"The Big Bang, which today we hold to be the origin of the world, does not contradict the intervention of the divine creator but, rather, requires it."
"... some theologians, at least, should be sufficiently well-versed in the sciences to make authentic and creative use of the resources that the best-established theories may offer them. Such an expertise would prevent them from making uncritical and overhasty use for apologetic purposes of such recent theories as that of the “Big Bang” in cosmology."
- — John Paul II
The Big Bang theory states that around 13.7 billion years ago the universe was condensed into an incredibly small, hot, dense "ball" of space and time. Some have speculated that it emerged from an infinitely dense and small object known as a singularity but most scientists prefer other hypotheses. 
The name "Big Bang" is somewhat of a misnomer, since the universe did not expand in a conventional sense and didn't explode or produce sound as we normally understand it. The first fraction of a second saw significant changes in the way forces, matter and energy existed and was very unlike the universe as we currently observe it. The universe expanded very rapidly by a process called "inflation". As the expansion continued, the universe cooled, eventually reaching a point at which long lived particles of matter could "freeze out" of a mixture of mass and energy which was previously in continual flux (see Wikipedia:Mass–energy equivalence) and collide with each other to form the first simple atoms. Over billions of years, these particles combined to form "clouds" of matter which further condensed, because of gravitational attraction, into stars and planets. (See Wikipedia:Physical cosmology for much more detail.) Atoms progressively formed "heavier" elements in stars through the process of nuclear fusion.
The history of the universe can be described in some detail back to the instant approximately 10-43 seconds after the big bang. What precisely occurred in the first 10-43 seconds (the Planck epoch) is current unknown with many competing theories, due to interactions between the theories of gravitation and quantum mechanics.
Did the universe have a beginning?
Perhaps. Or perhaps not.
The existence of the singularity is speculative. Singularity theorems are proved within the theory of General Relativity, but under the prevailing conditions one has to consider quantum effects. No one knows how to do that rigorously, but it is generally believed that quantum mechanics would not allow a singularity. The co-author of the universe from singularity model, Stephen Hawking, said: 
- "[An] interpretation of our results, which is favored by most scientists, is that it indicates that the General Theory of Relativity breaks down in the very strong gravitational fields in the early universe. It has to be replaced by a more complete theory."
There are several possibilities on what the real structure of existence may be. These include the idea that there are no boundaries at all, or that there is a sort of "fuzzy" boundary which is not a singularity. While a singularity or boundary are still possible, there is no good scientific reason in the present to suppose them. The universe may have pre-existed in some undiscovered state. This undermines the apologist's argument as this means there is no way to prove the existence of an "Unmoved Mover" or "First Event".
Events are usually preceded by earlier events. And yet, the singularity cannot be preceded by anything! How is this possible? Because the singularity isn't an "event" at all. The singularity is the limit of physical events. For an analogy, consider the series of numbers 1/1, 1/2, 1/3, 1/4... Each number in this series is followed by a smaller number, just like any event has a preceding event. If we follow the chain of numbers, it is easily seen that "at infinity" we reach the number 0. There is no actual Smallest Number within the series, however, and this is precisely why every given number in it is followed by a smaller one. The same is true for the singularity - it isn't a point in space-time, but rather a boundary of it. The singularity isn't a First State, there isn't a "First State". It isn't a state at all. Rather, it is the limit of states, with each state preceded by an earlier one.
An informed discussion of these issues is unfortunately beyond almost everyone, since the early universe was quite unlike the currently observed universe. Many of these issues are among the greatest unsolved problems of physics. 
No models predict an infinite universe
"The fact is, there is no mathematically and physically tenable model of the universe which is extrapolable to past infinity. If Goldstein thinks otherwise, let her tell us the model. The bottom line is: physics doesn’t deal in possibilities. Possibilities come cheap. What we want to know is where the evidence points."
No physical model is known with certainty to predict the universe's earliest stages or origin. While most models appear to imply a finite age, we simply do not know and known models are highly like to be incorrect; this subject is at the frontier of knowledge after all! There may be a correct model that corresponds to the actual universe having an infinite age that we simply has not discovered. We can only know a model is correct after appropriate observations, which are not possible at this time. Before Edwin Hubble’s observations, there were no tenable models of a finite aged universe, so it is quite possible scientific consensus may sift on this matter. Claiming that no model exists for an infinite universe itself implies a finite universe is an argument from ignorance.
The correct course in this case is to suspend judgement until we can validate a model as being correct. The lack of a model does not imply physical impossibility. If William Lane Craig bases his arguments on a premise that cannot be feasibly verified, his conclusions should also be considered as tentative (at best).
Did the universe come from ex nihilo "nothing"?
There is a common misconception that the Big Bang means that the universe "came from nothing." This is a straw man argument. What occurred in the very early universe is still a mystery. What, if anything, came before the Big Bang is a mystery.
Apologists claim that without God, the laws of thermodynamics would be violated.  At this point, we do not even know if the universe is a close system, which is fundamental assumption of thermodynamics. Also, thermodynamics have not been validated in the very early universe; physicists are still struggling with much more basic problems.
How did the universe get its physical laws and constants?
Scientists don't know that either. Many have been traced back to very fundamental principles, such as the laws of physics are invariant of the point of view of the observer, in a field of physics called gauge theory.  Physics can only hope to trace physical laws into a simple, unified set of principles called the "theory of everything" or ToE. However, we may never discover the ToE or the ToE might not even exist. If the ToE exists, it might be a "brute fact". If the ToE exists, and it is not a "brute fact", it is questionable what we mean by an "explanation" for the ToE.
No doubt, if the ToE is discovered, it will be used as another cosmological argument for God. The existence of natural physical laws is used in the natural-law argument.
Speculations on the pre-Big Bang universe
- The oscillatory universe is the hypothesis, attributable to Richard Tolman from 1934, that the universe undergoes an infinite series of oscillations, each beginning with a big bang and ending with a big crunch. After the big bang, the universe expands for a while before the gravitational attraction of matter causes it to collapse back in and undergo a bounce. (This theory has declined in popularity since 1998, when astronomers reported evidence that the acceleration of the universe's expansion continues unabated.) 
- The universe existed forever but somehow eventually developed into a Big Bang. Some models predict a beginning-less universe with no singularity but still featuring a Big Bang. 
- Cosmological natural selection is a speculative hypothesis proposed by Lee Smolin. Smolin speculates that every black hole might contain another universe inside it. Thus, our universe might be a black hole inside another universe. Each universe shares properties and fundamental constants with its "parent" universe, but may be slightly different. Thus--according to this theory--universes evolve over time, and the ones that are particularly well suited to produce black holes are the ones that thrive.
- The multiverse hypothesis suggests that there are already multiple parallel universes, generated in a meta-universe.
- There may be no such thing as a pre-Big Bang universe.
- Our intuition about time is based on the environment we live in (and evolved in), with small accelerations and a relatively flat spacetime. In highly curved regions of spacetime, those intuitions break down.
- As an analogy, "north" is a direction that is more or less constant in most cities. But the direction "north" in Los Angeles is not parallel to the direction "north" in Berlin because of the curvature of the earth. In fact, going north from any point will eventually lead to the north pole because the earth is a sphere. At the north pole itself there is no direction of "north" because every point around it is farther south than it is.
- Similarly, "towards the past" is not a direction that is the same for every point in spacetime, but rather going back in time from any point leads back to the big bang. If there is no previous universe from which our universe sprung, it may be that there is a "past pole", or a finite region of spacetime around which every other point is farther in the future. It would be meaningless to talk about what happened before this time, because there would be no such thing as a "before" that. The universe would either be uncaused or the "cause" would be something that did not precede it in time.
None of these concepts have been conclusively demonstrated, but they do illustrate that God isn't the only possible answer (See Wikipedia:Cosmogony for more information).
Creationists often object to the Big Bang theory on the grounds that it removes God's hand from creation. A common Creationist argument against it is the question "What caused the Big Bang?" and the closely related question "What happened before the Big Bang?". Various arguments rely on the universe having a beginning. This includes:
Apologist arguments assume far more than is currently known by science. Without their premises being established with any confidence, their conclusions are unreliable.
As a historical note, it is interesting to see very obscure areas of reality being used for an argument for God. This is a sign of God of the gaps in that earlier arguments for God from natural phenomena where discarded as science found a naturalistic explanation.
- ↑ 
- ↑ John Paul II, 1997, pp. M11, M12
- ↑ 3.0 3.1 Stephen Hawking, The Origin of the Universe 
- ↑ Wikipedia, List of unsolved problems in physics []
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- ↑ Jeff Miller, God and the Laws of Thermodynamics: A Mechanical Engineer’s Perspective 
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