Abstract:

Cosmological observations and terrestrial experiments disfavor natural explanations of the smallness of vacuum energy and of the weak scale. A fundamental theory that predicts a large landscape of three-dimensional vacua can explain the apparent fine-tunings. Theories of this type can be falsified by the standard approach: one must extract quantitative predictions and compare to observation. But the task is novel, and it faces formidable challenges. The underlying theory must be understood well, and statistical tools must be developed to derive the distribution of low energy parameters. Moreover, one must account for correlations between the distribution of observers in the multiverse and local low-energy parameters. Unexpected progress has come as a byproduct of the study of a different problem: the global divergences associated with accelerated expansion. The "causal patch", a regulator obtained by formal arguments, has turned out to yield robust predictions for the abundances of dark energy and dark matter. The analysis requires no specific anthropic assumptions, and only a simple parametrization of the statistical properties of the underlying theory. Compared to earlier work, quantitative agreement is much improved, even when Weinberg's assumption that observers require galaxies is dropped. Coincidence problems are directly addressed.