Listen to the whispers of the distant universe

Through one astronomical telescope after another, we know that the Milky Way above our heads is not the entire universe. So, scientists threw a "stone" into the center of time, and the "ripples" that emerged are gravitational waves. It can help us see the mysteries hidden in the dark depths of the universe.

Recently, the Chinese Pulsar Timing Array Research Team, composed of researchers from the National Astronomical Observatory of the Chinese Academy of Sciences and other institutions, used the "China Sky Eye" FAST to detect key evidence for the existence of nanohertz gravitational waves. The relevant research results were published online in the academic journal "Astronomy and Astrophysics Research" on June 29, Beijing time.

Chang Jin, an academician of the Chinese Academy of Sciences and director of the National Astronomical Observatory of the Chinese Academy of Sciences, pointed out that by using nanohertz gravitational waves, researchers can study supermassive celestial bodies in the universe, such as black holes and supermassive black holes, the formation, evolution, and merger of galaxies, as well as major scientific problems in astrophysics such as the early structure of the universe.

How difficult is it to detect nanohertz gravitational waves?

We know that about 95% of the universe is eternally "dark." Using gravitational wave observations, we can capture traces of this dark matter and dark energy.

In 1916, Einstein predicted the existence of gravitational waves based on his general theory of relativity. "Because gravitational waves are extremely weak, even Einstein himself did not believe that humans could detect them." Xu Heng, the first author of the paper and a special research assistant at the National Astronomical Observatory, told the Science Times reporter that although the detectable effect caused by gravitational waves is quite weak, on September 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States announced the first observation of gravitational waves.

"Gravitational waves are produced by the accelerated motion of matter. We can use gravitational waves to track the movement of massive matter in the universe." Xu Heng further explained that space-time can be curved. The mass of an object will cause the curvature of the space-time around it. The greater the mass, the greater the curvature of space-time.

In the vast universe, as long as there is accelerated motion of matter, there will be gravitational radiation. Xu Heng said that the frequency range of gravitational waves is very wide, ranging from kilohertz to 10-16 Hz, and the detection methods of gravitational waves in different frequency bands are different, because gravitational wave detectors are always only sensitive to gravitational waves within a certain frequency range, and they cannot replace each other.

"Detectors of high-frequency gravitational waves (in the hundred-hertz band) are mainly measured through ground-based laser interferometers, such as LIGO; low-frequency gravitational waves (in the millihertz band) are mainly detected through space laser interferometers, such as the European LISA project and my country's Taiji and Tianqin detectors," Xu Heng introduced.

The gravitational waves generated by more massive objects have lower frequencies. The gravitational waves generated by the rotation of the supermassive binary black hole system at the center of the galaxy, the most massive object in the universe, are mainly concentrated in the nanohertz frequency band.

"Gravitational waves in the nanohertz band have a period of several years to decades and a wavelength of tens of light years. So far, the only known means of detection is to monitor multiple pulsars with high timing accuracy through radio telescopes, namely pulsar timing arrays," said Xu Heng.

That is to say, the detection of nanohertz gravitational waves is "large" both in physical scale and time scale, which is not an easy task.

Why use pulsars?

Since it is very difficult to detect gravitational waves with frequencies as low as nanohertz, scientists need a reliable "assistant", so they set their sights on pulsars.

Pulsars are a type of compact celestial body, so named because their radiation beams periodically sweep across the Earth, causing Earthlings to see periodic pulses. The advantage of pulsars is that they rotate very stably and send out a pulse signal at regular intervals. If not affected by other factors, we can stably receive this signal on Earth.

Xu Heng added that there is no requirement for the distance of pulsars to detect nanohertz gravitational waves, and the more pulsars there are, the better, in order to distinguish gravitational waves from other noise.

In June 2016, the Chinese Academy of Sciences launched preliminary research on nanohertz gravitational waves and established the Chinese Pulsar Timing Array (CPTA) team in collaboration with Peking University, Xinjiang Astronomical Observatory, Yunnan Astronomical Observatory, Shanghai Astronomical Observatory, National Time Service Center, Guangzhou University and other related units.

FAST is currently the world's largest and most sensitive single-aperture radio telescope, and is also the most efficient radio telescope in the world for searching pulsars. So far, more than 740 new pulsars have been discovered.

Compared with other telescopes such as LIGO, how is FAST's detection method different? Xu Heng explained that the principles and essence of the two detections are the same, and both determine the existence of gravitational waves by monitoring the tiny changes in electromagnetic wave signals during propagation. Xu Heng gave an analogy that each pulsar is a reflector of a ground-based laser interferometer. "Of course, there will be some differences in the subsequent data processing and detection statistical methods."

Will detect a single gravitational wave signal

As a direct means of detecting non-luminous matter in the universe, gravitational wave detection has been a long-term pursuit of astronomers, especially wanting to reveal the appearance of black holes or the early universe by continuously "monitoring" these low "sounds" in the universe.

"Information such as the physical source of nanohertz gravitational waves requires precise measurement of the spectrum of nanohertz gravitational waves, which in turn strongly depends on the time span of our data. The larger the time span, the more accurately we can measure the spectrum of nanohertz gravitational waves." Xu Heng revealed that next, the team will need to continue to carry out observations according to the original plan, accumulate long-period data, and continuously increase their detection accuracy of nanohertz gravitational waves.

Xu Heng said that what is detected now is actually the background of countless nanohertz gravitational waves superimposed, which is the first step. The team's next goal is to detect the signal of a single nanohertz gravitational wave. "In this way, we can find out which supermassive binary black hole in the center of a galaxy produced this gravitational wave, and then all telescopes from gamma rays to radio will play a role.

In addition, it is also very important to accurately limit the information of the nanohertz gravitational wave spectrum and then determine the physical source of nanohertz gravitational waves or make precise restrictions. "Because in this frequency band, in addition to the gravitational waves generated by the rotation of the supermassive binary black hole system at the center of the galaxy, there are also gravitational waves from the early universe that have survived to this day and gravitational waves generated by strange objects such as cosmic strings," said Xu Heng. "However, at this stage we cannot distinguish them for the time being."

Today, we know that the universe is immersed in a background of gravitational waves, with long-standing low-frequency gravitational waves oscillating in every corner, opening a window to many unknown cosmic events.