What Are Black Holes And How Do They Affect The Universe?

For years, black holes have amazed many scientists involved in astrophysics. They are phenomenons which at one point were thought of as a ridiculous concept. However, in the present day, we now know that black holes not only exist, but there may be more black holes than we can even imagine - as there is a theory that there is a black hole at the centre of every galaxy. In basic terms, a black hole is a very dense body which exists in space. It is so dense that even light cannot escape from it. As nothing can travel faster than light, this therefore means that nothing can escape from a black hole. A black hole has a huge gravitational pull which causes light to bend as it enters it and also causes "space-time" to curve. Space-time is a complex concept, however - explained simply - Earth has 3 dimensions: length, width and depth. But space has a fourth dimension: time. This dimension of time is needed in space: if we wanted to meet at a certain co-ordinate in space, we would not only need the location of the co-ordinate, but the time we needed to meet as well. Collectively, these four dimensions are called "space-time". As black holes don't give off light, we cannot simply just look for them in the sky, and so can be fairly difficult to detect. One way of detecting black holes (and neutron stars) is by observing X-rays emitted from them. These X-rays are given off due to the intense gravitational force pulling in dust at such high velocities. These particles get so hot that they emit X-rays which cannot be detected from Earth as they don't enter our atmosphere, but astronomers use satellites and other technology to detect them. Alternatively, sometimes stars rotate around each other, such as the stars Sirius and Cygnus X-1. When astronomers cannot detect what the star is orbiting around, they suspect a black hole or neutron star. However, the most common way of detecting a black hole is a technique called "gravity lensing". When a massive object passes between a star and the Earth, the object acts like a lens: it focuses the light rays emitted from the star to the Earth. This results in the star appearing brighter. This suggests that the object passing between the Earth and the star is something so massive that it can bend the light rays from the star - either a neutron star or a black hole.

Black holes are formed by collapsing stars. In order to understand how they are formed, firstly we need to understand how stars are formed. The first stage of star formation is a cloud of gas and dust which are the remnants of a dead star, which we call a nebula. The nebula gets more and more dense and develops a gravitational pull, which causes the nebula to contract. Nuclear fusion in the star emits energy and heat, which causes outward pressure. This eventually balances out the gravitational pull in order for the star to become spherical. The nuclear fusion process continues throughout the star's life. The star begins by fusing Hydrogen - the first element in the periodic table - into Helium - the second element in the periodic table. Our Sun is still fusing Hydrogen, even though it is about 4.6 billion years old, therefore the fusion process can take a very long period of time. Once the star has finished fusing Hydrogen, it begins to fuse the Helium into Carbon. Carbon then fuses into Neon, Neon then fuses into Oxygen, Oxygen then fuses into Silicon and so on. This process usually carries on until the star has fused all the atoms into Iron - however it is dependent upon the size of the star as to when it runs out of fuel. When the star runs out of fuel, a few scenarios can arise. Physicist Chandrasekhar calculated how big a star needed to be in order for it to still be able to support itself against gravity after its fuel supply had diminished. He determined that a star more than 1.5 times the mass of the Sun would not be able to support itself against its own gravity - this amount is known as the Chandrasekhar limit. If a star's mass is less than this limit, then it would fuse only Hydrogen and once this was used up it would begin to collapse due to the imbalance between its gravity and the outward pressure. The star then becomes extremely dense with a glowing core and eventually turns into a white dwarf. In time, the core stops glowing. If the star was approximately the size of our Sun, then it would only fuse to Helium and would collapse after the Helium ran out and would become a white dwarf. This would then eventually fade into a black dwarf. If the star was a high-mass star - above the Chandrasekhar limit (e.g. red supergiant) - it would have a massive problem when it ran out of fuel. Some stars explode and emit vast amounts of light and energy: all its outer layers are blown off and the fusion process can speed up randomly and produce far heavier elements than Iron. This explosion is called a supernova. This occurs to avoid the star collapsing in due to its own gravity, however sometimes this cannot always happen. In 1939, Robert Oppenheimer said that as the star contracts, the gravitational force becomes stronger and stronger which causes the light to bend more which causes the light emitted from the star to be more difficult to escape. If the stars original mass is above 1.4 solar masses, it would collapse and become a neutron star. Neutron stars are very small; a radius of about 10 miles, but are very dense; hundreds of millions of tonnes per cubic inch. They also spin very fast on their axes. If the star is above 1.4 solar masses, but below 3 solar masses, there is a theory that it will become a hypothetical "quark star". This occurs when a process called neutron degeneracy pressure is too much for the neutron star's own gravity. It is then down to the quarks that make up neutrons to prevent the star from collapsing. Quarks are the particles which make up hadrons (protons, neutrons etc) there are two "down" quarks and one "up" quark. When quark degeneracy pressure occurs, these "up" and "down" quarks are converted into "strange" quarks. However, if the remains of the star are above 3 solar masses, the star shrinks to a critical radius; the gravitational force becomes so strong that light cannot now escape. Since nothing can travel faster than light (according to Einstein) then if light cannot escape, nothing else can. The star is now a black hole. A black hole has two main features: the event horizon and the singularity. Firstly, the event horizon is the point of which the gravitational pull is so immense that nothing can now escape from it. It is described as a "one-way" membrane: objects can fall in the black hole, but once they are past the event horizon they can never come back out. The radius of the event horizon is called the Schwarzschild radius. The centre of the black hole is called the "singularity": the point of infinite density. This is the point of the black hole where the laws of physics break down: they cease to exist. The singularity contains all the mass of the original star, but is distorted and so is very dense. The weak cosmic censorship hypothesis states that the singularity is always hidden from the observer as it is so dense. The hypothesis also states that there are no "naked" singularities - singularities which exist on their own, without the event horizon (except for the Big Bang) - therefore they only exist inside the event horizon of a black hole. This means the event horizon "protects" an observer from the singularity. The strong cosmic censorship hypothesis states that singularities exist either completely in the past - the Big Bang; or completely in the future - singularities inside black holes. If this hypothesis does not hold true, it would be catastrophic for an astronaut entering a naked singularity as this may allow us to travel in the past. This would be dangerous for everyone as no-one's life would be safe: someone could go into the past and kill your ancestors before you were even born. Sometimes, black holes also have a third feature called an accretion disk. This is a surrounding whirlpool of gas and dust, which eventually gets sucked into the black hole. An accretion disk is usually created around a supermassive black hole, as it has a massive gravitational pull.

It has been thought for many years that there could be a black hole at the centre of our galaxy, the Milky Way. This black hole is the most reasonable explanation for the distinctive spiral arms of our galaxy. The centre of the Milky Way is thought to lie in the constellation of Sagittarius, in the region Sagittarius A. When astronomers observed this region of the sky, a powerful radio source was detected which showed that there were complex structures in this region which are caused by rapid star formation and magnetic fields. The actual centre of the Milky Way is immensely crowded: there are an enormous number of stars which heat the dust and cause infra-red radiation. Also at the centre of our galaxy is a supermassive black hole called Sagittarius A*. Supermassive black holes are the largest type of black holes - they have masses of more than one million Suns put together (Sagittarius A* has a mass of approximately 4.5 million suns). It is relatively close to Earth - about 24,000 light years away. This does not sound very close, but when you consider the fact that to get from one end of the Milky Way to the other it takes 100,000 light years, in comparison it is not so far. Research has shown that supermassive black holes are very common - almost every galaxy has one. Also, the mass of the supermassive black hole is proportional to the mass of the galaxy, which suggests that the growth of the black hole is linked to the formation of the galaxy. Galaxies come in all shapes and sizes, our galaxy and our neighbour the Andromeda galaxy have a bulge and a disk in the centre. However, some galaxies only contain a disk in the centre and these are called ellipticals. Astronomers have found a supermassive black hole in every galaxy which has a bulge, but none in ellipticals. To me, this suggests that supermassive black holes must be linked to creating different types of galaxies. Perhaps, if a supermassive black hole is present at the birth of a galaxy, then it will influence the galaxy to contain a bulge and a disk. In the present day, it is generally accepted by astronomers that there is a black hole at the centre of almost every galaxy. But how did this happen? There are two possibilities; first, an average sized black hole formed at the centre of the galaxy due to gravitational collapse. Inevitably, this black hole was so powerful that it sucked in the stars and gas surrounding it, therefore causing it to grow and eventually become a supermassive black hole. Alternatively, the supermassive black hole was created in the early period of the Universe, when the pressure was immensely high. If this was the case, the black hole could have been the key to forming galaxies. Another question is which came first, the supermassive black hole, or the galaxy? Could the gravitational pull of the black hole be strong enough to create a galaxy, or could a juvenile galaxy's crowded centre give birth to a black hole? Astronomers have deduced that evidence points to neither of these scenarios. Their findings show that the supermassive black holes and their galaxies grow and evolve together at the same rate. The growth of supermassive black holes is thought to be linked to quasar activity. A quasar is a different type of black hole. It is a supermassive black hole surrounded by an accretion disk. When objects such as stars and even galaxies are sucked into the quasar, a huge collision of matter occurs and a massive explosion of energy and light results in a flare - a distinct characteristic of quasars. It is thought that quasars were created in the relatively early stages of the Universe. To observers on Earth, they appear red as they are very far away - about 10-15 billion light years away! Some scientists believe that quasars are actually infant galaxies. If this is true, this could then explain why there is a supermassive black hole at the centre of almost every galaxy. Shortly after the galaxy is born, it may enter a quasar phase. When it is in this quasar phase, it has an accretion disk containing lots of dust and gas. The black hole's gravitational pull is so strong that it pulls in surrounding matter, such as stars and planets and after a long period of time the black hole may go into a fairly dormant stage (like Sagittarius A*). This could be due to all the dust in the accretion disk being used up and so the quasar phase is ended. This results in the supermassive black hole still remaining at the centre of the galaxy, but it is calmer and so the galaxy is more stable. Therefore, if quasars are essential for galaxies to be born, then it appears that there must be a supermassive black hole at the centre of almost every galaxy. Black holes can come in many varieties: from the very large to the very small and also from the most brutal to the very timid. In 2011, astronomers found the most violent black holes ever: ten times the size of the Solar System, and has a mass of 21 billion Suns. This black hole is about 336 million light years away and is known as NGC 4889. Black holes can also exist in what is called a binary system. Astronomers have found two massive black holes orbiting each other at the centre of a galaxy. The two black holes are only separated by a relatively tiny amount - a tenth of the distance from Earth to the nearest star: Proxima Centauri. This could lead to a greater understanding of how supermassive black holes evolve at the centre of galaxies. Many galaxies are found in a group of galaxies, and could possibly collide with each other as they orbit in this group. However, what happens to these central black holes when the galaxies merge? Astronomers predict that they will orbit around each other and form into an even larger black hole. This suggests that there is an infinite size that a black hole could grow. As well as enormous black holes, the Universe may also be home to miniature black holes also. There is no evidence for their existence, but theory suggests that they were created in the early Universe. It is thought that these black holes contain as much matter as Mount Everest - relatively little compared to Sagittarius A*. Therefore, black holes can come in all shapes and sizes, and are relatively common in the Universe. So, exactly what effect do black holes have on the Universe? When black holes were first discovered, they were thought of as some sort of anomaly, and were thought to be of limited number in the Universe. But then, as mentioned before, research has shown that there appears to be a black hole at the centre of every galaxy. Surely then, this must mean they have some effect on the balance of the Universe? It appears that they help hold galaxies together, and ensure they function correctly. For example, if Sagittarius A* was not at the centre of our galaxy, then all the matter in the Milky Way would not orbit around the centre in a spiral shape. Instead, there may be chaos: different bodies colliding into each other, causing destruction. Therefore, if there was no black hole at the centre of our galaxy, then the Milky Way would be a completely different shape. This may not appear like such a big difference: however, if the Milky Way was a different shape, this may have resulted in the bodies inside the galaxy being in a different position. If - for example - the Sun was in a different position, it may have been too far away from the Earth, and so life on Earth may never have existed. Therefore, it appears that black holes may actually have been essential in order for us to be here today. This may also hold true for life in any other part of the Universe. It is suggested that black holes were needed shortly after the Big Bang, so that objects could take their shape and become calmer and could function properly. So, if black holes did not exist, then life as we know today may not be the same. Black holes are often perceived as "monsters" which will cause the destruction of the Universe. However, this is not the case the majority of the time. Yes, there are some very violent black holes out there which have destroyed many galaxies on one of their rampages. There are always sensationalist media journalists writing about how a black hole is going to kill us all. However, this is definitely not the case. Black holes are essential to the way the Universe is: if they were not present, who knows what the Universe would be like? Evidently they are needed for galaxies to function properly - why else would there be one at the centre of almost every galaxy? They are fascinating objects, which require a lot more study from astronomers. They may shed light on theories such as the Big Bang, or help us understand the elusive substance called dark matter. At one point, they were just thought to be a myth; even the legendary Einstein did not believe in their existence. However, after years of observation and research, we now know that they do exist, and there are more black holes in the Universe than people think. Maybe they are required for some higher purpose - perhaps to help the Universe function properly. Even though they are composed of such little parts - the event horizon and the singularity - they have the potential to wreak such destruction. Even though they can be destructive, they can also be constructive, such as initiating the formation of galaxies. The concept of time is also fascinating in black holes; they are the only places in the Universe where time appears to stop. This means that - to an observer - a person entering a black hole would just stay there, on the event horizon forever. This concept is one which only seems possible in some sort of science-fiction film. However, this concept would hold true in a black hole and, if phenomenal circumstances such as this can occur, what else could occur in our Universe? If an astronaut managed to survive hitting the singularity of the black hole, what happens then? Could she travel to a different Universe? Or even travel in time? This seems fairly far-fetched, but the singularity of the black hole is the point of which the laws of physics cease to exist: anything could happen. Therefore, in conclusion, black holes are the most interesting objects in our Universe. They have helped the Universe become shaped in the way that it is and without them we would not be here today. They have helped form our galaxy, and help us to stay in orbit so we can survive. Therefore, instead of people viewing black holes as "dangerous" objects, we should perceive them as captivating objects which help define the world that we live in: objects which require further study and could give us answers to the questions humans have asked for generations.