What is a black hole? What is the evidence that black holes actually exist, and where in the Universe do we find evidence of their existence?

It is also believed by astronomers that a black hole exists in almost all galaxies, usually in the centre, these are known as supermassive black holes. These can be millions if not billions times the mass of the Sun, however the formation of these supermassive black holes is still unexplained, with some scientists suggesting that they formed at the same time as their surrounding galaxies. Another way in which stellar-mass back holes are formed is when two orbiting neutron stars merge. Short gamma ray bursts are produced from what is left over from when the supernovas merge, that is excessive dense stellar cores. These rays create massive amounts of energy, such that it is detectable across the entire observable universe, making it easier to find and identify as well as explain the formation of black holes.

An artist's drawing shows a large black hole pulling gas away from a nearby star. 

Supermassive black holes are also known to be able to consume, from orbiting stars and surroundings gas clouds, matter which then piles into a disk outside its hole, which is called an accretion disk. This matter then orbits the black hole, at overwhelming speeds producing temperatures up to a million degrees resulting from the friction with the disk. This then results in high energy x-rays being emitted, which is the reason that these supermassive black holes are detectable from the observable universe. The point where the escape velocity is actually faster than light is referred to as the event horizons in black holes, this is the reason it is impossible to escape from a black hole.

Scientists and astronomers are provided with greater knowledge about black holes from studying x-rays emitted from them and using observatories such as EXIST. The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed NASA satellite, with the aim of learning and gaining further insight into what happens at the edge of a black hole, paying particular attention to time, space and matter as well as supermassive black holes. Exist enables scientists to locate black holes that may be hidden or obscured behind dust, this is an important feature because evidence shows that 80% of black holes are hidden behind dust, effectively providing more a more accurate count of black holes within the universe. Investigation and studies are still being undertaken by scientists regarding Einstein’s prediction that where ever a black hole is created a ripple effect is also created within in space, this is much like the ripple effect waves create on water. Though no gravitational waves have yet been detected, experiments continue to be undertaken to prove or disprove this prediction.

Evidence and Location

It is not possible for telescopes that detect x-rays to detect black holes; however scientists are able to assume the presence of black holes by analysing matter in the radius. Scientists infer that a black hole passes or has passed through the matter when it is detected as being drawn inwards. Furthermore, there is evidence that black holes drastically affect the area of space it is surrounded by, via the release of strong gamma ray bursts, which are capable of destroying neighbouring stars. Black holes are also known to be able to accelerate and decelerate the growth of new stars.

Evidence of black holes also comes from the gradational waves that arise when two black holes orbit each other. These gravitation waves are a result of this orbit, and are created through the accelerated masses which stream away. These waves are detected by a few different missions, including the Laser Interferometer Space Antenna (LISA). LISA is able to detect black holes by analysing the effect they have in and along space. Greater understanding regarding these gravitational fields that surround black holes is achieved, while the locations, rotations and relative mass can also be understood.

Another key player in the evidence of black holes is the Hubble Space Telescope and Chandra X-ray Observatory both providing the most significant evidence regarding the existence of an event horizon. This is an integral aspect of studying black holes because even horizons are the defining feature of black holes and because nobody of matter, except for a black hole can have an event horizon. Because these two observatories enable scientists to infer the existence of event horizons then by principle they also prove the existence of black holes. Scientists also discovered systems that contained potential black holes, and these systems only emit 1% as much energy as those with neutron stars. Because scientists are able to prove that very little energy is realised from these possible black holes and thus they are able to prove that event horizons also exist. It is clear that an event horizon must exist if only very little amounts of energy is able to escape from the objects and by principle black holes must also exist.

Evidence of black holes is also obtained from the Hubble Space Telescope by exploring different avenue in the detection of these black holes. Observing the ultraviolet pulses from bodies of hot gas, scientists were able to find that the gas on the surface and eventually disappears as it spins around a large object, via a black hole called Cygnus XR-1. It was also revealed, through the fluctuations in the measurement of ultraviolet lights that are from trapped gas in orbit around the black holes that, rapid decaying flashes of sequential light coming from the gas as it spins into the black hole. Gas would normally continue to brighten, continuing until it crashes onto the surface of a body of matter; however the existence of even horizons stops this from happening.

Because the existence of black holes has been proved, scientists are also able to approximately locate black holes. The centre of galaxies is the most probable location of black holes in different galaxies; this is because of the large concentration of stars that allow black holes to form. This is a result of the large possible implosion that can create a large black hole. Another known location of a black hole is the galaxy Andromeda, whereby stars located in the centre of the galaxy orbit a large mass. This mass is considered by scientists to be a black hole. Galaxy M87 is also thought to have a black hole, this is accounted by the jets of plasma that are released on opposite sides of the galaxy, this was observed by the Hubble Space telescope in 1994.

An accretion disk of a black hole has also been taken by the Hubble Space Telescope in the centre of the galaxy NGC 4261, where jets of plasma are also being released from the body. Rapidly orbiting stars has also been found in the Milky Way. This also holds a ring of orbiting gas clouds and jets of plasma; these also imply the existence of black hole. Black holes are also believed to exist in binary star systems, however only one star is visible. This implies that the second star is a black hole; this is further reinforced because it has been seen in the system Cygnus X-1. Scientists are also able to determine, via the observations of the orbit of the visible star in the system, that the second star is too large to be a white dwarf star or a neutron star, this proves with almost 100% certainty that a black hole exists.

Conclusion

In conclusion, black holes are very large bodies in which nothing, not even light, can escape, because of the gravitational force. These black holes are usually formed from collapsing stars. Black holes have been observed by the Chandra X-ray Observatory and the Hubble Space Telescope, both providing significant evidence to the existence and location of black holes. However further studies are needed to clarify and explain the strange existence of these black holes.