Chinese scientists have found key evidence for the existence of nanohertz gravitational waves.

The research was based on pulsar timing observations carried out with the Five-hundred-meter Aperture Spherical Telescope (FAST).

The detection of nanohertz gravitational waves can be very challenging due to their extremely low frequency, where the corresponding period can be as long as several years and wavelengths up to several light-years.

So far, using FAST-like large radio telescopes to conduct long-term timing observation of millisecond pulsars with extreme rotational stability is the only known method for effectively detecting nanohertz gravitational waves.

"In fact, the real gravitational wave detectors are those pulsars. Researchers use large telescopes to read the signals of these pulsars which act as a very standard 'clock,' to learn the time and to determine how space is affected by the gravitational waves," said Li Kejia, a researcher at the National Astronomical Observatories of Chinese Academy of Sciences (NAOC).

Taking advantage of FAST's high sensitivity, researchers from the Chinese Pulsar Timing Array (CPTA) collaboration monitored 57 millisecond pulsars with regular cadences for 41 months.

They found key evidence for quadrupole correlation signatures compatible with the prediction of nanohertz gravitational waves at a 4.6-sigma statistical confidence level, with a false alarm probability of two in a million.

Self-developed data analysis software and data processing algorithms have been adopted to achieve the breakthrough.

"Nanohertz gravitational waves open an important window for humans to observe the universe, which will certainly lead to many major discoveries in physics," said Chang Jin, the director of NAOC. "Using nanohertz gravitational waves, researchers can study supermassive objects in the universe, such as black holes, supermassive black holes, the formation, evolution and merger of galaxies, and the structure of the early universe."

The research was published online in the academic journal Research in Astronomy and Astrophysics.

Profound significance for human exploration of space 

Acceleration of massive objects disturbs the surrounding space-time and produces "ripples" which are known as gravitational waves.

Although such wave signals are extremely weak, they offer a direct method for probing masses that do not emit light. Therefore, astronomers have long aimed to use gravitational waves to aid in the study of key problems in contemporary astrophysics, such as supermassive black holes, the history of galaxy mergers and the formation of large-scale structures in the universe.

Regional pulsar timing array collaborations, including the North American Nanohertz Observatory for Gravitational Waves, the European Pulsar Timing Array and the Australian Parkes Pulsar Timing Array, have been collecting pulsar timing data for more than 20 years, with the aim of detecting nanohertz gravitational waves. Recently, several new regional collaborations have also joined this field, including CPTA, the India Pulsar Timing Array and the South Africa Pulsar Timing Array.

The Chinese Academy of Sciences deployed a strategic pilot project on the research of multi-band gravitational waves in June 2016. The CPTA research team started test observation as early as in September 2019 when FAST was still in the debugging stage, to accumulate observation data for the detection of nanohertz gravitational waves.

The researchers will further focus on nanohertz gravitational waves and open up a new scientific direction of nanohertz gravitational wave astronomy, and continue to maintain the country's leading position in low-frequency radio astronomy in the world, said Chang. (CGTN)