Here I paraphrase Einstein's famous question: "Did God have any choice in the creation of the Universe". I get rid of the God part, which Einstein only added to make it seem more whimsical, I am sure, because that just confuses the issue. The important question, perhaps the most important question facing physics today is the question of whether there is only one consistent set of physical laws that allow a working universe, or rather whether the constants of nature are arbitrary, and could take any set of values. Namely, if we continue to probe into the structure of matter and the nature of elementary forces will we find that mathematical consistency is possible only for one unique theory of the Universe, or not? In the former case, of course, there is hope for an exactly predictive "theory of everything". In the latter case, we might expect that it is natural that our Universe is merely one of an infinite set of Universes within some grand multiverse, in each of which the laws of physics differ, and in which anthropic arguments may govern why we live in the Universe we do.
The goal of physics throughout the ages has been to explain exactly why the universe is the way it is, but as we push closer and closer to the ultimate frontier, we may find out that in fact the ultimate laws of nature may generically produce a universe that is quite different from the one we live in. This would force a dramatic shift in our concept of natural law.
Some may suggest that this question is mere philosophical nonsense, and is akin to asking how many angels may sit on the head of a pin. However, I think that if we are lucky it may be empirically possible to address it. If, for example, we do come up with some fundamental theory that predicts the values of many fundamental quantities correctly, but that predicts that other mysterious quantities, like the energy of empty space, is generically different than the value we measure, or perhaps is determined probabilistically, this will add strong ammunition to the notion that our universe is not unique, but arose from an ensemble of causally disconnected parts, each with randomly varying values of the vacuum energy.
In any case, answerable or not, I think this is the ultimate question in science.
Lawrence Krauss is Professor of Physics at Case Western Reserve University and the author of Atom.