Must God exist to explain how the universe began?

But the fact of experience is not that everything we know gets its existence from a cause, but only every event or change does so. Nature has a permanent element, and also a changeable one; the changes are always the effects of previous changes, but so far as we know the permanent existences are not effects at all.

If everything must have a cause, then God must have a cause. If there can be anything without a cause, it may just as well be the world as God, so that there cannot be any validity in that argument. It is exactly of the same nature as the Hindu's view, that the world rested upon an elephant and the elephant rested upon a tortoise; and when they said, "How about the tortoise?" the Indian said, "Suppose we change the subject." The [First Cause] argument is really no better than that.

Because time is an aspect of the universe, it‘s hard to see how it can be said to have a “beginning” in the way the word is normally used. The concept of a “beginning” normally assumes a “time before” at which the object did not exist — but there was no “time before” the universe. Without a time before, the notion of "cause" no longer applies.

I'd probably speculate that near the big bang, the notion of time really breaks down, and so the question of what was the beginning and what was before the beginning - that kind of question - will ultimately turn out to not quite make sense.

In essence, [the Cosmological Argument] argues that everything has a cause, and since this chain of causality cannot go on forever, it must terminate in a first cause, which we call God. The idea of an ultimate cause is deeply attractive, and indeed the dream of elementary particle physics is to find the final theory at the root of all chains of explanation of what we see in nature. The trouble is that such a mathematical final theory would hardly be what anyone means by God.