He became one of the world’s most well-regarded scientists, especially after his 1988 book, A Brief History of Time, which became a worldwide bestseller.

Hawking was diagnosed with ALS, the degenerative nerve disorder also known as Lou Gehrig’s disease, in his early 20s. Doctors expected him to live for only a few more years, but Hawking survived for more than half a century.

Here is a list of Hawking’s great discoveries and theories that shaped modern science.

1. Gravitational singularity

One of Hawking’s most notable achievements was his work on singularities – a one-dimensional point that contains infinite mass, where spacetime appears to be infinitely curved and the laws of physics, as we know them, cease to exist.

After Einstein’s theory of general relativity, it was unclear whether singularities were real.

Hawking and his fellow scientists Roger Penrose proved Einstein’s theory allowed for gravitational singularities and there could have been one in our distant past — the big bang.

2. Hawking radiation

Hawking used quantum theory to explain that black holes are not actually completely black but they emit heat and eventually vanish in an extremely slow process. This theory went against classical physics, which stated black holes could not radiate heat.

The theory said the bigger the black hole, the more energy would be released.

3. How galaxies might arise

The British physicist also supported cosmological inflation, which is central to understanding how the universe expanded exponentially just after the big bang before slowing down to expand at a slower pace.

He was the first one to explain how quantum fluctuations during the period of expansion may give rise to different galaxies in the universe.

4. How the universe began

Hawking is also well-known for his attempts to combine two key theories of physics: Quantum theory and Einstein’s general relativity.

He suggested a new theory, known as quantum gravity, which could fill in many gaps in the current understanding of physics and the universe.

In a quest to create a quantum theory of gravity, Hawking collaborated with Jim Hartle in 1983 and proposed the wave function of the universe, which is known as the Hartle-Hawking state.

The theory states the universe has no origins and nothing existed before the big bang.

5. A Brief History of Time

In 1988, Hawking’s book A Brief History of Time was published. The book explains cosmology, black holes and the big bang in non-technical language in order for the average reader to understand it. The book became a bestseller and sold more than 10 million copies in 20 years.