Stephen Hawkings
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  1. Who is he ?
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  2. Stephen Hawking was a theoretical physicist and cosmologist best known for advancing theoretical models on black holes and cosmic inflation , which he discusses in his popular writing on time and space. Hawking's university education began in 1959, when, at age 17, he attended University College, Oxford, to study physics. Seeking a PhD at University of Cambridge, Hawking was disappointed to learn that the esteemed astronomer Fred Hoyle – who is now famous for coining the term ' Big Bang' theory by mocking it on radio – wouldn't be taking any more students. His supervisor was a relatively unknown researcher by the name of Dennis Sciama. What Sciama lacked in fame he more than made up for in mentorship, encouraging a young Hawking to follow his interests. Where Hoyle was dismissive of the Big Bang, Hawking became its champion. Between 1979 and 2009, he was the Lucasian Professor of Mathematics at the University of Cambridge. Stephen Hawking, who died Wednesday at the age of 76, had lived with the crippling disease ALS for 55 years.
6 important discoveries of Stephen Hawking
Singularities - 1970
Einstein's theory of gravity also tells about singularities i.e. points where space-time appeared to be infinitely curved. But that time it was unclear that whether singularities were real or not. Then, Roger Penrose proved that singularities would indeed form in black holes. Later, Stephen Hawking did path-breaking work on singularities and applied it for whole Universe and told that gravitation produces singularities. He also told that Einstein's theory predicted a singularity which was the big bang theory.
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Mathematics
In mathematics, a singularity is a point at which a given mathematical object is not defined, or a point where the mathematical object ceases to be well-behaved in some particular way, such as by lacking differentiability or analyticity.

For example, the real function

f(x)= 1/{x}

has a singularity at x=0, where the numerical value of the function approaches infinity ∞ so the function is not defined. The absolute value function g(x)=|x| also has a singularity at x=0, since it is not differentiable there.

The algebraic curve defined by {(x,y):y3-x2=0} in the (x,y) coordinate system has a singularity (called a cusp) at (0,0). For singularities in algebraic geometry, see singular point of an algebraic variety. For singularities in differential geometry, see singularity theory.
Cosmic Inflation Theory – 1982
This theory was introduced by Alan Guth in 1980 that in physical cosmology, cosmic inflation is a theory in which the Universe expands exponentially soon after the big bang. Further, Hawking was one of the first who calculate quantum fluctuations which means the small variations in the distribution of matter and shows that during inflation it might give rise to the spread of galaxies in the Universe.
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Model on the wave function of the Universe – 1983
He was interested in establishing a quantum theory of gravity but with James Hartle, he published a model the Hartle-Hawking state in 1983. This theory states that time did not exist before the big bang explosion and hence the concept of the beginning of the universe is meaningless. The Universe does not have any initial boundaries in time or space.
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Laws of Black hole mechanics – 1971-72
Stephen Hawking discovered the laws of black hole mechanics. - His first law states that the total surface area of the black hole will never get smaller. It is also known as the Hawking area theorem.
- Another law states that black holes were hot. But it is a contradiction of classical physics which states that black holes don't radiate heat.
- Another law is "no hair" theorem of black holes, which states that black holes can be characterised by three numbers; their mass, angular momentum and charge.
- Black holes emit radiation, which may continue till they exhaust their energy and evaporate. This is also known as Hawking radiation.
- In January 1971, he won the prestigious Gravity Research foundation Award for his essay titled "Black Holes".
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"A Brief History of Time" his famous book published on 1988
In this book he explains about the range of subjects in cosmology like big bang, black holes, light cones etc. This book became the bestselling book that breaks the record of 237 weeks and appeared on the British Sunday Times best-seller list.

“The twentieth century saw man's view of the universe transformed: we realized the insignificance of our planet in the vastness of the universe, and we discovered that time and space we curved and inseparable , that the universe was expanding, and that it had a beginning in time.”
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Along with Thomas Hertog, he proposed a theory "top-down cosmology" in 2006 which states that the Universe had not one unique initial state but consisted of a superposition of many possible initial conditions. Several other books were also published like The Universe in a Nutshell (2001), God Created the Integers: The Mathematical Breakthroughs That Changed History (2005) etc.Therefore, now we come to know about Stephen Hawking famous and important discoveries.
Black Hole
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A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.

Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

How Big Are Black Holes?
Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or "stuff," in an object.
Another kind of black hole is called "stellar." Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar mass black holes in Earth's galaxy. Earth's galaxy is called the Milky Way.

The largest black holes are called "supermassive." These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.

How Do Black Holes Form?

Scientists think the smallest black holes formed when the universe began.

Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space.

Scientists think supermassive black holes were made at the same time as the galaxy they are in.
If Black Holes Are "Black," How Do Scientists Know They Are There?
A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. But scientists can see how the strong gravity affects the stars and gas around the black hole. Scientists can study stars to find out if they are flying around, or orbiting, a black hole.

When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.

Could a Black Hole Destroy Earth?
Black holes do not go around in space eating stars, moons and planets. Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that.

Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now.

The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.