About 13.8 billion years ago, the universe began as a small, hot super-force that suddenly and rapidly expanded. Atoms formed, spiraling into stars and galaxies, kicking off a process of enlargement that continues today. That was the Big Bang, an origin story most scientists agree on. But what they still debate is what happened before the Big Bang. On Sunday’s season finale of StarTalk, famed physicist Stephen Hawking continued the debate, asserting he knows exactly what preceded the universe.

The answer, he said, is simultaneously simple and incredibly complex: There was nothing.

“Nothing was around before the big, Big Bang,” Hawking told StarTalk host and astrophysicist Neil deGrasse Tyson.

“According to Einstein’s general theory of relativity, space and time together form a space-time continuum or manifold, which is not flat but curved by the matter and energy in it.”

More specifically, Hawking doesn’t believe time existed before the Big Bang. Einstein’s equations predict that at the very beginning of the universe, all matter and energy were compressed into an extremely small point, but these equations do not provide a mathematical link between what happened before or after the Big Bang. Hawking reasons that because the equations cannot explain what happened before the expansion, the universe materialized out of nothing.

Hawking uses a Euclidean approach to quantum gravity, in which ordinary time is replaced by an imaginary time. This way of thinking imagines the space-time continuum as a closed surface with no end; that is, time began at a singular point, but it couldn’t have existed before the Big Bang. He continues:

“In this, ordinary real time is replaced by imaginary time, which behaves like a fourth direction of space. In the Euclidean approach, the history of the universe is imaginary time in a four-dimensional curved surface, like the surface of the Earth, but with two or more dimensions … The Euclidean space-time is a closed surface without end, like the surface of the Earth. One can regard imaginary and real-time as beginning at the South Pole, which is smooth of space-time where normal laws of physics hold. There is nothing south of the South Pole, so there was nothing around before the Big Bang.”

Stephen Hawking says "nothing" existed before the Big Bang.

While Hawking is confident in his theories, scientists still haven’t settled what actually happened before the Big Bang. Other theories include the cyclical model of the Big Bounce — the idea that our universe emerged from a different collapsing universe — and the multiverse theory, which argues that time couldn’t have begun with the Big Bang because it existed in other forms in other universes.

There are many arguments, some of which are more compelling than others. One Georgetown University physicist, Patrick Johnson, Ph.D., is even open to the idea that Star Wars could take place before the Big Bang.

But it’s Hawking’s habit to speak confidently and convincingly about most things — including his belief that A.I. will replace humans, aliens won’t kill us off, and that we need to get the hell off Earth as soon as possible. Whether he’s actually right may be beside the point.

Want more Stephen Hawking? Then watch this video on why he thinks humans “need to leave Earth.”