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Recently, a series of papers have been published by multinational scientific teams, including China, announcing that they have found evidence of the existence of ultra-low frequency gravitational waves for the first time. The wavelengths of gravitational waves previously detected by astronomers are usually tens to hundreds of kilometers, but this time the wavelengths of gravitational waves can reach several light-years. So what does the discovery of "monster-level" gravitational waves mean?

(by Chen Qiang / tr. by Robert Taylor)

According to Einstein's general theory of relativity, when a mass object moves with acceleration, it will produce ripples in space-time and propagate outward at the speed of light from its position. Such ripples in space-time are gravitational waves. Where gravitational waves pass through, space-time stretches and squeezes rhythmically, and its frequency is the frequency of gravitational waves.

The gravitational waves produced by most objects are very weak, and only those severe astronomical events, such as the merging of black holes or neutron stars, can produce gravitational waves that are strong enough to be observed. In September 2015, the Laser interference gravitational Wave Observatory (LIGO) detected gravitational waves generated by the merging of two black holes for the first time.

Since then, gravitational wave detectors on Earth have captured nearly 100 confirmed gravitational wave events. These events are caused by the merging of neutron stars or small and medium-sized black holes, and the gravitational waves they emit are high-frequency, so they can be detected by detectors a few kilometers from Earth. If it is a larger astronomical event, such as the merger of supermassive black holes in the center of the galaxy, the gravitational waves will be low-frequency, the frequency is on the nanohertz scale, and their wavelengths can be extended to several light-years. To detect such gravitational waves, you need a detector so long that the whole earth or even the entire solar system can't hold it.

Fortunately, there is a kind of celestial body in our Milky way that emits very precise pulses that are affected by low-frequency gravitational waves.

This kind of celestial body is a pulsar, which is a very fast rotating neutron star that emits a beam of electromagnetic radiation from the magnetic pole. As they spin, these beams of electromagnetic radiation sweep around the universe like lighthouses. When we sweep the earth, we will detect pulses one by one.

Because the pulsar's rotation period is very stable, we will detect pulses with stable intervals on Earth, usually in the range of milliseconds to tens of seconds. When the gravitational wave passes between the earth and the pulsar, the time of arrival of the pulse signal is disturbed.

The disturbance of a single pulse signal is not enough to indicate the passage of gravitational waves. If the radio telescope is used to observe a group of rotational stable pulsars for a long time, and it is found that the signals generated by multiple pulsars change regularly at some time, it means that gravitational waves have been detected. And this is what scientific teams around the world have been doing.

Recently, the Chinese Pulse Star timing Array (CPTA), the North American Nahertz gravitational Wave Observatory (NANOGrav), the European Pulse Star timing Array (EPTA) and the Australian Parks Pulse Star timing Array (PPTA) have detected relevant evidence of the existence of low-frequency gravitational waves using their respective radio telescopes.

Among them, the research team of the Chinese Pulse Star timing Array used the 500m aperture spherical radio telescope (FAST), known as the "Chinese Sky Eye", to systematically monitor 57 millisecond pulsars for a long time. After analyzing the data collected by FAST over a period of three years and five months, astronomers found evidence of low-frequency gravitational waves.

Other research teams have given similar evidence. It should be pointed out that gravitational waves discovered by pulsars are not usually generated by a single astronomical event, but are low-frequency space-time ripples caused by the combination of all the gravitational waves accumulated in the universe. Therefore, astronomers call them "gravitational wave background".

So far, the data given by these studies are slightly different from the scientifically recognized detection standards. As a result, astronomers working on the project modestly claim to have found only "evidence" of gravitational waves. However, more detection and data analysis are still under way, and most astronomers believe that the final results of the study will be confirmed within the next year or two.

Open a new chapter in gravitational wave detection if they are really low-frequency gravitational waves, how do they come about? The most obvious answer is the merger of supermassive black holes. After all, there are supermassive black holes in the center of almost every galaxy, and galactic merging is a very common phenomenon.

But it is not the only potential source of low-frequency gravitational waves. Another possibility is cosmic strings. Cosmic string, which was first conceived by British physicist Tom Tom Kibble in the 1970s, is essentially a rift in time and space caused by the expansion of the universe. Cosmic strings may kink and eventually break and produce gravitational waves. In addition, there are many strange physical mechanisms, such as some types of dark matter activity and the rapid expansion of space after the Big Bang, all of which can produce low-frequency gravitational waves.

Using pulsars to detect fixed gravitational waves does not require precision instruments like LIGO, just radio telescopes and enough patience. This new discovery opens a new chapter in gravitational wave detection. By analyzing these low-frequency gravitational waves, we can "listen" to more celestial bodies and events in the depths of the universe.

References:

Https://iopscience.iop.org/article/10.1088/1674-4527/acdfa5

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