The no boundary theory of the universe likens the universe to the Earth, in that is impossible to find any boundaries traveling around the Earth or to fall off it. This is possible as a result of theorizing that prior to the Big Bang, the generally accepted laws of physics did not apply and that the concept of time as we know it did not exist.
The no boundary theory of the universe was advanced by the physicists Stephen Hawking and James Hartle. It is called the no boundary theory because it posits that the universe has no boundaries.
The theory says the quantum singularity that gave birth to the universe via the Big Bang was of infinite density. In that strange state of flux, particles appear and disappear spontaneously, and space and time become separated. It is therefore at the moment that the Big Bang takes place that time starts to exist; before the Big Bang, it is nonsensical to talk about time.
Hawking uses the Earth once again to illustrate this point. While on Earth it is always possible to travel south, that is, until you reach the South Pole. Once on the South Pole, "south" doesn't exist. The same concept applies to time before the Big Bang — once you reach t=0, time ceases to exist.