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).

It can also be argued that we can never actually reach t=0, and yet at the same time that the universe has a finite age. The implication is that, since we can never reach t=0 by any means (science-fictionesque time travel, scientific probing, etc) that it is equally impossible for us to "look" at the universe prior to t=0 even if such a concept had meaning. This can be done without having transversed an infinite amount of time (a physical impossibility) by noting that time may slow as we approach t=0 (a theory which is consistent with general relativity). It's a sort of Zeno's paradox applied to time: our "steps" necessarily get smaller as we get closer to t=0 so that we never quite reach t=0.

In essence, both of these approaches argue that the universe is not infinitely old, and yet there is no "first cause" existing "before" the universe. That is to say, the universe has always existed, in the sense that "always" means "since the beginning of time": there was nothing before the universe--because "before" is here meaningless--and therefore the universe was not caused by a pre-existing entity (be it an agent, a non-personal "force", etc).

The Black Hole

Let's switch gears for a moment and consider something which is generally believed to exists in the universe: a black hole. Suppose in the distant future, mankind has sent a pair of probes--observers, really--to investigate a newly-discovered black hole. One probe, we'll call this observer probe A (or just OPA for short) is sent to descend into the black hole. The second probe, observer probe B (OPB) is stationed inertly one light year away, and watches OPA's descent.

What will each probe "observe"? Well, OPB will see OPA fall into the black hole, and as it falls it will emit radiation which become ever more energetic, until it passes beyond the event horizon. At this point, OPB will no longer be able to observe OPA, and OPA is effectively lost forever. However, OPB will also observe that OPA appears to "slow down" as it falls, and thus will never see OPA actually reach the event horizon. OPA, on the other hand, will see itself pass quite rapidly through the event horizon (assuming that these futuristic probes are also strong enough not to be crushed by the gravitational forces of the black hole!).

Now, let's reverse this process: OPA is given some large initial velocity in an attempt to escape from the black hole. The time reversal would require that from OPB's frame of reference, OPA slowly breaks free of the event horizon (beginning somewhere outside of it, by the way). If OPB begins at the event horizon, then from the perspective of OPA, it will require an infinite amount of time to leave the event horizon. If OPA begins outside of the event horizon, the escape would appear very fast to OPA but very slow to OPB.

The Big Bang

In the case of the Big Bang, essentially the entire universe may act as OPA. Any outside observer--OPB--by necessity must exist outside of the universe: hence, the point about steps needing to be smaller as time t=0 is approached are really more applicable to extra-universal observers (who in turn may or may not actually be governed by the laws of physics operating within our universe). In any case, OPB may be God, another universe (if you subscribe to multiverse theory), etc, but not really anybody in and of this universe.

As for the universe itself--the OPA reference frame--there is are really three possibilities which make sense. The first possibility is that the matter of the universe existed within the event horizon of a primordial black hole* and that the laws of physics are bent "in the beginning" so that this matter might escape. The second is that the universe's "beginning" consisted of matter in a state outside of the event horizon of any primordial black hole. The third is that there is no primordial black hole.

The first possibility would make any physicist uneasy, because physics and indeed all hard sciences are based on the idea that the laws of nature are immutable. What's physically true now must have been true yesterday and must be true tomorrow. Light has a speed which is invariant in a vacuum, and which cannot be reached (let alone exceeded) by matter; therefore, it is impossible to reach an escape velocity during the Big Bang if matter was within the event horizon of the primordial black hole: an outside source of energy must continue to supply additional energy to the matter, and must do so with enough acceleration to break through the event horizon--itself a physical impossibility. Under such a view, the laws of special and general relativity, at least, and perhaps of Newtonian gravity as well, would need to be suspended until some time after the Big Bang.

The second possibility is problematic, but for a different reason: the arrow of time requires that the universe be ever-expanding as time from the initial event (Big Bang) increases. If matter exists initially outside of the primordial black hole, then it is possible for the universe to have been decreasing initially: "negative time" (alternatively, the Big Bang occurs some time after t=0). This is to say, time is older than space, or than the universe. If this is the case, then the universe came into existence necessarily by and outside cause, and there did exist something "before" the universe. This means that all of the "stuff" which exists in the universe did not exist at all times--there is a point in time during whcih it did not exist, and this point is the point prior to the Big Bang event. If this is the case, then the universe is necessarily caused, be it by God, branes, the multiverse, etc.

The third possibility is that there is no "primordial black hole." This is the say, the universe"begins" as a collection of matter which is already both expanding and somewhat separated. This might be implied, for example, by the Hartle-Hawking State (as I understand it). Here, the universe "begins" as a wave-function which, when collapsed, yields our universe. Quentin Smith, a disciple of Hawking, argues--albeit incorrectly--that this latter model is a refutation of theism**.

In any case, this last view also pre-supposes a universe which is in an "uncompressed" state, that is, it yields a universe whose beginning is similar to that of the second scenario. Thus, the time of the "Big Bang" is not t=0 absolute, because the universe may be further compressed, and thus the arrow of time may perhaps be made to reverse direction from the time of the Big Bang.

All of these scenarios lead, if not to the absolute necessity then at least to the strong possibility of a universe which is caused, in the sense that it needs an external agent or force to bring about its existence. This may be as the first observer, or the source of energy for expansion and the cause of the righting of our physical laws, or as the thing which brings about the existence of matter in its initial state (if not at the actual time t=0). Because the universe had a beginning, it must have a cause.

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*When I mentioned the primordial black hole here, I don't mean "a primordial black hole" in the sense often commonly used: that is, a black hole formed early in the universe's lifetime by some matter forming more densely in one region of space, thus creating a black hole which may be smaller that the 3 solar masses generally required to cause a star to collapse into a black hole at the end of its "life." Rather, I am referring to the condition of the universe prior to the Big Bang, during which the density of matter exceeds that necessary to be a black hole, implying thereby that the universe itself began as a large psuedo-black hole and exploded as an instant evaporation of Hawkings-radiation.

Another way of saying this is: what was the initial radius of the universe? If it was initially so small that it is impossible to further compress, then for all intents and purposes, we have scenario 1. If further compression of the matter and energy of the universe is possible, then we have scenario 2.
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**Another interesting aside: According to the widely-accepted Copenhagen Interpretation of Quantum Mechanics, and indeed to most other interpretations, wave-functions don't collapse into a particular eigenstate until an outside observer makes an observation. If this is true, then a wave-function beginning to the universe actually implies an observer from beyond the universe-otherwise, the wave function would not collapse to create the universe in the first place! Of course, there are other interpretations of quantum mechanics, but as far as I am aware, none of them allows a wave to collapse of its own accord (e.g. without an outside observer making an observation), save for possibly the highly problematic Objective Collapse Theory.

With that said, the late Fr Stanley L Jaki argued that quantum wave interpretations of the universe would not only imply the need for an external observer, but would moreover yield only a virtual universe. This, in an essay in his collection "The Limits of a Limitless Science."