Pulsar clocks open new window on gravitational waves

A team of European astronomers, together with Indian and Japanese colleagues, has for the first time found strong evidence of ultra-low-frequency gravitational waves, which probably come from pairs of supermassive black holes at the centre of merging galaxies.

It is the result of more than 25 years of observations with the most sensitive radio telescopes in Europe and India, including the Westerbork Synthesis Radio Telescope (WSRT). In doing so, they have opened a new window on gravitational wave research. These gravitational waves contain information about the universe’s best-kept secrets. The research has been published in a series of articles in the scientific journal Astronomy & Astrophysics.

Researcher Anuradha Samajdar was involved in the project from her previous position as a Humboldt Fellow at the University of Potsdam, Germany. Since May 2023, Samajdar is assistant professor at Utrecht University’s Institute for Gravitational and Subatomic Physics (GRASP). She is also affiliated with Nikhef, the Dutch National Institute for Subatomic Physics.

Pulses from extinct stars

The scientists work together within the European Pulsar Timing Array (EPTA) and the Indian Pulsar Timing Array (InPTA). The EPTA is a collaboration of scientists from more than 10 institutions across Europe in which astronomers and theoretical physicists use observations of very regular pulses from pulsars – extinct stars – as a gravitational wave detector the size of our Milky Way galaxy. From the Netherlands, astronomers from ASTRON and Radboud University are involved. Other teams from around the world have independently obtained the same results, also announced today.

Unique sources and phenomena

The astronomically large gravitational wave detector, which spans twenty-five specifically chosen pulsars, scattered across the Milky Way, gives the possibility to investigate gravitational waves at lower frequencies (wavelengths of a few light years) than is possible with other detectors such as LIGO and Virgo (wavelengths of several kilometres). At these nanohertz frequencies, unique sources and phenomena can be observed.

New exploration of the universe

The gravitational waves now seen are most likely a sum of signals from a very large number of supermassive black holes orbiting each other very slowly. The researchers see the results as the beginning of a new exploration of the universe.

It is exciting to see the hints of supermassive black holes, with a million up to tens of billions of solar masses

“It is exciting to see the hints of supermassive black holes, with a million up to tens of billions of solar masses,” says Samajdar. “Less massive black holes, of the order of tens of times the mass of the sun, have already been detected and studied by LIGO and Virgo. Being able to investigate supermassive black holes as well will give us valuable information in a new mass regime where we do not know much yet.”

Neutron star collisions

Samajdar is member of EPTA and of the Virgo and the Einstein Telescope collaborations. She continues: “The future Einstein Telescope will be able to also detect neutron star collisions for a much longer duration than is possible with current detectors. Neutron stars are compact objects with masses much lower than that of black holes. With the bandwidth of Einstein Telescope we will observe these signals for hours, giving valuable information due to the length of the signal and the improved sensitivity of the instrument. So in the future we will be able to study the population of compact objects in the universe in a whole range of masses with many different observation methods.”