unique data set

The results are based on decades of coordinated observing campaigns using the five largest radio telescopes in Europe: the 100-m Effelsberg Radio Telescope in Germany, the Lovell Telescope of the Jodrell Bank Observatory in the United Kingdom, the Nançay Radio Telescope in France, the Sardinia Radio Telescope in Italy and the Westerbork Radio Synthesis Telescope in the Netherlands. Once a month, these telescopes are additionally recording data together to enhance sensitivity, comparable to the largest radio telescope on Earth. These observations have been complemented by data collected with the Giant Metrewave Radio Telescope by the InPTA, leading to the development of a uniquely sensitive dataset. In the current set of papers, results were based on a subset of 25 pulsars chosen to give the greatest sensitivity to a gravitational wave background.

Michael Kramer, director at the Max Planck Institute for Radio Astronomy in Bonn, Germany, emphasizes, “Data from the Effelsberg telescope stretches back more than 25 years. This is important, as it makes the EPTA uniquely sensitive to the lowest frequencies probed”.

The announcement of the EPTAs results are coordinated with similar publications by other collaborations across the world, namely the Parkes-based Australian, Chinese, and  North-American pulsar timing array (PTA) collaborations, abbreviated as the PPTA, CPTA and NANOGrav, respectively. Astronomers are confident that what they see are signatures of gravitational waves as their results are consistent with and supported by similar data and results across all PTA collaborations.

"In our last data set we saw evidence of a common noise in the pulsars, but could not say what its origin might be,” explains Lorenzo Speri, PhD student in the “Astrophysical and Cosmological Relativity” department and co-author of the study. “It is very exciting that we are now starting to see the characteristic correlation pattern that suggests it is a gravitational-wave background. We are looking forward to new insights that will come from the interpretation of results over the coming years."

The uniquely long and dense data set of the EPTA provides sensitivity to a wide range of frequencies, over which to probe the physics of merging galaxies. The length of the data set allows it to probe frequencies of the gravitational waves as slow as 1 oscillation every 30 years, while the cadence of the data makes it possible to study frequencies as high as several oscillations per month. Overall, this allows the EPTA data set to probe black hole systems with orbital periods from months up to tens of years.

The analysis of the EPTA data presented today is in line with what astrophysicists expect. Nevertheless 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 result reported by the EPTA – as well as by the other international collaborations – does not yet meet this criterion, but work is already in progress: Scientists from most of the leading PTAs are combining their data sets under the auspices of the International Pulsar Timing Array. The aim is to expand the current datasets, by exploiting an array consisting of over 100 pulsars, observed with thirteen radio telescopes, and agglomerating more than 10,000 observations for each pulsar, which should allow the astronomers to obtain irreproachable proof of the presence of a gravitational-wave background at nanohertz frequencies.