Disclaimer: This is a bit speculative on my own part. As a disclaimer, I am a physicist, but I study lasers and plasma physics, not astronomy and astrophysics. Thus, I have some knowledge about General Relativity, the Big Bang, black holes, etc, but this is not the subject matter of my particular expertise. Nevertheless, I got to musing about this stuff, and thought it was kind of interesting.

In the Beginning...

A question has been posed concerning the universe's existence and the specific requirement that it has a beginning. It is sometimes asked, "what happens before the Big Bang?" Alternatively, it may be made as a statement:
There is no point in time when the "stuff" that comprises the universe did not exist. Why must it have a cause? Or, why must a finite being be caused by something else in order to exist?

The background of the statement is the assumption that time is inextricably interwoven with space, so that both space and time came into existence with the beginning of the universe. Thus, time itself cannot be older than the universe: no universe, no time. The Big Bang is the first event, before which nothing occurred; indeed, "before" is a meaningless concept (since it implies a progress of time) until after the Big Bang.

As a simple example of how this can make sense, image that the universe has a sort of "time line" along which it travels. There are, indeed, several "arrows" which give the "direction" of time: as time increases, the universe expands; as time increases, entropy increases; etc. The assumption being that time t=0 is the Big Bang, as as t approaches a value of 0, the universe by necessity approaches the instant of the Big Bang and its initial extent (which is not necessarily zero), and entropy by necessity approaches its minimum value (perhaps somewhat larger than zero).