**Time’s arrow**
Hawking was studying the work of

Roger Penrose, which proved that if Einstein’s

general theory of relativity is correct, at the heart of every black hole must be a point where space and time themselves break down – a singularity. Hawking realised that if time’s arrow were reversed, the same reasoning would hold true for the universe as a whole. Under Sciama’s encouragement, he worked out the maths and was able to prove it: the universe according to general relativity began in a singularity.

Hawking was well aware, however, that Einstein didn’t have the last word. General relativity, which describes space and time on a large scale, doesn’t take into account

quantum mechanics, which describes matter’s strange behaviour at much smaller scales. Some unknown “theory of everything” was needed to unite the two. For Hawking, the singularity at the universe’s origin did not signal the breakdown of space and time; it signalled the need for

quantum gravity.

Luckily, the link that he forged between Penrose’s singularity and the singularity at the big bang provided a key clue for finding such a theory. If physicists wanted to understand the origin of the universe, Hawking had just shown them exactly where to look:

a black hole.

Black holes were a subject ripe for investigation in the early 1970s. Although Karl Schwarzschild had found such objects lurking in the equations of general relativity back in 1915,

theoreticians viewed them as mere mathematical anomalies and were reluctant to believe they could actually exist.

Read more:

https://www.newscientist.com/articl...en-hawking-a-legacy-of-paradox/#ixzz6qdaTbNBn
A brief history of Stephen Hawking: A legacy of paradox