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Xinfan Technology News, on August 27, Beijing time, reported that among all astronomical concepts, black holes might be the most peculiar. The density of black holes is extremely high, even light cannot escape, like a terrifying giant dark trap. As common laws of physics don't apply in black holes, black holes seem as if they were born for science fiction. However, various direct and indirect evidence indicate that black holes indeed exist in the universe.

Einstein's Prophecy

Black holes are an inevitable result of Einstein's general relativity.

German astronomer Karl Schwarzschild first predicted the existence of black holes in 1916, considering it to be an inevitable result of Einstein's general relativity. In other words, if Einstein's theory is correct (and all evidence points to this), then black holes must exist. The research of Roger Penrose and Stephen Hawking further solidified the theoretical foundation for the existence of black holes. Their research showed that any star collapsing into a black hole will form a singularity, and traditional physical laws completely fail at this point.

Gamma-ray Bursts

Terrestrial observation equipment has already detected some gamma-ray bursts produced in the process of black hole formation.

In the 1930s, Indian astrophysicist Subrahmanyan Chandrasekhar conducted research on the fate of stellar nuclear fuel depletion, discovering that the final outcome depends on the star's mass. If a star is very massive, say 20 times the mass of the Sun, its dense core (whose mass alone may reach two or three times the Sun's mass) will continue to collapse until it forms a black hole. The core's collapse speed is extremely fast, lasting only a few seconds, releasing an astonishing amount of energy in the form of a gamma-ray burst, equivalent to the total energy a normal star emits throughout its lifetime. Terrestrial telescopes have detected multiple gamma-ray bursts, some of which were emitted by galaxies billions of light years away, indicating that indeed we have observed the process of black hole formation.

Gravitational Waves

Pictured is an artist's impression of gravitational wave ripples. The gravitational interaction between two black holes creates ripples in time, spreading outward in the form of gravitational waves.

Black holes are not always solitary; sometimes they appear in pairs, orbiting each other. The gravitational interaction between two black holes creates ripples in time, spreading outward in the form of gravitational waves, which is another one of the predictions of Einstein's relativity. With the help of observatories like LIGO and Virgo, we now have the ability to detect gravitational waves. In 2016, scientists announced for the first time the discovery of gravitational waves produced by the merger of two black holes. Since then, more gravitational wave events have been detected. As the sensitivity of the detectors continues to improve, scientists have also detected gravitational waves from events other than black hole mergers, such as black holes colliding with neutron stars and so on.

Invisible Companion

Pictured is an imagined depiction of the orbital motion in the trinary system HR6819.

The events capable of producing gamma-ray bursts or gravitational waves occur in a short time and can be detected from halfway across the universe. However, considering their nature, most black holes are undetectable. Black holes do not emit any light or radiation, thus they can silently lurk in the darkness, without astronomers noticing their existence. However, there is a method to detect their presence: using the gravitational influence a black hole exerts on other stars. In 2020, while observing the seemingly ordinary stellar system HR6819, astronomers noticed unusual movements in the trajectories of two stars, which could only be explained by the presence of a completely invisible object in the system. Calculating its mass, researchers realized there was only one possibility: this object must be a black hole. It is only a thousand light years away from Earth, located in the Milky Way Galaxy, and is the closest black hole discovered to Earth so far.

X-ray

The black hole Cygnus X-1 is currently consuming its giant blue supergiant companion star.

In 1971, while studying a stellar system in the Milky Way Galaxy named Cygnus X-1, scientists first observed evidence of a black hole's existence. The X-rays generated by this system are extremely bright, but these rays do not come from the black hole or its visible companion star; they are produced by the accretion disk formed as the black hole consumes stellar material. As in the case of the HR6819 system mentioned earlier, astronomers can also use the orbital trajectory of the visible star to estimate the mass of the invisible object in the Cygnus X-1 system. The final calculation result is approximately 21 times the mass of the Sun, and considering the relatively small space it occupies, it can only be a black hole, without the need to consider other possibilities.

Supermassive Black Holes

There's also a supermassive black hole at the center of the Milky Way Galaxy. 𝚏𝗿𝗲𝐞𝐰𝚎𝕓𝐧𝚘𝘃𝗲𝐥.𝐜𝚘𝕞

Besides black holes formed by stellar collapse, there is evidence that supermassive black holes, with masses reaching millions or even billions of times that of the Sun, might be lurking at the centers of galaxies and may have existed since the early universe. In the so-called "active galaxies," the evidence for the existence of these supermassive black holes is astounding. NASA indicates that around the black holes at the centers of such galaxies are accretion disks that emit extremely intense radiation across various spectral bands. There is a black hole at the center of the Milky Way Galaxy because we have observed that the star rotation speed in that region is alarmingly high, reaching 8% of the speed of light, indicating they must be orbiting around a massively dense but extremely small-object. Current estimates suggest that the black hole at the center of the Milky Way Galaxy has a mass approximately 4 million times that of the Sun.