Scientists find key evidence for the existence of nanohertz gravitational waves

(Nanowerk News) A group of Chinese scientists recently found key evidence for the existence of nanohertz gravitational waves, heralding a new era in nanohertz gravitational wave research. This research is based on pulsar timing observations made with the Five hundred meter Aperture Spherical Telescope (FAST).

The research was carried out by the cooperation of the Chinese Pulsar Timing Array (CPTA). Researchers (Prof. Kejia Lee, Post-Doc. Siyuan Chen, PhD students Jiangwei Xu, and Zihan Xue) from the Astronomy Department of the Physics School and Peking University’s Kavli Institute of Astronomy and Astrophysics played a key role in the collaboration.

Their findings are published online in an academic journal Astronomy and Astrophysics Research (“Searching for the Nano-Hertz Stochastic Gravitational Wave Background with the Release of China Pulsar Time Array Data I”).

The acceleration of a massive object disrupts the surrounding spacetime and generates “ripples,” or gravitational waves. The detection of nanohertz gravitational waves will help astronomers understand the formation of the universe’s structure, and investigate the growth, evolution, and mergers of supermassive black holes, the most massive celestial bodies in the universe. It will also help physicists gain insight into the basic physical laws of space-time.

The CPTA collaboration used FAST to perform long-term systematic monitoring of the 57-millisecond pulsar. This millisecond pulsar forms a Galactic-scale gravitational-wave detector that is sensitive to nanohertz gravitational waves. Based on data collected via FAST spanning 3 years and 5 months, the CPTA team found signature evidence of a quadrupole correlation that is compatible with predictions of nanohertz gravitational waves at the statistical confidence level of 4.6 sigma (2 million false alarm probability).

The gravitational wave signal is very weak, but it directly probes the non-light-emitting masses in the universe. Opening the cosmic observation window for gravitational waves has been one of the main goals that astronomers have long pursued. Between the 1970s and 1980s, the existence of gravitational waves was indirectly confirmed by observing the changing orbits of a pulsar binary system, which earned Hulse and Taylor the 1993 Nobel Prize in Physics.

In 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the detection of gravitational waves from merging stellar-mass binary black holes in the 100 Hz frequency band, and immediately won the 2017 Nobel Prize in Physics. More massive objects produce gravitational waves with lower frequencies. For example, the most massive celestial bodies in the universe, the supermassive (100 million to 100 billion solar masses) black hole binaries at the center of galaxies mainly generate gravitational waves in the nanohertz band, and the time scale of the corresponding signals is from years to decades. This frequency band also contains gravitational wave contributions from processes in the early universe and exotic bodies such as cosmic strings. Similar to LIGO, PTA is a direct GW detection method, where LIGO measures the laser phase to detect GW and PTA measures the rotational phase of the pulsar.

Taking advantage of FAST’s high sensitivity, the CPTA research team has been monitoring a 57-millisecond pulsar with a regular rhythm for 41 months. The team found key evidence for a signature quadrupole correlation that is compatible with predictions of nanohertz gravitational waves at the 4.6-sigma statistical confidence level (with a two-in-million false-alarm probability).

The team uses independently developed data analysis software and data processing algorithms to achieve its breakthroughs at the same time as other international groups. Independent data processing pipelines produce compatible results.

The time span of the CPTA data set is much shorter than the time spent by other telescopes detecting gravitational waves. However, due to the high sensitivity of the FAST telescope, in only 3.5 years the CPTA achieved the same sensitivity compared to other PTAs. Future observations will quickly expand the range of CPTA data and help identify astrophysical or current signal sources.

Objects with greater mass generate gravitational waves of lower frequency. For example, the most massive celestial bodies in the universe, the supermassive black hole binaries (with 100 million to 100 billion solar masses) at the center of galaxies, generate primarily gravitational waves in the nanohertz band, with signal timescales corresponding to years to decades. This frequency band also includes gravitational wave contributions from the processes of the early universe as well as exotic objects such as cosmic strings.

Using nanohertz gravitational waves in cosmic observations is critical to studying key issues in contemporary astrophysics such as supermassive black holes, the history of galactic mergers, and the formation of large-scale structures in the Universe.

The detection of nanohertz gravitational waves is very challenging, due to their very low frequency, where the corresponding period can be as long as several years and the wavelength up to several light years. So far, long-term observation of millisecond pulsars with extreme rotational stability is the only known method for effectively detecting nanohertz gravitational waves.

Hunting for these waves is one of the main focuses of physics and astronomy today. Regional pulsar timing collaborations, including the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the European Pulsar Timing Array (EPTA), and the Australian Parkes Pulsar Timing Array (PPTA), have been collecting pulsar timing data for more than 20 years, with the goal of detecting nanohertz gravitational waves.

Recently, several new regional collaborations have also joined forces in this field, including the CPTA, India Pulsar Timing Array (InPTA), and South Africa Pulsar Timing Array (SAPTA). NANOGrav, EPTA, and PPTA will publish similar results compatible with the current CPTA findings on the same day. The detection sensitivity of the pulsar’s time series to nanohertz gravitational waves is highly dependent on the observation time span—that is, the sensitivity grows rapidly with increasing observation time ranges.

In the future, this regional cooperation will promote international pulsar timeframe cooperation and expand exploration of the universe through observing nanohertz gravitational waves.