What evidence refutes a "pre-eternally existent cosmic egg"? I've seen many Young-Earth Creationists attempt to discredit Big Bang cosmology. For example, an article by Cal Thomas in the 1990's stated that it meant there was a "pre-eternally existing 'cosmic egg' that decided to explode." A new refrain I keep hearing that there was pre-eternally existent energy.
My layman's understanding of Big Bang cosmology is that there was a "start" to matter, space, time, and energy.  The Young Earth retelling, in contrast, would say "pre-eternally existent energy, then turning into these other things."
Even the theme song for Big Bang Theory says:

Our whole universe was in a hot dense state,
Then nearly fourteen billion years ago expansion started, wait

Which, seems to be a popular re-telling of what I presumed was the YEC distortion of the Big Bang model itself. Temperature and density pre-existed, according to the song.
QUESTION: What references or very short articles explicitly define the Big Bang model in terms of "what-was-before"?  Was there energy sitting around?
 A: Before getting into the physics, I have to say this particular line of argument from YECs is pretty ironic. Big bang cosmology was originated by a Belgian Catholic priest and astronomer, Lemaitre. Before Lemaitre's work, the Catholic church had already come to the conclusion that time had to have had a beginning, based on theological arguments. I suppose the YECs are either ignorant of this history or treat it with suspicion because their fundamentalist Protestant outlook is so different from the kind of non-literalist, intellectually sophisticated tradition that Lemaitre was a part of (his early education was Jesuit).
Anyway, there are serious problems with this interpretation.
Back in the 1930's, Richard Chace Tolman worked on cyclical cosmologies, in which the universe expanded and recontracted indefinitely. His conclusions, summarized in a book (Tolman 1934) systematically worked out all the thermodynamics of these models, and concluded that they were not viable based on the second law of thermodynamics. Although various later people came up with theoretical tricks to get around these issues, the generic and very clear expectation is that if the universe had been past-eternal, and operated according to the known laws of physics during that infinite period of time, it would have reached thermodynamic equilibrium infinitely far in the past. This means that if these particular YECs want to have this particular idea taken seriously, the burden of proof is on them to say which theoretical trick they propose in order to evade this issue. But of course (1) they have in mind a supernatural rather than naturalistic description of the pre-big bang period, and (2) YEC is not a logically coherent framework but a random collection of ideas, which the YECs can propose and then later disclaim when it's convenient.
Another problem with this proposal is that, unlike the kind of cyclical cosmologies Tolman worked on, it seems to be based on a common popular misunderstanding of the big bang as an explosion that occurred at some particular starting point (their "egg") in a preexisting space. Such a picture is not consistent with general relativity, which is at this point a very well tested and successful theory. Modern cosmology is a high-precision science, so there are tight constraints here. Measurements of the spatial curvature of the universe show it to  be very nearly zero, and this implies a lower limit of the size of the universe, assuming approximate homogeneity and isotropy. This size is too big to make it possible for all the matter to have gotten to where it is starting from the same point. The only way around this would be to abandon isotropy and homogeneity, but there is strong observational evidence for isotropy and homogeneity. People have proposed things like models with big voids in them, but my recollection is that these did not really end up being viable based on the latest observations. Again, the problem is that YECs don't play by the rules. If they want to propose some kind of inhomogeneous cosmology, they need to specify what their model is and show that it's compatible with observation.
Tolman, 1934, Relativity, Thermodynamics, and Cosmology, http://books.google.com/books?id=1ZOgD9qlWtsC&lpg=PP1&pg=PP1#v=onepage&q&f=true
A: The Big Bang Theory itself does not say anything about what came "before" the Big Bang. There are various conjectures, mostly having to do with the idea that vacuum is unstable, which is an accepted fact: a complete vacuum would violate the Uncertainty Principle, and as a result particles are constantly popping in and out of existence. This has been proven with the demonstration of the Casimir Effect.
An interesting book to read is Lawrence Krauss's "A Universe from Nothing".
A: When physics was just a descriptive  science one did not even have a strict definition of energy.
When observations became  quantified then mathematics, used as a tool, started modeling observational data with accuracy and giving prediction for observations not yet seen.
Thus we have the success of Newton et al   in describing our planetary system.
The mathematical models have a region of validity though. The validity of Newtonian mechanics ends at accuracies where Special Relativity or even General Relativity holds.
The validity of statistical mechanics and thermodynamics ends where quantum dimensions become important. A relevant example is black body radiation:


The amount of radiation emitted in a given frequency range should be proportional to the number of modes in that range. The best of classical physics suggested that all modes had an equal chance of being produced, and that the number of modes went up proportional to the square of the frequency.
But the predicted continual increase in radiated energy with frequency (dubbed the "ultraviolet catastrophe") did not happen. Nature knew better.

The quantum line was crossed and the ultraviolet catastrophe is just an infinity coming from an extrapolation of a model  invalid for that regime.
A gedanken experiment:  the potential of a point charge goes as 1/r. If we go to r=0  the potential becomes infinite. Again the quantum nature of Nature makes nonsense of this extrapolation, though it works extremely well till we reach  quantum dimensions where quantum field theory takes over.
The Big Bang is an extrapolation of a successful model of the universe where at the core, time=0, lies a huge singularity. Usually in physics this means that the region of validity of the model has stopped being relevant to the real world.
The problem is that it is not possible to do experiments approaching t=0 of the Big Bang. If we go by the way nature solved the problem on other infinities we could expect quantum mechanics to come to the rescue. We do not yet have a consistent framework for GR and QM.

Before a time classified as a Planck time, 10^-43 seconds, all of the four fundamental forces are presumed to have been unified into one force. All matter, energy, space and time are presumed to have exploded outward from the original singularity. Nothing is known of this period.

In the future we might get more accurate observational data that might distinguish different models ( maybe they predict ringing, or special concentrations of matter that might be checked in a few centuries etc). We are now close to the science fiction stage. The ideal would be that the model unifying quantum mechanically all four forces also solves or at least gives a satisfying glimpse for the time t<0, but I would not hold my breath.
A: If you're looking for the scientific community to come to a consensus on what existed before the Big Bang, well, you're going to be disappointed.
As others have alluded to, the Big Bang is not quite the creation story of the universe. Rather, it is the result of an extrapolation that we know we can't continue indefinitely.
This all began back in the early 20th century, when astronomers like Hubble realized the universe seemed to be expanding. At the same time, theorists working on developing general relativity realized the idea of an expanding universe could fit quite nicely in their models.
At this point, it wasn't much of a leap to ask, "Well, if the universe is expanding, what happens if we go back in time? It must have been smaller then." And in fact, those theorists realized the "size" of the universe goes to zero a finite time in the past. At the same time, the temperature and density must necessarily go to infinity.
Now no one really ever thought these zeros and infinities actually described nature. Rather, they were either:


*

*(1) A sign that the theory was completely off the mark. This was the position of advocates of the steady-state cosmology, one of whom coined the term "Big Bang" just to highlight how ridiculous the notion was.

*(2) A sign that the theory was incomplete. At early enough times, with temperatures and densities unfathomably large, new physics would come into play. The only reason the theory seemed to give infinities was our ignorance of some of the physics. This is comparable to how basic theories predicted an aircraft would experience infinite drag as it approached the speed of sound, and so the sound barrier could only be broken with the inclusion of better physical models in the regime.


The scientific community has reached a consensus that option (2) is right, abandoning option (1).1
How do we know? Well, even if the theory doesn't reliably extend to the beginning of all things, it still makes quantitative predictions based on instances where we can reliably extrapolate back in time. In particular, taking what we know about particle physics, we can ask, "If the universe were originally too hot to form nuclei (it doesn't matter how much hotter), and then it expanded and cooled, what distribution of nuclei would we expect to condense out?" That predicted distribution matches the observed distribution quite well. You can read more about Big Bang nucleosynthesis for instance in this answer to What has been proved about the big bang, and what has not?
But at some point in the extrapolation the consensus ends. If you ask, "What was the universe like in the first $10^{-50}\ \mathrm{s}$?" you will be told, "We have ideas, but we need more of a theoretical framework before we say anything definitive."
If you go even further and ask, "What came before the Big Bang?" of ten cosmologists, you'll get at least a dozen different answers. Some of the more popular (or at least more discussed) theories are:


*

*This question is not even well defined. In this traditional, pure general relativity viewpoint, asking, "Is this model for the existence of stuff before stuff existed correct?" is pure nonsense. It's like saying "Let ☢ be the greatest positive integer less than 0. Is ☢ a prime number?" Asking about properties of things that don't exist doesn't even make sense, and there is no more "before the Big Bang" than there is a positive integer less than 0 or a real number less than $-\infty$.

*Eternal inflation: Roughly the idea that everything is just expanding, and every once in a while a quantum fluctuation slows down the expansion in a small patch, which becomes the visible universe.

*A cyclic universe: A contracting universe gets very small, undergoing a "Big Crunch," and, once it has extremely high but not mathematically infinite density it "bounces" back in a Big Bang, leaving little trace of the previous incarnation of the universe.

*The ekpyrotic universe: Two string-theory-esque branes collide in some "pre-existing" background, with the result being the birth of the universe as we know it.


So there are many scientific ideas about what came before the Big Bang, but you can't point to a collection of conflicting theories as evidence that a separate theory is wrong.2

1 I feel obliged to point out that supporters of the steady-state theory, while it was still popular, ended up making great contributions to physics all the same. In particular, they pioneered the field of stellar nucleosynthesis in an effort to explain how stars could convert hydrogen to heavy elements. As a result, they also pushed ahead the field of general nuclear physics considerably.
2 If you want to challenge a theory, the following is how you put it to the test. First, the theory must be well defined. It cannot have dangling poetic phrases that don't mean anything beyond sounding "sciency." Second, it must be internally self-consistent, at least within its purported domain of applicability. Most theories rejected by the scientific community right off the bat fail these tests. If the theory passes, the third step is to ask if it can explain observations already explained by the theories it claims to replace. If not, then it is deemed a nice attempt, but not in line with nature. Finally, if the theory passes the first three tests, you ask it to predict something new in such a way that its competitors will disagree with it. If this cannot be done, the theory is not new but rather just a rewording of an old theory (which may or may not be useful to have). If it can be done but the subsequent experiment disproves the new theory, then it is deemed a very nice attempt that just happens to not work with nature. Only if it passes this fourth test is it accepted as a new, working scientific theory, but even then it may be superseded in the future by better theories.
A: The moment of the Big Bang was a singularity, which means (with respect to time, anyways) that the chain of cause-and-effect is broken at that point. That doesn't necessarily mean that nothing caused the Big Bang, or that there was nothing before it. All it means is that there is no way to trace any of the effects that happened after that moment back to any particular cause that happened before it. This is (of course) explained and explored in Stephen Hawking's a Brief History Of Time.
A: I will focus only on the part of the question about the theory of the "exploding egg" as an alternative to the Big Bang theory. In a nut shell the exploding egg says, that we don't need a Big Bang, we can just assume that all the stuff was along ago very close to each other and than there was just some explosion that has blown the parts away. Without deeply thinking about it the exploding egg might appear possible, but there are some problems with it. The easiest to explain might be the one presented very comprehensively with visuals in the online lecture series by Paul Francis and Brian Schmidt, see here - an explosion requires that there is a difference in the density of the blown away matter, either you would have faster moving things and slower moving things (hence more density towards the explosion center) or you would have all parts with the same speed (hence everything having the same density, but only in a shell and outside of the shell there is just density=0). But this is not what we observe. We observe that the universe is homogenous and isotropic in any direction. And this is something that the exploding egg doesn't explain, but Big Bang does. How? Here comes what I think is the main point of the Big Bang - it says that there was not an explosion IN space, but rather a start of the expansion OF space. And because space itself has been and is still expanding ever since you see that things move away from each other (if they are sufficiently far away from each other!), but the decrease in density over time is the same in every direction of the universe
A: Creationist arguments are pretty much always debunked somewhere in An Index to Creationist Claims. See here and here for original-energy concerns. That said:
The goal of cosmology is to understand the history of the Universe as far back as possible${}^\dagger$, in a manner that fits observations and the rest of our physical theory. The Universe has spent the last 13.8 Gigayears or so expanding from a very dense state. This fact about its history correctly predicts various observable facts. It was first postulated, without such a precise estimate of how long it's been happening, to explain Hubble's law, but it has since made sense of other facts, such as CMB radiation and primordial nuclide ratios.
We can't currently support with observational data any specific claims about what the Universe was doing before this expansion began, or even whether it existed prior to this; nor does other theory obligate a specific answer. (See also here.) We simply use the term "age of the Universe" as shorthand for how long ago this expansion started, partly because a state that dense would observationally screen any hypothetical earlier period from us.
But the Big Bang is compatible with both observation and theory; indeed, such expansion is required by general relativity if, on large length scales, the Universe is homogeneous and isotropic. There is no theoretical reason it wouldn't be, and observations don't challenge that either.
${}^\dagger$ And its present, and future as far forward as possible, but let's both of those considerations aside for now.
