While the evidence for the Big Bang model is very convincing (see posts below), many questions remain. The first and most obvious is the singularity problem. In essence, the problem is that while the BB model gives us much information on the evolution of the universe after the Bang, it gives little information about the Bang itself. What banged? What does time zero really mean?
The Big Bang model predicts that the matter, energy, space and time of our universe all came into being in a faction of the first instant and that the universe has been expanding and cooling ever since. The problem is that as we rewind the clock of an expanding universe and the intergalatic distances shrink to zero, relativity predicts that the temperature and density of the universe must increase to infinity (just as the function f(x) = 1/x approaches infinity as x approaches zero – known as a singularity in mathematics). What does this mean physically?
A singularity appears as the distance approaches zero
At first, it was thought that the singularity might be a facet of the simplfying assumptions used in applying the equations of general relativity to the universe. However, in the 1970s, Hawking and Penrose published a number of theorems showing that classical relativity predicts that an expanding universe must originate in a singularity, assuming only some very general conditions. (The most important general condition is a restriction on the behaviour of matter at high energy known as the strong energy condition – of course one way out of the problem is to assume that this condition does not apply, more on this later).
Hawking: an expanding universe must begin in a singularity
The most obvious solution to the singularity problem is to realise that when one is considering an extremely young universe (i.e. a universe of atomic dimensions), quantum effects will become important. However, these cannot be described as we do not have a quantum theory of gravity (i.e. we do not yet have a version of general relativity that takes quantum physics into account, hence the name classical general relativity). Until we do, we can say little of the universe in the time when it was of atomic dimensions or smaller.
In other words, the prediction of an initial singularity may well be a limitation of current theoretical physics, rather than a facet of reality. This incompleteness of the Big Bang model is well recognized, and it is the main reason one talks about the Big Bang model rather than the Big Bang theory. ..
Note on theory: one of the great challenges of modern physics is to reconcile general relativity and quantum theory, the two great pillars of modern physics. Gravity is now the only one of the four interactions that we cannot currently describe as a quantum field theory. Most of the time, this doesn’t matter, as gravity typically deals with the world of the very large, while quantum theory deals with the world of the very small. However, when we when we attempt to describe gravity on small scales – (i.e.black holes or Big Bangs), the mutal incompatibility of the two theories becomes a major stumbling block…