One of the most common questions asked in popular lectures on astrophysics and cosmology - and one that often gives lecturers the most trouble in answering - is the simple question of "What came before the Big Bang?".

According to the laws of general relativity, and to observations made by Hubble and others in the last century, the Universe itself is constantly expanding. At first some physicists thought that perhaps the Universe still possessed an infinitely long past, with it being small and slow growing at first, such models are inconsistent with both theory and experiments.And in the past century, more data and better models have been generated which further support the idea that the Universe and time itself began roughly fourteen billion years ago. Therefore the Universe has to have had a beginning.

And it would seem natural that anything with a beginning must also have something that preceded it. The day begins when the last one ends. Each year begins only when the past one ends. So then what ended when all of time began?

The answer is, we just don't know. No experiment yet devised has ever been able to explore anything outside of our own Universe. However that hasn't stopped the theoretical physics research community from developing a number of possible explanations. And while these are too technical and too numerous to review in detail here, the most interesting fall into four categories:

  1. Everything started at time zero: Perhaps there really is nothing before the Big Bang. Everything we know about the laws of physics works just fine with the Big Bang truly being the starting point of everything - although there are obvious philosophical issues. One way to visualize this is to take a globe, and imagine that instead of north and south you have 'future' and 'past'. At nearly any point on the surface, you can imagine the past as being the area south of that point and the future is everything north of that point. Then ask "What is further south than the south pole?". The answer is clear in this case, that nothing is south of the south pole. Many viable models of the Universe treat time in exactly the same way.
  2. The Universe bounces: Another possibility is that the Universe didn't really begin at the Big Bang, but rather at a Big Bounce. It is possible that our Universe will eventually start to contract again, and in the distant future everything will collapse to a single tiny, high energy, high density point. It will then explode outward again and create an entirely different Universe which will expand and contract again, and form yet another Universe. This process may have been happening into the infinite past. There are problems with this model though, in that each Universe is effectively hotter than the previous one, so if it has happened forever then it should be far too hot for any structure (ie stars and galaxies) to form. However those problems can be resolved if the Universe has hidden higher dimensions which can effectively dilute the extra entropy. 
  3. We are in a bubble: A third option is that what we think is the Universe is only a bubble in a bigger Universe. Since nothing can travel faster than the speed of light, we can really only view a section of the Universe a little less than fourteen billion lightyears across. And yet we are confident that the Universe is at least a few hundred billion lightyears across, based on its small amount of curvature. And yet it is only inside our own little section of the Universe that we can be confident that it began with a Big Bang at a certain time in history. Perhaps the Universe has been around for infinite time, but small regions of it suddenly explode into large bubble sub-Universes. The inhabitants of these bubbles would be unaware of anything outside the bubble. In these models, there exist an infinite number of other bubbles which have different ages and chemical compositions, different values of particle masses and interaction strengths, and possible even have different laws of physics!
  4. The Multiverse: And finally, there is the very real possibility that we exist in a Universe that itself exists in a higher-dimensional space and time which is filled with other Universes. (It should be noted that in the previous example, all the bubbles exist in the same four-dimensional Universe and have boundaries with neighbouring bubbles, while in this model each Universe has its own set of dimensions, and are completely disconnected from each other). In this model, there are an infinite number of Universes, each with its own number of space and time dimensions, each with its own laws of physics, and completely disconnected from all other Universes. In this case, what we perceive as the Big Bang is either the formation of our own Universe in the Multiverse, or it could have been a collision between two older Universes. Unfortunately this model is also the least testable of the four, but the most interesting mathematically and philosophically.
So what did come before the Big Bang? We just don't know. But perhaps with more experiments, and more calculations by the theorists, we may one day find not only new planets to explore, but entirely new Universes...