Initial Conditions of the Universe
An old dream of science is to find an ultimate theory by which one can explain and in principle predict everything. Some branches of quantum gravity have, in the context of string theory, relived this idea in a modern form, but recent research revealed how far we really are from its realization: Looking for solutions which can describe a universe accelerating at late times, much too many of them turned up without, so far, a way to select one among them. Thus, while one may believe that certain theoretical constructions are unique, too many solutions result to be compared with observations. Some of these solutions may be close to what we see in our universe, but this will be only a small fraction of all possibilities. A newly proposed viewpoint by Hawking and Hertog, combining different properties of quantum gravity theories, claims to solve this problem. Based on quantum cosmology, conditions for unique solutions, appearing as quantum superpositions of classical alternatives, can be devised and used for predictions. However, the procedure raises more questions regarding whether physics is to just predict or to explain observed phenomena. These issues are discussed in Nature 442, 988. Nature News & Views, M. Bojowald.