Approximately 13.8 billion years ago the Big Bang started it all. But what was the universe like before the Big Bang? In short, no one knows. But there are many theories
Experts don’t know exactly what was happening until 1 second after the Big Bang occurred. This was when the universe cooled off enough for protons and neutrons to crash and stick together. Many scientists believe that the universe underwent a process of exponential expansion known as inflation when that first second took place. This could be why matter is so evenly distributed in today’s universe
It's possible that before the Big Bang, the Universe was controlled by quantum mechanics, says Sean Carroll, a theoretical physicist from the California Institute of Technology and the author of "The Big Picture: On the Origins of Life, Meaning and the Universe Itself," explaining that “The Big Bang is a moment in time, not a point in space.”
If so, the Big Bang would have represented the moment when classical physics took over from quantum mechanics. And that’s the only moment that mattered for Stephen Hawking. Before the Big Bang, everything was unmeasurable and undefined. Hawking called this the no-boundary proposal explaining that time and space were finite, but didn’t have any boundaries, starting points, or endpoints. It’s similar to how Earth is finite but has no edge.
Carroll said that at the moment of the Big Bang the Universe might not have been that small; in fact, it didn't expand into space. Space itself expanded.
"No matter where you are in the Universe, if you trace yourself back 14 billion years, you come to this point where it was scorching, dense and rapidly expanding," he said.
"Since events before the Big Bang have no observational consequences, one may as well cut them out of the theory and say that time began at the Big Bang," Hawking explained during an interview on the National Geographic’s "StarTalk" in 2018.
Although scientists have no way of looking back to the instant of the Big Bang, much less, what came before, they are still exploring several theories. For example, they have identified gravitational waves from powerful galactic collisions in 2015, which has opened up the possibility that these waves could help solve fundamental puzzles about how the universe expanded in that first critical second.
Theoretical physicists are also working on making more precise studies of how quantum forces, such as quantum gravity, might function.