The astrophysicist bases his answer on a theory known as the 'proposal without limits', in which there are no boundaries in the universe.
We know that the universe is constantly expanding. As we go back in time, the universe is contracting. If we rewound long enough, around 13,700 million years, at the time of the Big Bang, all the matter in the universe was transformed into a speck of incredibly hot and infinitely dense matter. That is, the entire universe reduced to the size of a single atom. This subatomic ball is known as singularity.
But what happened before that? What happened before the Big Bang? The famous physicist Stephen Hawking has an answer for it. He offered it in an interview with the popular astrophysicist Neil deGrasse Tyson in the latest 'Star Talk' program on the National Geographic Channel.
Hawking's response to the question "What was there before there was something?" Is based on a theory known as the "limitless proposal": "The boundary condition of the universe is ... that has no border," he simply said. British physicist to Tyson. This is, nothing.
But just because your answer can be summed up in one word, does not mean it is not complicated. Luckily for us, Hawking is one of the best science communicators in the world and in his interview with Tyson, Hawking used the form of the Earth as an analogy for the curved shape of the space-time continuum.
The time began in the Big Bang
Within this small and enormous mass of heat and energy, the laws of physics and time as we know them cease to function. In other words, time as we understand it literally did not exist before the universe began to expand. On the contrary, the arrow of time contracts infinitely as the universe gets smaller and smaller and never reaches a clear starting point.
"According to Einstein's theory of general relativity, space and time come together for a space-time continuum that is not flat but is curved by the matter and energy it contains," Hawking said.
To explain it further, Hawking used the Euclidean approach to quantum gravity. In the Euclidean approach, ordinary real time is replaced by imaginary time, which behaves like a fourth dimension.
"In the Euclidean approach, the history of the universe in imaginary time is a fourth-dimensional curved surface like the surface of the Earth, but with two more dimensions," Hawking clarified.
So, does that mean that there are six dimensions in all?
According to Hawking, the universe has no limits. In other words, the Euclidean space-time continuum is an endless closed surface, something like the surface of the Earth. "There is nothing south of the South Pole, so there was nothing before the Big Bang," says the physicist.
Just as there is nothing south of a point further south, time can not exist before the Big Bang. Instead, time and space expand and extend outward from this singular point in time, such as the degrees of latitude on planet Earth.