1) If the universe has only been in existence for a finite amount of time, then it is rational to believe that the universe began to exist. If the big bang theory is true, it provides strong evidence that our universe has only existed for a finite amount of time and, therefore, that our universe began to exist.
2) The universe is expanding and hence becoming less dense. This means that in the past it was much more dense. If we extrapolate back far enough, we reach a state when the universe was extremely dense and hot and finally a state of infinite density where the theory of general relativity breaks down (a state known as a singularity). The big bang theory describes the expansion of the universe from this early hot, dense phase. According the standard account , not just energy, but space and time came into existence at the singularity.
3) The exact nature of such a singularity is far from clear, but if the universe had a beginning such a singularity would mark the beginning of the universe (or else the time immediately after which the universe began to exist). If we consider the timeline of the universe, the singularity would simply be the time t=0.
4) We cannot appeal to the singularity as the cause of the universe. If the big bang singularity is precisely nothing, we are left with the question of how the universe then came into existence out of nothing. Others have argued that a big bang singularity would be a real physical state; but if so it would still just exist at the time t=0. In that case we have to ask “how did the singularity come into existence out of nothing?”
5) Some speculate that future scientific research will provide strong evidence in favour of cosmologies that avoid a beginning of the universe. For example, in the oscillating universe model the universe expands, then collapses back on itself, then expands again, and so on. However, many such models turn out to be incompatible with an infinite number of cycles and so do not avoid the beginning. Furthermore, our current evidence indicates that our universe will not collapse back in on itself.
6) Research by Borde, Guth and Vilenkin has shown that, under reasonable assumptions, an expanding universe will have a finite past. [i]
7) If a proposal is intended to undermine the idea that the universe had a beginning, there must be some good reason to think that the proposal is true or likely to be true. Merely appealing to the possibility that the universe might not have had a beginning would be a very weak response to the argument being proposed here.
8) While there is no conclusive proof that the big bang theory is true or that the universe had a beginning, the scientific evidence does strongly point in that direction. Sir Martin Rees has written:
The empirical evidence for a Big Bang ten to fifteen billion years ago is as compelling as the evidence that geologists offer on Earth’s history … A few years ago, I already had ninety per cent confidence that there was indeed a Big Bang … The case now is far stronger: dramatic advances in observations and experiments have brought the broad cosmic picture into sharpfocus during the 1990s, and I would now raise my degree of certainty to ninety-nine per cent.[ii]”
9) The scientific evidence for the beginning of the universe provides evidence for theism and against materialistic atheism. Consider the following argument:
(A) Whatever begins to exist has a cause;
(B) The universe began to exist;
(C) Therefore, the universe has a cause.
There can be very little doubt about (A) and we have a good deal of scientific evidence for (B); therefore, there is not a great deal of uncertainty about the conclusion.
10) The cause of the universe could not be physical and would exist outside our space and time. This is certainly much more consistent with belief in God than it is with materialistic atheism.