Millions of celestial objects are swirling across the cosmos right now; black holes are one among them. A black hole is a region of place in space, which has an extremely high gravitational force where not even light can escape. The term black hole was coined by John Wheeler, a theoretical physicist. The idea that the existence of a body that is extremely dense was given by John Michell in a letter published in November 1784. The signs of an object of this kind were observed in Einstein’s general theory of relativity (GR). Karl Schwarzschild found a solution, which describes gravitational field of a point by solving GR. Black holes are formed from the collapse of a star of a huge mass. A star emits heat and light by the process of nuclear fusion, which constantly takes place in its core. There is a constant battle between the pressure exerted by the fusion outwards by the gravitational pull of the core inwards. The fusion occurs for billions of years producing heavier elements at its core starting from hydrogen and stops until it reaches iron. Iron is a highly unstable element, because of which no further fusion reaction takes place. When the star reaches its end, the core cannot sustain the pressure exerted inwards and the star implodes within. The star then explodes into a spectacular supernova releasing material into the outer space. The star’s core then implodes into a singular point, which is extremely dense, leading to the formation of a black hole. The debris that is near to it falls into the black hole because of its gravity. A boundary where nothing can escape the pull of a BH known as event horizon is formed. Anything can become a BH if its mass is confined to a tiny region that can be calculated by Schwarzschild radius.
On earth, particle accelerators produce microscopic black holes for a fraction of time as they evaporate by a process known as Hawking Radiation, which was proposed by the most renounced theoretical physicists Stephen William Hawking. Black holes by the name are black and cannot be observed directly. Astrophysicists observe the effects produced on neighboring objects, such as planets and stars. They would study the motion of stars for years and calculate from the data gathered. The supermassive black hole at the center of our galaxy is being observed and images are being taken by this method. They observed the motion of a particular star, which was orbiting at an enormous speed. The only thing which can make anything orbit at that speed would be a body of extremely high density, a black hole named Sagittarius A* which is located about 26,000 light years from earth in the center of the milky way. The other way to detect black holes is by gravitational waves. Gravitational waves occur when two extremely dense bodies such as black holes or neutron stars collide. The energy released during the collision at that particular time would be extremely humongous for short period. In the year 2015 the first ever detection of gravitational waves was observed by LIGO. All for now we know is what happens around the black hole because we cannot see what happens inside the BH as not even light can escape from its pull.
Inside the BH, the laws of physics as we know would collapse, but near it, some unique features are observed such as time dilation, length contraction which were given by GR. Near the BH quantum effects are observed which was first seen by Hawking radiation. Many theories are being proposed for the theory of everything, which would have two of the most intruding theories in physics GR and quantum mechanics. One of the most promising theory is the string theory, where many scientists are still working on it. The holographic principle which states that we live in a multi-dimensional hologram has emerged as a consequence of string theory, Dark matter and dark energy are also few of the most mysterious happenings in the cosmos, which are being studied and are the hottest topics in the field of theoretical physics.