It is difficult enough to imagine a time, roughly 13.7 billion years ago, when the entire universe existed as a singularity. According to the Big Bang theory, one of the main contenders vying to explain how the universe came to be, all the matter in the cosmos -- all of space itself -- existed in a form smaller than a subatomic particle [source: Wall].
Once you think about that, an even more difficult question arises: What existed just before the big bang occurred?
The question itself predates modern cosmology by at least 1,600 years. Fourth-century theologian St. Augustine wrestled with question of what existed before God created the universe. His conclusion was that the Biblical phrase "In the beginning" implied that God had made nothing previously. Moreover, Augustine argued that the world was not made by God at a certain time, but that time and the universe had been created simultaneously [source: Villanova University].
In the early 20th century, Albert Einstein came to very similar conclusions with his theory of general relativity. Just consider the effect of mass on time. A planet's hefty mass warps time -- making time run a tiny bit slower for a human on Earth's surface than a satellite in orbit. The difference is too small to notice, but time even runs more slowly for someone standing next to a large boulder than it does for a person standing alone in a field. [source: Redd].
Based upon Einstein's work, Belgian cosmologist Rev. Georges Lemaître published a paper in 1927 that proposed the universe started out as a singularity and that the Big Bang led to its expansion [source: Soter and Tyson].
According to Einstein's theory of relativity, time only came into being as that primordial singularity expanded toward its current size and shape.
Case closed? Far from it. This is one cosmological quandary that won't stay dead. In the decades following Einstein's death, the advent of quantum physics and a host of new theories resurrected questions about the pre-big bang universe. Keep reading to learn about some of them.