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) Bradley Monton and Graham Oppy correctly point out that we do not know all the relevant physics and so there is no guarantee that a theory of quantum gravity will include a beginning of the universe, but Monton’s claim that “we ought to conclude that it’s unknown whether there’s a big bang and hence we ought to conclude that it’s unknown whether the universe began to exist”[ii]seems too sceptical.
It is not a serious objection to the argument to point out the mere possibility that it might turn out that the universe had no beginning; some good reason would need to be given for thinking that that was the case.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. The relevant evidence is what we would expect if the universe had a beginning and so rival theories are in the difficult position of having to account for this evidence while also denying that such a beginning occurred. On the basis of the evidence there is plenty of reason to affirm, and little, if any, to deny, that the universe had a beginning.
8 ) A key point here is that things do not come into existence completely uncaused. Although quantum theory does allow for particles to appear in a quantum vacuum it does not give us any reason to believe that particles (or universes for that matter) can spontaneously come into existence out of literally nothing.
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. The argument is not based on scientific ignorance, but depends only on whether the universe had a beginning, which is a question science can address.
[i] ‘Inflationary Spacetimes are Incomplete in Past Directions’, Physical Review Letters, 90(15) (2003), 151301.