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Xinfan Technology News, at 8:27 on Beijing time, among all astronomical concepts, a black hole might be the most peculiar one. A black hole's density is extreme, even light cannot escape from it, like a dark and terrifying giant prison. Since ordinary physical laws do not apply within a black hole, it seems as if black holes were tailor-made for science fiction. However, numerous direct and indirect pieces of evidence suggest that black holes do indeed exist in the universe.

Einstein's prediction

A black hole is a necessary outcome of Einstein's General Theory of Relativity.

German astronomer Karl Schwarzschild was the first to predict the existence of black holes in 1916, positing that it was 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 by Roger Penrose and Stephen Hawking further solidified the theoretical foundation of black hole existence. Their research shows that any stellar collapse into a black hole will result in a singularity, where all traditional laws of physics fail.

Gamma-ray bursts

Earth-based observational equipment has already detected some gamma-ray bursts generated during the formation of black holes.

In the 1930s, Indian astrophysicist Subrahmanyan Chandrasekhar researched the fate of a star's core fuel depletion. It was discovered that the outcome depended on the star's mass. If a star is very massive, say about 20 times the mass of the Sun, the star's dense core (whose mass alone could be two or three times that of the Sun) would continue to collapse until it forms a black hole. The collapse speed of the stellar core is extremely fast, happening in just a few seconds, during which time it releases an astonishing amount of energy in the form of gamma-ray bursts, equivalent to the total energy released by an ordinary star over its lifetime. Earth's telescopes have detected multiple gamma-ray bursts, with some originating from galaxies billions of light-years away, indicating that we have indeed observed the process of black hole formation.

Gravitational waves

The illustration shows an artist's concept image of gravitational waves. The gravitational interaction between two black holes can create time ripples, which diffuse outward as gravitational waves.

Black holes are not always solitary; sometimes they appear in pairs, rotating around each other. The gravitational interaction between two black holes can create time ripples, which diffuse outward as gravitational waves. This is also one of the predictions raised by Einstein's relativity. With the help of observatories such as LIGO and Virgo, we now have the capability 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, we have detected multiple gravitational wave events. As the sensitivity of detectors continues to improve, scientists have also detected gravitational waves from events other than black hole mergers, such as those involving black holes and neutron stars.

Invisible companion stars

The image is an artistic impression of the orbital imagination of several celestial bodies in the triple star system HR6819.

Phenomena capable of producing gamma-ray bursts or gravitational waves are short-lived and intense events, potentially visible from halfway across the universe. But considering their nature, most black holes are undetectable. Black holes do not emit any light or radiation, thus they can quietly lurk in the universe without astronomers being aware of their existence. However, there is one method to detect them: using the gravitational effect of black holes on other stars. In 2020, when astronomers were observing what appeared to be an ordinary star system HR6819, they noticed something odd about the orbits of two stars. Unless there is an entirely invisible celestial body in the system, this phenomenon cannot be explained. Upon calculating its mass, researchers realized there was only one truth: this celestial body must be a black hole. It lies a thousand light-years away from Earth, located in the Milky Way Galaxy, and is the closest black hole to Earth discovered so far.

X-ray emissions

The black hole Cygnus X-1 is devouring the nearby giant blue companion star.

In 1971, when scientists studied a stellar system in the Milky Way Galaxy called Cygnus X-1, they first observed evidence of a black hole's existence. The X-ray emissions from this system were extremely bright, but these emissions did not originate from the black hole or its visible companion star. Instead, they came from the accretion disk formed as the black hole devoured stellar material. Much like the previously mentioned star system HR6819, astronomers can also use the orbital trajectory of the visible star to estimate the mass of the invisible celestial body in the Cygnus X-1 system. The final calculation results in a mass of approximately 21 times that of the Sun, and considering the relatively small space it occupies, indicates that it can only be a black hole; no other possibility is worth considering.

Supermassive black holes

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

Besides black holes formed by stellar collapse, evidence suggests that the centers of galaxies may also harbor supermassive black holes, with masses potentially reaching millions, even billions of times that of the Sun, and they may have existed since the early universe. In so-called "active galaxies," evidence of the existence of these supermassive black holes is spectacular. NASA points out that there's a dense accretion disk around the black holes in the center of these galaxies, emitting extremely intense and wide-ranging radiation. The center of the Milky Way Galaxy also contains a black hole because we observe a surprisingly high speed of stellar rotation in this region, reaching 8% of the speed of light, indicating they must be orbiting a highly dense yet massive celestial body. Current estimates suggest the mass of the central Milky Way black hole is about 4 million times that of the Sun.