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Xinfan Technology News, Beijing Time, August 27. Among all astronomical concepts, black holes may be the most peculiar of all. Black holes have extreme density, even light cannot escape, like a terrifying giant black receptacle. Since ordinary physical laws fail in black holes, they seem to be tailored for sci-fi stories. However, a great deal of direct and indirect evidence confirms that black holes indeed exist in the universe.

Einstein's Prophecy 𝒻𝘳𝘦𝘦𝘸ℯ𝒷𝘯𝘰𝑣ℯ𝑙.𝘤𝑜𝘮

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

German astronomer Karl Schwarzschild first predicted the existence of black holes in 1916, considering them an inevitable result of Einstein's theory of general relativity. In other words, if Einstein's theory is correct (all evidence points to this), then black holes must exist. The works of Roger Penrose and Stephen Hawking gradually solidified the theoretical foundation for black holes. Their research showed that any celestial collapse forming a black hole would lead to a singularity where conventional physics laws entirely break down.

Gamma-ray bursts

Ground-based observation equipment has detected gamma-ray bursts generated during the formation of black holes.

In the 1930s, Indian astrophysicist Subrahmanyan Chandrasekhar studied the fate of exhausted stellar nuclear fuel. It was discovered that the ultimate result depends on a star's mass. If the star is massive, say twenty times the mass of the Sun, its dense core (which alone can reach two to three times the solar mass) will directly collapse, eventually forming a black hole. The collapse speed is extremely fast, happening in a matter of mere seconds, releasing astounding energy in gamma-ray bursts equivalent to the energy released by an ordinary star throughout its long life. Ground-based telescopes have detected multiple gamma-ray bursts, some even emitted from galaxies billions of light-years away, confirming the observation of black hole formation processes.

Gravitational waves

Pictured is an artist's rendering of the concept of gravitational waves. The gravitational interaction between two black holes will create ripples in space-time that radiate outward in the form of gravitational waves.

Black holes are not always solitary; sometimes they appear in pairs and rotate around each other. The gravitational interaction between two black holes will create ripples in space-time that radiate outward in the form of gravitational waves, one of Einstein's relativity predictions. With help from observatories like LIGO and Virgo, we now have the ability to detect gravitational waves. In 2016, scientists announced the first discovery of gravitational waves created by the merger of two black holes. Since then, multiple gravitational wave events have been detected. With continuously improving detector sensitivity, scientists have even detected gravitational waves generated by events other than black hole mergers, such as collisions between black holes and neutron stars.

Hidden companion stars

Pictured is an imagination of the star trajectories in the three-star system HR6819.

Gamma-ray bursts or gravitational waves are both events that occur in a short time and can be detected across half the universe. But considering their nature, most black holes are hard to detect. Black holes do not emit any light or radiation and can silently lurk in space, with astronomers often unaware of their existence. Yet, there is a method to detect their presence: using the gravitational effects of black holes on other stars. In 2020, when astronomers observed the seemingly ordinary HR6819 star system, they found the trajectories of the two stars somewhat strange, unless there was a completely invisible celestial body at play. When calculating its mass, researchers realized the only truth: this celestial body must be a black hole. It is located just a thousand light-years away from Earth, right in the Milky Way Galaxy, making it the closest black hole discovered so far.

X-ray emissions

The black hole CygnusX-1 is devouring material from its huge blue companion star.

In 1971, scientists found evidence of black hole existence while researching a star system in the Milky Way Galaxy known as CygnusX-1. The X-ray emissions from this system are extraordinarily bright, but these emissions are not directly from the black hole or its visible companion star. Instead, they are generated by the accretion disk formed when the black hole devours stellar material. Just as mentioned for the HR6819 star system, astronomers can also use the visible star's trajectory to estimate the mass of the hidden celestial body in the CygnusX-1 system. The final calculated result shows the mass is about 21 times the solar mass, considering the relatively small space occupied by this celestial body, indicating it can only be a black hole, without the need to consider other possibilities.

Supermassive black holes

The center of the Milky Way Galaxy also contains a supermassive black hole.

Aside from black holes formed by stellar collapse, evidence suggests that galactic centers may harbor supermassive black holes, with mass reaching several million or even billions of times that of the Sun, possibly existing since the early universe. In so-called "active galaxies," the evidence of the existence of supermassive black holes is spectacular. According to NASA, these galactic centers surrounded by accretion disks release extremely strong emission across various wavelengths. There is also a black hole in the center of the Milky Way Galaxy, as we observe the astonishing rotation speeds of stars in that region, reaching 8% of the speed of light, indicating they are surely orbiting a celestial body that is extremely small but extremely massive. Current estimates suggest the black hole at the center of the Milky Way Galaxy is about 4 million times the mass of the Sun.