According to world-famous physicist Stephen Hawking, before the Big Bang occurred, launching the present-day universe, time existed, but not the time we conceive of as linear time. Rather, Hawking posits, there was imaginary time, which he described as “just as real as what we call real time.”
Speaking with physicist Neil deGrasse Tyson on National Geographic’s "Star Talk," Hawking said paradoxically, “The boundary condition of the universe ... is that it has no boundary,” meaning that there was no “time before time” because time always existed.
Hawking opined that time existed in a "bent” state, relentlessly approaching non-existence but never reaching it. Thus the Big Bang did not arise from nothing.
Hawking has said, “There must have been a beginning. Otherwise, the universe would be in a state of complete disorder by now, and everything would be at the same temperature. In an infinite and everlasting universe, every line of sight would end on the surface of a star. This would mean that the night sky would have been as bright as the surface of the Sun. The only way of avoiding this problem would be if, for some reason, the stars did not shine before a certain time.”
But at the Big Bang? “The density would have been infinite. It would have been what is called, a singularity. At a singularity, all the laws of physics would have broken down. This means that the state of the universe, after the Big Bang, will not depend on anything that may have happened before, because the deterministic laws that govern the universe will break down in the Big Bang.”
Hawking continued, “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. Events before the Big Bang, are simply not defined, because there’s no way one could measure what happened at them.”
Hawking offered an analogy to explain “imaginary time”:
Quantum theory introduces a new idea, that of imaginary time. Imaginary time may sound like science fiction, and it has been brought into Doctor Who. But nevertheless, it is a genuine scientific concept. One can picture it in the following way. One can think of ordinary, real, time as a horizontal line. On the left, one has the past, and on the right, the future. But there’s another kind of time in the vertical direction. This is called imaginary time, because it is not the kind of time we normally experience. But in a sense, it is just as real as what we call real time.