Black holes is perhaps the most bizarre concept in astronomy. These dark behemoths are a region in space where matter is so dense that no light can escape. This makes them a very frightening prospect. It's easy to dismiss black holes and the rest of the rules of physics as science fiction. There is ample evidence, both direct and indirect, that black holes do indeed exist in the universe.
Einstein's "robust prediction"
Albert Einstein's theory general relativity led to the discovery of black holes. (Image credit: Bettmann / Contributor)
Black holes are a theoretical possibility. Karl Schwarzschild predicted them in 1916 as a result of Einstein's theory general relativity. Black holes are a consequence of Einstein's theory of general relativity, which all evidence supports. Roger Penrose, Stephen Hawking and others proved them even more solid. They showed that any object falling to a blackhole will create a singularity, where traditional laws of physics are broken down. This was so well accepted that Penrose received a share of the 2020 Nobel Prize in Physics "for the discovery of black hole formation as a robust prediction for the general theory of relativity."
Gamma-ray bursts
Earth-based equipment has detected gamma-ray bursts that are caused by the formation of black holes. (Image credit: NASA/Swift/Cruz deWilde)
Subramanian Chandrasekhar, an Indian astrophysicist, studied what happens to a star after it exhausts all its nuclear fuel. This was in the 1930s according to NASA. He found that the star's mass is a key factor in the final result. NASA says that if a star is large, such as 20 solar masses, its dense core, which could be three times or more the sun's mass, will collapse to form a black hole. In a matter of seconds, the final core collapse takes place. It then releases tremendous amounts of energy as a gamma radiation burst. This burst can emit as much energy as an ordinary star in its lifetime. Telescopes around the world have seen many of these bursts; some are from galaxies billions light-years distant. We can therefore see black holes being formed.
Gravitational waves
Artist's impression on gravitational waves. Black holes orbiting one another create ripples of space-time that propagate outwards as gravitational wave. Image credit: R. Hurt/Caltech JPL
Sometimes black holes are not isolated. They can sometimes be found in pairs orbiting each other. The gravitational interaction between them causes ripples in spacetime, which propagate as gravitational waves. This is Einstein's prediction. These waves can now be detected by observatories such as the Laser Interferometer Gravitational Wave Observatory and Virgo, Space.com's sister site Space.com reported. In 2016, the first black hole merger discovery was made. Many more have since been made. Live Science reported that other wave-generating events, such as black hole mergers, are becoming more common. One example is a collision between a blackhole and a neutron Star. This occurred at a distance far beyond our galaxy, at approximately 650 million to 1.5billion light-years from Earth.
Invisible companion
This artist's impression shows how the orbits of objects in the triple system HR 6819 are displayed. (Image credit: L. Calada/ESO)
While gammaray bursts or gravitational waves can be seen halfway across the universe, their short-lived and high-energy counterparts, black holes, due to their nature, will remain invisible for most of their lives. They don't emit light or radiation so they could be lurking in the cosmic vicinity without astronomers being aware. The only way to spot these dark creatures is through their gravitational effects upon other stars. Astronomers observed an ordinary-looking binary system or pair of orbiting star, HR 6819, in 2020. They noticed strange motions in the visible stars. This could only be explained if there were a third, completely invisible object. Researchers discovered that the mass of this object was at least four times the sun's. Live Science reported that it had to have been a black hole, the closest known to Earth. It is located just a thousand light-years from Earth in our galaxy.
X-ray vision
The black hole Cygnus X-1 pulls material from a large blue companion star. Image credit: NASA/CXC
In 1971, the first observational evidence of a black hole was made. This too was from a binary star system in our galaxy. The system, Cygnus X-1 is responsible for some of the most brilliant X-rays in the universe. NASA says that these X-rays don't come from the black holes themselves or from their visible companion star, which is 33 times larger than our sun. NASA stated that the Xrays are actually emitted from matter being constantly removed from the giant star. Astronomers can use the observed motion of stars to determine the mass of Cygnus X-1's unseen object, just as they did with HR 6819. Live Science reported that the latest calculations placed the dark object at 21 solar mass concentrated in such a small area that it could not be any other than a Black Hole.
Supermassive black hole
The supermassive black spot in the middle of our galaxy, known as SagittariusA, is located in the region. Image credit: ESAC. Carreau
Live Science reported that in addition to black holes formed by stellar collapse, evidence indicates that supermassive dark holes, each with millions, or even billions, of solar masses, have been lurking at the centers of galaxies from the beginning of the history of the universe. These heavyweights are evident in the case of active galaxies. NASA states that the galaxies' central black holes are surrounded in accretion discs that emit intense radiation at all wavelengths. Evidence also shows that our galaxy contains a central black hole. We can see stars in the region moving at 8% speed of light. This means they must orbit something small and massive. According to current estimates, the central black hole of the Milky Way is approximately 4 million solar masses.
Spaghettification
Spaghettification is another evidence of the existence of black hole. You might be wondering what spaghettification is. It is what happens when you are thrown into a black hole. The explanation is quite simple. The black hole's extreme gravitational force can cause you to be stretched into thin strands. Live Science reported that although it's unlikely that this will happen to you, or anyone else you know, it could be the fate for a star that wanders too closely to a supermassive dark hole. Astronomers saw this star being ripped apart, or, at the very least, witnessed it shredding in October 2020. The spaghettifying did not occur near Earth. It took place in a galaxy 215,000,000 light-years distant.
Finally, a direct image
This is the first direct image ever taken of a black hole. (Image credit: Event Horizon Telescope Collaboration)
We have a lot of convincing indirect evidence to support black holes. These include bursts or gravitational waves or dynamical effects that aren't possible with any other known object. The final clincher was April 2019, when a direct photo of the supermassive dark hole at the centre of the active galaxy Messier 87 was taken. This amazing photo was taken using the Event Horizon Telescope, a name that is a bit misleading as it comprises a large network instead of one instrument. NASA states that the image quality is better if there are more telescopes participating and they are more spread out. As reported by Live Science, the result clearly shows the shadow of the 6.5-billion-solar-mass blackhole against the orange glow from its surrounding accretion disc.
Original publication on Live Science
