The discovery of dark energy has presented both observational and theoretical cosmologists with a win-win scenario. On the observational side, we will either verify to high precision the existence of a truly constant vacuum energy representing a new fundamental constant of nature and a potentially crucial clue to the reconciliation of gravity with quantum field theory, or we will detect variations in the dark energy density which indicate either a new dynamical component or an alteration of general relativity itself. On the theoretical side, we have been given invaluable insight into one of the most perplexing issues in theoretical physics (the cosmological constant problem), and we are now faced with a brand new issue (the coincidence problem) whose resolution will necessarily involve exciting new theoretical developments.
Nobody who took arguments of naturalness and fine-tuning seriously would have expected to discover a small but nonzero dark energy density (1) We should not conclude from this that such arguments have no value, but that we should always be prepared for surprises. One way of characterizing our current inventory of the universe is to divide it into ordinary baryonic matter, comprising 5% of the energy density of the universe, and a "dark sector" comprising the remaining 95%. In this classification, the role of the recent discoveries has been to reveal that the dark sector includes at least two distinct components, the dark matter and the dark energy. Who is to say that future experiments will not reveal further structure within this sector, perhaps including interesting interactions between components? It is safe to say that the future of dark physics looks very bright.
This work was supported in part by the U.S. Dept. of Energy, the Alfred P. Sloan Foundation, and the David and Lucile Packard Foundation.
1 This is a rhetorical exaggeration. It has been pointed out to me, quite correctly, that people who took naturalness arguments seriously might have expected a nonzero cosmological constant if they also took the anthropic principle seriously. Back.