Was the big bang the beginning? There have been plenty of theories put forth that suggest there was something before the big bang, like the cyclic universe and ekpyrotic universe models. They all tend to involve some exotic physics like the collision of branes (multidimensional sheets of spacetime intersecting, nothing to do with zombies) or other ideas that would take a book rather than a short piece here to explain.
This morning I heard more about an interesting addition to the list of pre-big bang models that involves what the speaker Abhay Ashtekar, of Penn State University, called a "quantum bounce." Ashtekar described some work in loop quantum gravity, one of the more promising alternatives to string theory as a possible theory of quantum gravity. The idea has been floating around for a while but he has made considerable progress in developing the model.
When physicists studying the big bang talk, they tend to run the universe in reverse and they talk about the evolution of the universe going backward so you hear descriptions about the universe getting closer to the big bang, but it is coming from this end of time, not the beginning. It takes a minute to adjust your head to, but it starts to make sense eventually.
Ashtekar talked about how within loop quantum gravity, the universe can look like a normal universe governed by Einstein's general theory of relativity until very close to the big bang, at which point the quantum aspects of loop quantum gravity take over. In general relativity everything would become what is called a singularity at the point of the big bang. A singularity is a place where the mathematics turns, to use a technical term, "stupid." There are infinities to deal with and the laws of physics break down.
But within loop quantum gravity, no singularity appears. Without having to invent any new rules, the theory transitions from being like the classical laws into the quantum laws and there is essentially a patch of spacetime, ruled by the laws of quantum geometry, that joins it all together and prevents a singularity appearing.
However, if you push this to its conclusion, you find that you come out the other side of the quantum geometry region with a bounce--the quantum bounce Ashtekar talked about. Then the universe turns back into something like a classical universe. This process can repeat many times over with the universe repeatedly undergoing these cosmic quantum bounces. During the transition, the laws of loop quantum gravity are well-behaved and deterministic (not random).
Furthermore, within the classes of models Ashtekar and his coworkers have investigated, the quantum bounce seems to happen in every case. The research comes with a lot of other technical results that were very interesting but that I don't understand well enough to talk about here! This is all early work and you can't take it as a given that this is the way the universe works.
It is, however, intriguing to consider a universe where, as Ashtekar said, "Physics does not end at the big bang." Nor does it begin there.
See all posts from the American Physical Society April 2008 conference here.
