Black Hole: The monster of the Universe

 

A black hole is a region in spacetime where the gravitational pull is so strong, that no matter can escape from it. The gravity in such a region is of such magnitude that not even electromagnetic radiation (light) can escape from it. Theoretically, black holes were predicted from the solutions of the field equations of general relativity, as regions in spacetime with infinite curvature, such that gravity in the region becomes extremely large.

The boundary of a black hole is called the event horizon, which has great significance for a particle falling into the black hole. The event horizon is basically a point of no return for the infalling bodies, because once a particle crosses the event horizon it does not have any possibility of making it back, and is destined to be engulfed by the black hole. For an external observer, the infalling body will appear to fall into the black hole for eternity due to time dilation near the black hole. Time dilation is caused by the extreme gravity of the black hole and the flow of time is slowed down drastically. The object that is falling into the black hole will appear to fall very slowly to an external observer and the event of falling will seem to occur for eternity.

Using quantum field theory in curved spacetime famous cosmologist Stephen Hawking predicted that a particular type of radiation is emitted from the event horizon of a black hole. This radiation is called the Hawking radiation, whose temperature is inversely proportional to the mass. This temperature is of the order of a billionth of a kelvin for stellar black holes due to which it is almost impossible to detect it directly.

Black hole as a Mathematical entity

In 1916,  Karl Schwarzschild derived the first solution of the Einstein's equations of general relativity which directly showed the presence of a black hole theoretically. For a long time black holes just remained a mathematical curiosity with no physical reality associated with it. With the discovery of neutron stars by Jocelyn Bell Burnell in 1967, new interest was generated in the field of gravitationally collapsing bodies. 

Formation of a black hole

A star at the end of its lifetime undergoes gravitational collapse and forms a white dwarf which is a compact astronomical object. Further collapse from this state will occur if the mass of the white dwarf exceeds 1.4 solar mass (Chandrasekhar limit) and the resulting object will be a highly compact neutron star. If this state is not in equilibrium, then further collapse is a possibility, which will result in a singularity. If the singularity is shrouded by an event horizon, then it is a black hole. The first black hole known to us was Cygnus X-1 identified in 1971.

After the formation of a black hole

After a black hole is formed it can grow by absorbing the surrounding mass due to its immense gravitational pull. Supermassive black holes (millions of solar masses) can absorb stars or even other black holes. The presence of a black hole is generally inferred by the way it interacts with the surrounding matter and electromagnetic radiation. Any infalling matter in the black hole forms an accretion disk which is enormously heated due to friction. Stars which are in the close vicinity of a black hole will be shredded and engulfed by it, whereas stars which are at a considerable distance will orbit the black hole. The mass of the black hole and its location can be inferred from the orbit of these stars. It is believed that supermassive black holes exists at the centres of most galaxies. There is a radio source at the heart of our galaxy Milky Way known as the Sagittarius A*. It is believed that it houses a supermassive black hole of 4.3 million solar masses.

Recent Advancement

On February 11, 2016 gravitational waves due to a black hole merger was recorded by the LIGO and Virgo scientific collaboration. On April 10, 2019 the first direct image of a black hole was recorded by the Event Horizon telescope. It was situated in the M87 galaxy around 50 million lightyears away from us. Till date gravitational lensing is possibly the best method to infer the presence of a black hole. Although only around 20 black holes have been detected so far, it is expected that there are hundreds of millions of such monsters roaming in the universe. Having an encounter with such a thing is not going to be a pleasant experience at all!!

By

Prabir Rudra

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