BLACK HOLE FORMATION:-
Black holes are some of the strangest & most fascinating objects of the universe. They are so dense & have such a gravitational pull that nothing , not even light rays can escape from it.
According to NASA our own Milky way could have over 100 million black holes .At the heart of the milky way lies a Super Massive Black Hole called a Sagittarius A*.It is about 4 million times the mass of the sun & approximately 26000 light- years away from earth.
A stellar black hole can be formed when a massive star has exhausted the internal thermonuclear fuel in its core, the core becomes unstable & gravitationally collapses inward upon itself & the star's outer layer blown away in space.The core ( dying star) becomes of point of zero volume & of infinite density called the singularity of the black hole.
In the 18 th century John Mitchell & Pierre - Laplace first predicted the object whose gravitational field is so strong that not even light can escape. But the Modern concept of black hole came to light due to Einstein's theory of general relativity.
According to general relativity , a sufficiently compact mass can deform space time in such a manner that nothing can escape from it.The boundary of no escape is called the eventhorizon of the black hole. In 1916 a German physicist karl schwarzchild found the first solution of Einstein’s equations for spherical symmetric objects. It is said to be the first modern solution of a black hole.
The centre of a black hole is hidden by the event- horizon, particles to escape from inside to event -horizon require more velocity than speed of light.The radius of the event -horizon is called schwarzchild radius .The size of this radius depends upon the mass of the collapsing Star. For a black hole having a mass 10 times greater than that of the sun , its Schwazrchild radius would be 30 km.
For a far away observer , clocks near a black hole would appear to tick more slowly than the observer near the black hole. Due to the effect called gravitational time dilation, an object falling into a black hole appears too slow as it approaches the event -horizon, taking infinite time to fall into the black hole.The light emitted from the vicinity of the black hole becomes redder & dimmer from the perspective of the observer far away, hence all the processes near event -horizon appears to slow down.This effect called gravitational Red shift.
While from the perspective of the near horizon observer, nothing fancy happens to him , as he crosses the event -horizon. His clock ticks normally, he will hit the singularity in the finite time.Due to infinite density he crushed before as he moves in due to growing tidal forces he will be torn apart, which is known as spaghettification.
In 1958 , David Finkelstein proposed the Schwazrchild surface as an event -horizon that is perfect unidirectional membrane ; causal influences can cross it in only one direction.Finkelstein’s solution extended Schwazrchild solution for the future observer falling into the black hole.
From the 1960s onward believed to be the golden age of general relativity , since black hole is the solution of Einstein’s equations, it became the main stream area of research.The pace of research triggered by the discovery of pulsar by Jocelyn john bell in 1967, pulsar was rapidly rotating neutron star .once neutron star detected, astronomers started firmly believe in the existence of the black hole.
Amongst more black holes solutions , one in 1963 by Roy kerr found for rotating black holes.Then Neumann found solutions for rotating & electrically charged black holes.
Through the work of Werner Israel , Brandon Carter and David Robinson, black hole no - hair theorem came into existence. According to which black holes can be completely characterized by mass , electric charge & angular momentum like parameters.
Some of the physicists were suspicious about singularity , they called it a symmetric artefact of black holes solutions , but from work of Rozer Penrose & Stephen Hawking made it clear that singularity is quite evident in black hole formation. Which awarded the Penrose 2020 Noble prize in physics.
On 11 Feb 2016 , LIGO & VIRGO announced the first gravitational waves detection, representing the first observation of the black holes merger.
In April 2019 , Event Horizon - Telescope (EHT) published the image of M 87 super Massive Black Hole.
As per the size of the black holes , they can be characterized in four categories.
(1) stellar mass- black holes:-
If a massive star is about 20 times more massive than the sun , collapse due to its own gravity & form a stellar mass black hole.
(2) Intermediate Mass black holes:-
They are formed from two or more stellar black hole mergers, after a series of merger a final black hole becomes 100 to 1 million times more massive than the sun.
(3) Super Massive Black Holes:-
At the centre of each Galaxie, there exists a SMBH .when a stellar black hole & intermediate black hole are merged SMBH is formed.
(4) Primordial black holes:-
Mere second after the Big - bang the primordial black holes believed to be formed.They could range from about 0.00001 times the mass of a paper clip to about 100,000 times more massive than the sun.
Over the years,black holes puzzled physicists a lot both in theortical & in the experimental realm.Due to their intriguing mathematics, still physicists are exploring so many unveiled facts about these beasts objects of the universe. Lot to uncover about the black holes.