Question: Why do you say that the laws of physics are not sufficient to explain the existence of the universe?
Answer: Let us say that a man, John Doe, generally goes to bed at approximately 9:00 pm. Is the fact that (John Doe generally goes to bed at 9:00 pm) sufficient to explain why John Doe came into existence? Of course not. But that is precisely what this question is asking.
The “laws of physics” are really just our best description of how the universe behaves, based on our own necessarily limited observations. The fact that the pressure of a gas tends to decrease as the volume the gas fills increases (Boyle’s Law) no more explains “why the universe exists” than “when John Doe goes to bed” explains why John Doe exists. It has no causal power, not even an actual existence of its own. It is purely descriptive. More importantly, since they are descriptive of how the universe operates, and causes cannot come after their effects, the “laws of physics” by definition cannot have caused the universe. So far as they may be attributed any meaningful existence at all, they are ontologically dependent upon the universe. We will return to this momentarily.
Speaking of cause-and-effect, we have reached the problem of time. Many people have the misconception that “time” is a thing-in-itself, but this is not the case. Even in describing it as one of four contiguous dimensions which began to expand with the Big Bang, gives the illusion of positive existence. In fact, the space-time dimensions have a negative existence: they exist only as the separation between physical objects. I am currently seated approximately two meters from a door. There are not really two of a thing called “meters” between the door and myself; rather, I have arbitrarily assigned a measurement (“meter”) to describe the separation between subject (myself) and object (the door).
This is important because, at the moment of the Big Bang, none of the dimensions, including time, had expanded. There was no “here” or “there”, no “before” or “after”, no subject nor object. There was only “is”. Since all physical causes must precede their effects, and since the idea of “preceding” the Big Bang is meaningless, the cause of the universe cannot have been physical. It must necessarily be metaphysical (and I hope you are well-educated enough to understand that “metaphysical” has nothing to do with “supernatural”, whatever your online dictionary may tell you), existing outside of the timeless “is” which expanded into the universe.
So, we have identified one characteristic of First Cause: it must be metaphysical. But we have identified a second characteristic, have we not? Just as “John goes to bed at 9:00 pm” cannot have caused John, because it does not exist without John, so First Cause cannot be ontologically dependent upon the universe. A thing cannot be caused by something it has, itself, caused.
And this brings us to the third characteristic of First Cause. Human logic, and therefore science, depend upon the universe behaving in a rational manner. This is always observed to be the case. If the universe were irrational, logic (and therefore mathematics, science, and all things which depend from them) would be impossible. Since the universe is rational, First Cause must also be rational. Just as hurricanes do not build hotels, irrational causes do not produce rational effects.
Now, we have identified that First Cause must be rational, metaphysical, and ontologically independent. A rational, metaphysical, uncaused cause is the definition of God.