One of the most productive areas of research in modern cosmology is the application of quantum mechanics to an analysis of the origins and very earliest stages of the universe. It is important to note that our observational knowledge of the origins and early stages of the universe is very limited. But we can argue back quite rigorously to the physical conditions which characterized those stages by applying physics and mathematics to what we observe in the universe today. Amidst the myriads of such observational data there are three principal observations which emerge and which allow us to reconstruct the early universe: (1) from the measurements of distant galaxies and clusters of galaxies we know that the universe is expanding with very precise conditions and, indeed, accelerating; (2) from the measurement of the abundances of helium, lithium, deuterium and other light elements, we know that much of that material had to be created under extremely high temperature and density conditions in the early universe; (3) from a measurement of the current temperature of the universe, the so-called cosmic background radiation, we can establish the temperature of the early universe. When we combine all of this and other observations we can determine the age of the universe, its approximate mass and its mean density.
This summary of the results of modern cosmology represents an amazing feat in the combination of our knowledge of elementary particle physics and observational astrophysics. But the nagging questions remain: how did it all begin? When it began were there not certain initial conditions which determined how it would evolve? Did the universe really come to be in all its specificity from quantum fluctuations at its origin?
What is the best approach to such questions? How do we know we are on the path to the truth? In other words how do we judge what is the best model for now. Cosmology shares, with the other natural sciences, a number of criterion whereby a model is judged to be best. I would list the principal criteria as the following: (1) verifiability or falsifiability; (2) predictability; (3) simplicity or economy, i.e., the least assumptions are made to get the greatest explanatory power; (4) beauty, i.e., the model has an aesthetic quality about it; (5) unifying explanatory power; i.e. the model not only explains the observations at hand but it also is in harmony with all else that we know, even with that which we know outside of the natural sciences.
It is this last criterion which I would like to discuss, since it appears to me to extend the epistemological nature of the natural sciences towards the realm of other disciplines, such as philosophy and theology. Put in very simple terms this criterion is nothing else than a call for the unification of our knowledge. One could hardly be opposed to that. The problem arises with the application of this criterion. When is the unification not truly unifying but rather an adulteration of knowledge obtained by one discipline with the presuppositions inherent in another discipline. History is full of examples of such adulterations. It is for this reason that scientists have always hesitated to make use of this criterion. And yet, if applied cautiously, it appears to me to be a most creative one for the advancement of our knowledge.
In the Hot Big Bang cosmological models the universe had a beginning. That beginning at time zero is a mathematical singularity. It cannot be addressed by classical mathematics or physics. To avoid that singularity it is claimed that quantum gravity must be applied at the extreme conditions of the universe's beginning. During this quantum gravity regime, however, the concept of time is not applicable in any simple way. Most approaches require an origin of our specific universe from quantum fluctuations of a previous state: a collapsing previous state, a region of flat space-time, a previous black hole final state, etc. Such approaches, therefore, only address relative beginnings. They still leave us wondering about the origins of the previous state upon which the quantum fluctuations played out their game. A suggestion has been made that the origin of the universe occurred in a state where time did not exist. There were four spatial dimensions instead of three spatial dimensions plus time. In such a model there was no beginning to the universe in the usual sense of the word. Rather time came to be. What, if anything, do these quantum gravity considerations of the origin of the universe have to do with, for instance, the theological considerations of the creation of the universe in time and from nothing (creatio ex nihilo)?
Any attempt to simply identify the nothing (nihilo) of the theologians with the quantum fluctuations of one of the preexisting states or with the unbounded regime of quantum cosmology would, to my mind, create nothing but confusion. But the one concept may illuminate the other. The thrust of the "in time" and "from nothing" of the theologians is to assert the total and exclusive dependence of the universe upon God the Creator. There was no rival to God preexisting before the universe began and in its beginning and continuation it depends on God. I cannot see how the scientific concepts deny or challenge the theological ones and they may even be illuminated by them. It would be equally confusing to deny the existence of God by stating that, at least in the quantum cosmological conception, since no boundary conditions were required for the origin of the universe, God is not required. The God of the theologians is not a boundary condition for the universe. He is the creator, whatever content that notion of creator might have. It would be sufficient, I think, to state that God willed that the Universe come to be in the quantum cosmology model, if that proves indeed to be the correct one.