1) Everything that begins to exist has a cause of its existence.
2) The universe began to exist.
3) Therefore the universe has a cause of its existence. (Click to learn more about the surprising Islamic origins of this argument and what Ludwig Wittgenstein had to say about it.)
There are many options for attacking the logic of this cosmological argument, and contemporary opponents of theism have tried them all.
If everything needs a cause for its existence, then so does God. (More frequently heard in the form "But Mummy, who made God?") This objection fails because it gets Premise 1 wrong. The premise does not say that everything needs a cause but that everything that begins to exist does. God never began to exist--he is eternal. So he does not need a cause for his existence.
Maybe the universe had a natural cause. But the big bang could not have been caused by prior physical processes. That is because it began with pointlike singularity, which, according to relativity theory, is not a "thing" but a boundary or an edge in time. Since no causal lines can be extended through it, the cause of the big bang must transcend the physical world.
W ell, then, perhaps it had no cause at all. It is hard to think of a principle more amply confirmed by our experience than that things do not just pop into existence uncaused. No one can really pull a rabbit out of a hat. Ex nihilo nihil fit. Yet something of the sort does seem to happen in the quantum world, where, owing to Heisenberg's uncertainty principle, tiny "virtual particles" spontaneously appear and disappear all the time. An entire universe could do the same, claim some cosmologists. Calling themselves "nothing theorists," they have produced models showing how the cosmos could have burst into being all by itself out of a patch of "false vacuum," or a 3-D geometry of zero volume, or--in the case of Alexander Vilenkin of Tufts University--literally nothing at all (this took Vilenkin four pages of math). So the universe is summoned out of the void by the laws of physics. But this can't be right. The laws of physics are just a set of equations, a mathematical pattern. They cannot cause the world to exist. As Stephen Hawking has written, "A scientific theory ... exists only in our minds and does not have any other reality (whatever that might mean)."
Just because the universe is temporally finite does not mean it had a beginning. Speaking of Hawking, this is his famous "no boundary" proposal. "So long as the universe had a beginning, we could suppose it had a creator," Hawking wrote in A Brief History of Time. "But if the universe is completely self-contained, having no boundary or edge, it would have neither beginning or end: it would simply be. What place, then, for a creator?" In Hawking's quantum cosmology, the pointlike singularity of the big bang is replaced by a smooth hemisphere in which space and time are commingled. "Time zero" becomes an arbitrary point, not a true beginning; it is no more a boundary than the North Pole is.
Hawking's proposal is extremely popular with laymen who are hostile to the cosmological argument, judging from the mail I get. Apparently they enjoy being baffled by "imaginary time," a theoretical fiction Hawking uses to redescribe the big bang so that there is no beginning. In real time there still is a beginning. Sometimes Hawking says that imaginary time is "earlier" than real time, which is a logical contradiction; sometimes he suggests it might be more real than real time, which is an absurdity.
OK, so the universe had a beginning, and hence a First Cause, which is, moreover, transcendent. How does it follow that this cause is God, or even God-like? Now there is an acute question. Philosopher Thomas Nagel has suggested that something humanly inconceivable lies behind the big bang. What, if anything, can really be inferred about the First Cause? Well, suppose that it were something mechanical. An ideal machine produces its effect either always or never; it does not just suddenly start to operate at some moment, unless someone gives it a kick. If a mechanical cause produced the universe at time T, there is no reason it should not have done so at time T minus 1. The argument can be repeated to T minus infinity: A mechanical cause would have either produced the universe from eternity or not at all. But the universe was created at one moment out of an infinity of other indistinguishable moments. This implies that the moment was freely chosen, and hence that the creator had a will, and to that extent a personal nature. And power.
Yet the big-bang cosmology has one unwelcome consequence for theists. It seems to suggest that the Creator was a bungler. A singularity is inherently lawless. Anything at all can come out of one. It is exceedingly unlikely that a big-bang singularity should give rise to a universe whose conditions are precisely suitable for life, let alone the best of all possible worlds. As the American philosopher Quentin Smith has pointed out, "If God created the universe with the aim of making it animate, it is illogical that he would have created as its first state something whose natural evolution would lead with high probability only to inanimate states." The only way God could have ensured the appearance of creatures in his own image was by repeatedly intervening and making adjustments to steer the evolution of the world away from lifeless disaster. But "a competent Creator does not create things he immediately or subsequently needs to set aright," observes Smith. (Remember, we are talking about the universe's physical infrastructure, not sinners with free will.)