The latest season of Star Talk ended with a big bang, with host Neil deGrasse Tyson quizzing physics heavyweight Stephen Hawking on a little matter called the origins of the universe. While even top physicists are completely stumped as to what happened before the Big Bang, the cataclysmic event that set our universe in motion 13.8 billion years ago, there are multiple proposals floating around and Hawking has one of his own.
So, what came before the Big Bang according to Hawking?
Hawking proposes something called the "No Boundary" condition, which he worked on with his collaborator Jim Hartle, a professor of physics at the University of California in Santa Barbara. The proposal is based on Einstein's theory of relativity, which supposes that space and time form a curved space-time continuum, and a Euclidean approach to quantum gravity, which rejects the existence of a singularity.
The proposal involves replacing real time with a concept called imaginary time. Imaginary time is not imaginary in the sense that it is made up, but in the sense that it is expressed in imaginary numbers. To calculate imaginary time, real time undergoes a Wick rotation, meaning that its coordinates are multiplied by the imaginary root. Imaginary time operates like the fourth dimension of space.
Sounds complicated? Hawking uses the shape of the Earth as a metaphor.
“In the Euclidean approach, the history of the universe in imaginary time is a four-dimensional curved surface like the surface of the Earth, but with two more dimensions,” Hawking explained.
“In order terms, the Euclidean space-time is a closed surface without end, like the surface of the Earth.”
Meaning, essentially, there are no boundaries or sharp points of singularity from which space-time and all the matter of the universe erupted. There is no Big Bang as such. Rather, time slows down as it reaches the South Pole or singularity, getting slower and slower so that there isn't a clear beginning. Or, as the Tech Times puts it, "Time was distorted along another dimension, it was always reaching closer to nothing but didn't become nothing."
“One can regard imaginary and real time as beginning at the South Pole, which is a smooth point of space-time where the normal laws of physics hold," said Hawking. "There is nothing south of the South Pole, so there was nothing around before the Big Bang.”
That’s it. In a nutshell, it is impossible to measure events before the Big Bang because time in the way we recognize it simply didn’t exist.
As of right now, this is just pure speculation and one of several hypotheses attempting to explain what triggered the universe into being, including the multiverse model, the collision model, and the bouncing universe model.
Hawking explains his "No Boundary" condition in more detail in a free-to-watch 50-minute documentary on YouTube.