Here is a new theory of the universe, twenty years in the making, by Stephen Hawking and his close collaborator Thomas Hertog. Perhaps the biggest question Stephen Hawking tried to answer in his extraordinary life was how the universe could have created conditions so perfectly hospitable to life. Pondering this mystery led Hawking to study the big bang origin of the universe, but his early work ran into a crisis when the maths predicted many big bangs producing a multiverse - countless different universes, most of which were far too bizarre to harbour life.