Recently, the Chinese pulsar time array research team composed of researchers from the National Astronomical Observatory of the Chinese Academy of Sciences and other units used the Chinese sky eye FAST to detect key evidence of the existence of nanohertz gravitational waves, indicating that the research of mainland nanohertz gravitational waves has reached the leading level in international synchronization. The paper was published online in the continental astronomical journal Astronomy and Astrophysics Research (RAA).
Nanohertz gravitational waves are a type of gravitational wave
According to Einstein's general theory of relativity, space-time can be bent, and mass objects accelerate their movement in it, disturbing the surrounding space-time, and gravitational waves will be generated. This is like a stone thrown into the water to produce water waves, so gravitational waves are often called "ripples in space-time".
Li Kejia, a researcher at the National Astronomical Observatory and corresponding author of the paper, said: "Nanohertz gravitational waves are a kind of gravitational waves. Detecting gravitational waves as low as nanohertz will help astronomers understand the origins of the structure of the universe. ”
The gravitational wave signal is extremely weak, but it is a direct means of detecting non-luminous matter in the universe, and detecting gravitational waves and opening a new window for gravitational waves to observe the universe is the goal that astronomers have long pursued. In the 70s and 80s of the 20th century, the existence of gravitational waves was indirectly confirmed by observing the orbital changes of pulsar binary star systems, and won the 1993 Nobel Prize in Physics. In 2016, the US Laser Interferometer Gravitational-Wave Observatory announced the detection of gravitational waves produced by the merger of star-level massive double black holes in the hundredhertz band, and won the 2017 Nobel Prize in Physics. In August 2017, the US Laser Interferometer Gravitational-Wave Observatory detected the gravitational waves generated by the merger of two neutron stars, and in addition to gravitational wave signals, full-band electromagnetic radiation was also observed, indicating the advent of the era of multi-messenger astronomy.
Gravitational waves produced by more massive objects are less frequent. "For example, the supermassive binary black hole system at the center of the galaxy is the most massive object in the universe, with a mass of hundreds to hundreds of billions of times that of the sun," Li said. The gravitational waves generated by its rotation are mainly concentrated in the nanohertz band, and the corresponding signal time scale is years to decades. In this frequency band, there may also be gravitational waves generated by parts of the original gravitational waves of the early universe that have survived to this day and strange objects such as cosmic strings. ”
Opening a new window for nanohertz gravitational waves to detect the universe is of great significance for understanding supermassive black holes, the history of galaxy mergers, and the formation of large-scale structures of the universe.
Detecting nanohertz gravitational waves is challenging
The discovery of nanohertz gravitational waves is one of the focal points of international competitions in the field of physics and astronomy.
Xu Heng, assistant researcher at the National Astronomical Observatory and first author of the paper, said: "Nanohertz gravitational waves are very challenging to detect due to their extremely low frequency and period of several years, and their wavelengths can reach several light years. The use of large radio telescopes to conduct long-term time observation of a group of millisecond pulsars with extremely regular rotation is the only known detection method of nanohertz gravitational waves. ”
Internationally, the North American Nanohertz Gravitational Wave Observatory, the European Pulsar Time Array, and the Australian Parkes Pulsar Time Array have used their respective large radio telescopes to carry out the search for nanohertz gravitational waves for 20 years. Recently, a number of new forces have gradually joined this field, including the Chinese pulsar timing array on the mainland, the Indian pulsar timing array and the South African pulsar timeless array.
In this study, the scientific research team used the Chinese sky eye to conduct long-term systematic monitoring of 57 millisecond pulsars, and formed these millisecond pulsars into a gravitational wave detector the size of the Milky Way to search for nanohertz gravitational waves. Based on independently developed software, the researchers analyzed and studied the data collected by China's Sky Eye over a period of 3 years and 5 months, and found evidence of quadrupole correlation signals with nanohertz gravitational wave characteristics at a 4.6 sigma confidence level (false alarm rate less than 1 in 500,000).
The sensitivity of pulsar time-array detection of nanohertz gravitational waves is strongly dependent on the observation time span – that is, the sensitivity increases rapidly as the observation time span increases.
Faced with the unfavorable situation that the observation time span is much shorter than the three international teams in the United States, Europe and Australia, the research team made full use of the advantages of high sensitivity of China's sky eye, large number of monitorable pulsars and higher measurement accuracy, systematically monitored a large number of millisecond pulsars for a long time, independently developed independent data analysis software, and made up for the gap in time span with the advantages of data accuracy, number of pulsars and data processing algorithms, so that the sensitivity of mainland nanohertz gravitational wave detection quickly reached the same level as that of the United States, Europe and Australia. Thus achieving this major scientific breakthrough at the same time. However, limited by the short time span of the current observation data, the team is temporarily unable to determine the main physical source of gravitational waves in the nanohertz band.
"But this will be solved as the time span of subsequent observations increases." "Because our team's existing data span is shorter, the effect of data time span growth will be more obvious, for example, if the data time span grows for another 3 years and 5 months, our data time span will double, while other international teams will only grow by less than 20%. ”
A whole new window opens for gravitational-wave astrophysics
In September 2020, the North American Nanohertz Gravitational-Wave Observatory used 12.5 years of observations from 47 pulsars to claim that the central amplitude of the nanohertz gravitational wave was measured to be 1.92 x 10^-15, and the frequency spectral index was consistent with the theoretical expectation of a supermassive binary black hole system of -2/3, but the Hellings-Downs curve of the quadrupole moment correlation, which is the key evidence of gravitational waves, was not seen in the signal.
Cai Ronggen, an academician of the Chinese Academy of Sciences, said: "The results of the Chinese pulsar timing array research team are consistent with the theoretical expectations of the supermassive double black hole system. Of particular importance, the team found the existence of quadrupole moment correlations in the signal within 4.6 standard deviations. ”
Cai Ronggen told reporters that through the further accumulation of data, a key next step is to determine the gravitational spectrum index. "If the spectral index is -2/3, it is certain that the gravitational wave source detected is a supermassive binary black hole system, which is the first time such a supermassive binary star system has been observed in the universe, which is of great significance for understanding the formation and evolution of these binary star systems." In addition, in addition to the supermassive double black hole system, theoretical studies have shown that there are also rich physical processes that produce gravitational waves in the nanohertz band, such as cosmic strings, cosmic walls, cosmological phase transitions, random gravitational waves caused by curvature fluctuations during cosmic inflation. The key to distinguishing these physical processes is to determine the gravitational spectral index. ”
He Zishan, chair professor at Peking University, said: "The generation of nanohertz gravitational waves is a unique prediction of the merger of supermassive black holes. This long-awaited prediction was finally studied by the Chinese pulsar time array to obtain observational evidence by using the Chinese sky eye. This is a major scientific breakthrough with lasting and enormous significance. Not only does it have profound implications for the broad field of galaxy evolution and supermassive black hole research, but it also opens a whole new window into gravitational-wave astrophysics. ”
Subsequently, the National Astronomical Observatory of the Chinese Academy of Sciences will give full play to the international leading advantage of China's sky eye pulsar timing accuracy, accelerate the scientific research of nanohertz gravitational wave detection, accumulate longer-term observation data, gradually publish higher precision detection results, and open a new window for human beings to use nanohertz gravitational waves to detect the universe.
Chang Jin, director of the National Astronomical Observatory of the Chinese Academy of Sciences and academician of the Chinese Academy of Sciences, said: "The National Astronomical Observatory will also actively promote the expansion and upgrading of China's sky eye, based on the pulsar timing array method, to achieve routine observation of nanohertz gravitational wave events, so as to build a nanohertz gravitational wave observatory, and open a new era of low-frequency radio observation and research with higher sensitivity and higher resolution, and accelerate the construction of the mainland into a powerful country in gravitational wave astronomy and radio astronomy." ”
Source: People's Daily client